Abstracts

Introduction

There is a growing interest in the association between immigration and food consumption. Food and beverages play a key role in maintaining social and cultural connections among immigrants. Over time, food consumption among immigrants tends to change due to acculturation and lack of availability of typical food products from their home country ¹1. Lv N, Cason KL. Dietary pattern change and acculturation of Chinese Americans in Pennsylvania. J Am Diet Assoc 2004; 104:771-8.^,22. Méjean C, Traissac P, Eymard-Duvernay S, Delpeuch F, Maire B. Influence of acculturation among Tunisian migrants in France and their past/present exposure to the home country on diet and physical activity. Public Health Nutr 2008; 12: 832-41.^,33. Ayala G, Baquero B, Klinger S. A systematic review of the relationship between acculturation and diet among latinos in the United States: implications for future research. J Am Diet Assoc 2008; 108:1330-44..

An association has been observed between dietary acculturation and immigrant integration into the host country and a deterioration of diet among immigrants ⁴4. Satia JA. Dietary acculturation and the nutrition transition. Appl Physiol Nutr Metab 2010; 35: 219-23.. Immigration often leads to changes in the quality of food consumed and cooking methods. Immigrants are susceptible to a rapid change in emphasis towards convenience foods, such as snacks and sweets, while the consumption of staple foods like rice and corn remains unchanged over longer periods of time ⁵5. Gilbert PA, Khokhar S. Changing dietary habits of ethnic groups in Europe and implications for health. Nutr Rev 2008; 66:203-15.. Studies show that the factors that affect eating habits among immigrants include food availability, income, beliefs relating to food, religion and "food laws", and the generation and age of immigrants ⁵5. Gilbert PA, Khokhar S. Changing dietary habits of ethnic groups in Europe and implications for health. Nutr Rev 2008; 66:203-15..

Dietary acculturation does not always have negative results. In a study of Moroccan immigrants carried out in Spain, food choices were more varied and nutritionally-dense among of the most acculturated immigrants ⁶6. Sáez PPM, Cantero AMT, Isasa MET. La alimentación de los inmigrantes marroquíes de la Comunidad de Madrid: factores que influyen en la selección de los alimentos. Aten Primaria 2001; 27: 264-70.. The authors also concluded that recent immigrants with poor fluency in Spanish may have difficulties in accessing a large variety of foods.

Other studies suggest that the initial health status of immigrants is better than that of the population of the host country. This phenomenon is known as the "healthy immigrant effect" ⁷7. Argeseanu Cunningham S, Ruben JD, Narayan KMV. Health of foreign-born people in the United States: a review. Health Place 2008; 14:623-35.. However, this effect often disappears as length of residence increases ⁷7. Argeseanu Cunningham S, Ruben JD, Narayan KMV. Health of foreign-born people in the United States: a review. Health Place 2008; 14:623-35.^,88. Oza-Frank R, Cunningham S. The weight of US residence among immigrants: a systematic review. Obes Rev 2010; 11:271-80.. On the other hand, comparative studies show that after immigration the risk of diseases associated with obesity, such as cardiovascular disease, diabetes, and cancer, may change over time ⁷7. Argeseanu Cunningham S, Ruben JD, Narayan KMV. Health of foreign-born people in the United States: a review. Health Place 2008; 14:623-35.. Susceptibility to obesity [defined as body mass index (BMI) ≥ 30 kg/m²] ⁹9. World Health Organization. Obesity: preventing and managing the global epidemic. Geneva: World Health Organization; 2000. (Technical Report Series, 894). increases with immigration and length of residence in the host country. Evidence from studies carried out in the United States suggests that recent immigrants generally have a lower BMI than the American population. However, BMI tends to increase as the length of residence in the host country increases, progressively converging towards the levels of the American population ⁷7. Argeseanu Cunningham S, Ruben JD, Narayan KMV. Health of foreign-born people in the United States: a review. Health Place 2008; 14:623-35.. The same phenomenon has been observed by studies in Canada, Australia and Europe ⁸8. Oza-Frank R, Cunningham S. The weight of US residence among immigrants: a systematic review. Obes Rev 2010; 11:271-80..

Studies of this phenomenon in other countries are scarce. Four of the five systematic reviews of this issue concentrate exclusively on obesity among the US immigrant population. Misra & Ganda ¹⁰10. Misra A, Ganda OP. Migration and its impact on adiposity and type 2 diabetes. Nutrition 2007; 23:696-708. analyzed this issue from a global perspective. However, the study was published in 2007 and lacks a qualitative assessment of the papers included. The present article is the result of an updated systematic review of studies to analyze the association between length of time since immigration and body weight among first generation immigrants.

The review was worldwide, including countries for which data was available, and focused on two approaches to research which address the links between immigration, acculturation and weight gain among first generation immigrants. The first uses length of residence in the host country as a proxy measure of acculturation. Although a very common approach, the validity of this assumption has yet to be fully demonstrated in the literature ⁹9. World Health Organization. Obesity: preventing and managing the global epidemic. Geneva: World Health Organization; 2000. (Technical Report Series, 894).. The second area of research compares immigrants living in the host country with their counterparts living in their home country.

Methods

A systematic review was conducted in accordance with PRISMA guidelines ¹¹11. Moher D, Liberati A, Tetzlaff J, Altman DG. Preferred reporting items for systematic reviews and meta-analyses: the PRISMA statement. BMJ 2009; 339:b2535. through a search of published studies contained in the PubMed, JSTOR and EBSCO databases, with no publication date restrictions. The search was conducted initially on July 27^th 2011 and updated on August 8^th 2014. The following combined keywords were used as alternatives in order to obtain as many studies as possible: "immigrant"; "migrant", "foreign-born"; "country of birth", "country of origin"; "region of birth", "region of origin"; "BMI", "body mass index"; "overweight", "obesity"; "body weight" and "duration", "length of stay"; "time"; "acculturation".

The titles and abstracts of the identified papers were independently screened by the first two authors who arrived at a consensus on eligibility and content. The full texts of potentially relevant papers were assessed for eligibility based on the following criteria: (i) papers must describe original work; (ii) studies include adults born in foreign countries (first generation immigrants) and respective specific data about the variables of interest; (iii) measure of overweight or BMI (or change in these variables) is a key/endpoint variable - whether self-reported or not; (iv) studies include length of residence in the host country or amount of time spent in the host country as a key/independent variable; (v) studies include data that allows comparison of immigrants groups and nonimmigrant counterparts from the immigrant's country of origin; (vi) full texts are in English, Spanish or Portuguese. Studies that focused only on children or teenagers, regardless of whether they were first generation immigrants, were excluded. Generally this group is not entirely independent in their food and leisure physical activity choices, and choice of where to live, and therefore the factors that influence the variables in this group are different to those influencing adult immigrants.

Data was extracted into a standardized matrix that included the title of the article, name of the scientific journal and publication, host country, population and comparison groups (if applicable), main outcome (BMI, overweight/obesity, change in BMI), statistical analysis, covariates used, and main results.

The first two authors independently appraised the quality of the evidence produced by studies focussing on the impact of immigration on obesity based on the following criteria: (a) recruitment strategy; (b) sample size; (c) whether participation rate was reported; (d) participation rate; (e) well defined eligibility criteria; and (f) overall adequacy of methods. The results for each criteria were used to generate a final score, ranging from 0 (low relevance) to 7 (high relevance).

Results

The study selection process is shown in Figure 1. A total of 39 articles from an initial list of 365 citations were considered eligible for this review. These articles were published between 2000 and 2014 and based on studies in the following countries: the United States (25), Canada (4), the Netherlands (1), Spain (1), Italy (1), Israel (1), France (1), Norway (2), Luxembourg (1), Sweden (1) and the United Kingdom (1).

Sample sizes varied between 53 ¹²12. Regev-Tobias H, Reifen R, Endevelt R, Havkin O, Cohen E, Stern G, et al. Dietary acculturation and increasing rates of obesity in Ethiopian women living in Israel. Nutrition 2012; 28:30-4. and 478,276 individuals ¹³13. Singh GK, Siahpush M, Hiatt R, Timsina LR. Dramatic increases in obesity and overweight prevalence and body mass index among ethnic-immigrant and social class groups in the United States, 1976-2008. J Community Health 2011; 36:94-110.. Table 1 shows that a number of papers included in this review did not include relevant details about methodology, particularly sampling and recruitment techniques and participation rates, thus compromising the quality of the evidence.

Length of residence in the host country as a determinant of increases in BMI

Significant disparities were observed between immigrant sub-groups with regard to the association between length of time since immigration and BMI ¹³13. Singh GK, Siahpush M, Hiatt R, Timsina LR. Dramatic increases in obesity and overweight prevalence and body mass index among ethnic-immigrant and social class groups in the United States, 1976-2008. J Community Health 2011; 36:94-110.. Results are therefore presented by immigrant group (Table 2) showing the results for specific countries of origin where available. Some studies therefore appear more than once. We show the most prevalent religion among immigrants and first language based on the region of birth to facilitate the grouping of immigrants and the interpretation of potential acculturation barriers. Four of the studies ¹²12. Regev-Tobias H, Reifen R, Endevelt R, Havkin O, Cohen E, Stern G, et al. Dietary acculturation and increasing rates of obesity in Ethiopian women living in Israel. Nutrition 2012; 28:30-4.^,1414. Wolin KY, Colangelo LA, Chiu BC, Gapstur S. Obesity and immigration among latina women. J Immigr Minor Health 2009; 11:428-31.^,1515. Himmelgreen DA, Pe R, Martinez D, Bretnall A, Eells B. The longer you stay, the bigger you get: length of time and language use in the U.S. are associated with obesity in Puerto Rican women. Am J Phys Anthropol 2004; 125:90-6.^,1616. Miller AM, Chandler PJ, Wilbur J, Sorokin O. Acculturation and cardiovascular disease risk in midlife immigrant women from the former Soviet Union. Prog Cardiovasc Nurs 2004; 19:47-55. used only female samples. All but one of the studies ¹⁷17. Roshania R, Narayan KM, Oza-Frank R. Age at arrival and risk of obesity among US immigrants. Obesity (Silver Spring) 2008; 16:2669-75. sampled only adults. All but one of the studies ¹⁸18. Albrecht SS, Diez Roux AV, Kandula NR, Osypuk TL, Ni H, Shrager S. Immigrant assimilation and BMI and waist size: a longitudinal examination among Hispanic and Chinese participants in the multi-ethnic study of atherosclerosis. Obesity (Silver Spring) 2013; 21:1695-703. were cross-sectional. One study consisted of a repeated cross-sectional survey to evaluate changes in the prevalence of obesity among immigrants and US-born individuals ¹⁹19. Park J, Myers D, Kao D, Min S. Immigrant obesity and unhealthy assimilation: alternative estimates of convergence or divergence, 1995-2005. Soc Sci Med 2009; 69:1625-33.. BMI was self-reported in the majority of the studies. Ten of the studies measured weight and height ¹⁴14. Wolin KY, Colangelo LA, Chiu BC, Gapstur S. Obesity and immigration among latina women. J Immigr Minor Health 2009; 11:428-31.^,1616. Miller AM, Chandler PJ, Wilbur J, Sorokin O. Acculturation and cardiovascular disease risk in midlife immigrant women from the former Soviet Union. Prog Cardiovasc Nurs 2004; 19:47-55.^,1818. Albrecht SS, Diez Roux AV, Kandula NR, Osypuk TL, Ni H, Shrager S. Immigrant assimilation and BMI and waist size: a longitudinal examination among Hispanic and Chinese participants in the multi-ethnic study of atherosclerosis. Obesity (Silver Spring) 2013; 21:1695-703.^,2020. Park Y, Neckerman KM, Quinn J, Weiss C, Rundle A. Place of birth, duration of residence, neighborhood immigrant composition and body mass index in New York City. Int J Behav Nutr Phys Act 2008; 5:1-11.^,2121. Fu H, Vanlandingham MJ. Disentangling the effects of migration, selection and acculturation on weight and body fat distribution: results from a natural experiment involving Vietnamese Americans, returnees, and never-leavers. J Immigr Minor Health 2012; 14:786-96.^,2222. Alkerwi A, Sauvageot N, Pagny S, Beissel J, Delagardelle C, Lair M-L. Acculturation, immigration status and cardiovascular risk factors among Portuguese immigrants to Luxembourg: findings from ORISCAV-LUX study. BMC Public Health 2012; 12:864.^,2323. Gele A, Mbalilaki AJ. Overweight and obesity among African immigrants in Oslo. BMC Res Notes 2013; 6:119.^,2424. Garduño-Diaz SD, Khokhar S. South Asian dietary patterns and their association with risk factors for the metabolic syndrome. J Hum Nutr Diet 2013; 26:145-55., while one study measured only height and weight was self-reported ²⁵25. Fuentes-Afflick E, Hessol N. Acculturation and body mass among Latina women. J Womens Health 2008; 17:67-73.. In another study, weight and height were measured but change in BMI (outcome variable) was calculated based on self-reported past weight and height ²⁶26. Iversen T, Ma C-TA, Meyer HE. Immigrants' acculturation and changes in body mass index. Econ Hum Biol 2013; 11:1-7.. Only one study reported to have imputed missing data ²⁷27. Creighton MJ, Goldman N, Pebley AR, Chung CY. Durational and generational differences in Mexican immigrant obesity: is acculturation the explanation? Soc Sci Med 2012; 75:300-10.. The variable length of residence in the host country was always defined using an interval scale, generally divided into recent (< 5 years) and long-term (> 15 years). Eleven ¹³13. Singh GK, Siahpush M, Hiatt R, Timsina LR. Dramatic increases in obesity and overweight prevalence and body mass index among ethnic-immigrant and social class groups in the United States, 1976-2008. J Community Health 2011; 36:94-110.^,1717. Roshania R, Narayan KM, Oza-Frank R. Age at arrival and risk of obesity among US immigrants. Obesity (Silver Spring) 2008; 16:2669-75.^,2020. Park Y, Neckerman KM, Quinn J, Weiss C, Rundle A. Place of birth, duration of residence, neighborhood immigrant composition and body mass index in New York City. Int J Behav Nutr Phys Act 2008; 5:1-11.^,2828. Koya DL, Egede LE. Association between length of residence and cardiovascular disease risk factors among an ethnically diverse group of United States immigrants. J Gen Intern Med 2007; 22: 841-6.^,2929. Sanchez-Vaznaugh EV, Kawachi I, Subramanian SV, Sánchez BN, Acevedo-Garcia D. Differential effect of birthplace and length of residence on body mass index (BMI) by education, gender and race/ethnicity. Soc Sci Med 2008; 67:1300-10.^,3030. McDonald JT, Kennedy S. Is migration to Canada associated with unhealthy weight gain? Overweight and obesity among Canada's immigrants. Soc Sci Med 2005; 61:2469-81.^,3131. Oza-Frank R, Narayan KMV. Effect of length of residence on overweight by region of birth and age at arrival among US immigrants. Public Health Nutr 2009; 13:868-75.^,3232. Gutiérrez-Fisac JL, Marín-Guerrero A, Regidor E, Guallar-Castillón P, Banegas JR, Rodríguez-Artalejo F. Length of residence and obesity among immigrants in Spain. Public Health Nutr 2010; 13: 1593-8.^,3333. Tremblay MS, Pérez CE, Ardern CI, Bryan SN. Obesity, overweight and ethnicity. Health Rep 2005; 16:23-34.^,3434. Goel MS, McCarthy EP, Phillips RS, Wee CC. Obesity among US immigrant subgroups by duration of residence. JAMA 2004; 292:2860-7.^,3535. Chiu M, Austin PC, Manuel DG, Tu JV. Cardiovascular risk factor profiles of recent immigrants vs long-term residents of Ontario: a multi-ethnic study. Can J Cardiol 2012; 28:20-6. studies included all immigrant groups resident in the host country. Five studies ¹⁷17. Roshania R, Narayan KM, Oza-Frank R. Age at arrival and risk of obesity among US immigrants. Obesity (Silver Spring) 2008; 16:2669-75.^,2828. Koya DL, Egede LE. Association between length of residence and cardiovascular disease risk factors among an ethnically diverse group of United States immigrants. J Gen Intern Med 2007; 22: 841-6.^,3232. Gutiérrez-Fisac JL, Marín-Guerrero A, Regidor E, Guallar-Castillón P, Banegas JR, Rodríguez-Artalejo F. Length of residence and obesity among immigrants in Spain. Public Health Nutr 2010; 13: 1593-8.^,3636. Cairney J, Ostbye T. Time since immigration and excess body weight. Can J Public Health 1999; 90:120-4.^,3737. Martin-Fernandez J, Grillo F, Tichit C, Parizot I, Chauvin P. Overweight according to geographical origin and time spent in France: a cross sectional study in the Paris metropolitan area. BMC Public Health 2012; 12:937. showed the increase in length of residence in the host country but did not stratify the results by immigrant group. Another study ²⁸28. Koya DL, Egede LE. Association between length of residence and cardiovascular disease risk factors among an ethnically diverse group of United States immigrants. J Gen Intern Med 2007; 22: 841-6. presented results by region of origin, concluding that this variable had a significant effect on the prevalence of cardiovascular risk factors (including overweight) among immigrants. A study by Martin-Fernandez et al. ³⁷37. Martin-Fernandez J, Grillo F, Tichit C, Parizot I, Chauvin P. Overweight according to geographical origin and time spent in France: a cross sectional study in the Paris metropolitan area. BMC Public Health 2012; 12:937. of immigrants living in France found that the association between length of time spent in the host country and BMI/overweight was only evident among older men.

• Hispanics

Nineteen mentions of the Hispanic subgroup involving different countries of origin were made in 17 studies, of which 15 showed a positive association between length of residence and BMI/rate of overweight 79%). All except one of the studies (from Canada) were from the United States. Sample sizes varied between 32 and 273,188 immigrants.

A cross-sectional population based study carried out by Sanchez-Vaznaugh et al. ²⁹ 29. Sanchez-Vaznaugh EV, Kawachi I, Subramanian SV, Sánchez BN, Acevedo-Garcia D. Differential effect of birthplace and length of residence on body mass index (BMI) by education, gender and race/ethnicity. Soc Sci Med 2008; 67:1300-10.in California showed that increases in BMI with length of residence and general mean BMI were greatest among Hispanic immigrants. This association varies within the Hispanic subgroup, with Mexican immigrants experiencing the greatest increase in BMI and waist circumference over time ¹⁸18. Albrecht SS, Diez Roux AV, Kandula NR, Osypuk TL, Ni H, Shrager S. Immigrant assimilation and BMI and waist size: a longitudinal examination among Hispanic and Chinese participants in the multi-ethnic study of atherosclerosis. Obesity (Silver Spring) 2013; 21:1695-703.. Albretch et al. ¹⁸18. Albrecht SS, Diez Roux AV, Kandula NR, Osypuk TL, Ni H, Shrager S. Immigrant assimilation and BMI and waist size: a longitudinal examination among Hispanic and Chinese participants in the multi-ethnic study of atherosclerosis. Obesity (Silver Spring) 2013; 21:1695-703. conducted the only longitudinal study on this matter involving Asian and Hispanic immigrants living in six different regions of the United States. Mean annual changes in BMI were statistically significant among Mexican immigrants, but not in other Hispanic subgroups. However, these changes are not significantly different from those observed among US-born Mexican-Americans.

The findings suggest differences between genders. Barcenas et al. ³⁸38. Barcenas CH, Wilkinson AV, Strom SS, Cao Y, Saunders KC, Mahabir S, et al. Birthplace, years of residence in the United States, and obesity among Mexican-American adults. Obesity (Silver Spring) 2007; 15:1043-52. reported that among males BMI was significantly in long-term immigrants (resident for ≥ 15 years) than among short-term immigrants (< 5 years), while among females BMI was significantly higher in all length of residence categories (5 to 9 years; 10 to 14 years; ≥ 15 years) than in the short-term category ³²32. Gutiérrez-Fisac JL, Marín-Guerrero A, Regidor E, Guallar-Castillón P, Banegas JR, Rodríguez-Artalejo F. Length of residence and obesity among immigrants in Spain. Public Health Nutr 2010; 13: 1593-8..

• Asians

Results were less consistent, showing differences according to region of birth. Sample sizes varied from 261 to 254,153 individuals. Studies covered the following host countries: United States (13), Canada (3), Norway (1), and the Netherlands (1). Of the 34 mentions made of the Asian subgroup (corresponding to 18 different studies about Asian immigrants), 13 (38.2%) found a positive and significant association between length of residence and BMI/rate of overweight. McDonald et al. ³⁰30. McDonald JT, Kennedy S. Is migration to Canada associated with unhealthy weight gain? Overweight and obesity among Canada's immigrants. Soc Sci Med 2005; 61:2469-81. found that there was little change in weight with increasing length of residence in the host country among Chinese immigrants living in Canada. The only longitudinal study included in this review showed that there was no increase in BMI or waist circumference among Chinese immigrants after a five-year follow-up ¹1. Lv N, Cason KL. Dietary pattern change and acculturation of Chinese Americans in Pennsylvania. J Am Diet Assoc 2004; 104:771-8., while three studies ¹1. Lv N, Cason KL. Dietary pattern change and acculturation of Chinese Americans in Pennsylvania. J Am Diet Assoc 2004; 104:771-8.^,1313. Singh GK, Siahpush M, Hiatt R, Timsina LR. Dramatic increases in obesity and overweight prevalence and body mass index among ethnic-immigrant and social class groups in the United States, 1976-2008. J Community Health 2011; 36:94-110.^,3939. Lauderdale DS, Rathouz PJ. Body mass index in a US national sample of Asian Americans: effects of nativity, years since immigration and socioeconomic status. Int J Obes (Lond) 2000; 24:1188-94. found a low prevalence of overweight in this group. Evidence from two studies ³⁰30. McDonald JT, Kennedy S. Is migration to Canada associated with unhealthy weight gain? Overweight and obesity among Canada's immigrants. Soc Sci Med 2005; 61:2469-81.^,31 31. Oza-Frank R, Narayan KMV. Effect of length of residence on overweight by region of birth and age at arrival among US immigrants. Public Health Nutr 2009; 13:868-75.shows a positive or null association between the two main variables among immigrants from the Middle East and West Asia. While no increase in BMI with increasing length of residence in the host country was shown for Indians living in California ⁴⁰ 40. Bharmal N, Kaplan R, Shapiro MF, Kagawa-Singer M, Wong MD, Mangione CM, et al. The association of religiosity with overweight/obese body mass index among Asian Indian immigrants in California. Prev Med 2013; 57:315-21., BMI and insulin-resistance tends to be greater in this group than in their counterparts living in India ¹⁰10. Misra A, Ganda OP. Migration and its impact on adiposity and type 2 diabetes. Nutrition 2007; 23:696-708.. A positive association between increase in BMI and increasing length of residence in the host country was found among Vietnamese immigrants living in United States ²¹21. Fu H, Vanlandingham MJ. Disentangling the effects of migration, selection and acculturation on weight and body fat distribution: results from a natural experiment involving Vietnamese Americans, returnees, and never-leavers. J Immigr Minor Health 2012; 14:786-96.^,4141. Choi JY, Hwang J, Yi J. Acculturation, body perception, and weight status among Vietnamese American students. J Immigr Minor Health 2011; 13:1116-24. and this group was more likely to be overweight than returnees or counterparts who never left the country ²¹21. Fu H, Vanlandingham MJ. Disentangling the effects of migration, selection and acculturation on weight and body fat distribution: results from a natural experiment involving Vietnamese Americans, returnees, and never-leavers. J Immigr Minor Health 2012; 14:786-96.. This effect was not confirmed in Norway ²⁶26. Iversen T, Ma C-TA, Meyer HE. Immigrants' acculturation and changes in body mass index. Econ Hum Biol 2013; 11:1-7.. Dijkshoorn et al. ⁴²42. Dijkshoorn H, Nierkens V, Nicolaou M. Risk groups for overweight and obesity among Turkish and Moroccan migrants in The Netherlands. Public Health 2008; 122:625-30. found a high prevalence of obesity in Turkish immigrants living in the Netherlands, but there was no evidence of an association with length of residence in the host country.

• Europeans

Sample sizes varied between 218 and 163,797. Only two studies included the immigrants' country of origin ¹⁶16. Miller AM, Chandler PJ, Wilbur J, Sorokin O. Acculturation and cardiovascular disease risk in midlife immigrant women from the former Soviet Union. Prog Cardiovasc Nurs 2004; 19:47-55.^,2222. Alkerwi A, Sauvageot N, Pagny S, Beissel J, Delagardelle C, Lair M-L. Acculturation, immigration status and cardiovascular risk factors among Portuguese immigrants to Luxembourg: findings from ORISCAV-LUX study. BMC Public Health 2012; 12:864.. Eleven mentions were made of the European subgroup in 10 different studies, of which eight reported a positive association between the two variables of interest (72.7%).

A cross-sectional population-based study in the United States carried out by Sanchez-Vaznaugh et al. ²⁹29. Sanchez-Vaznaugh EV, Kawachi I, Subramanian SV, Sánchez BN, Acevedo-Garcia D. Differential effect of birthplace and length of residence on body mass index (BMI) by education, gender and race/ethnicity. Soc Sci Med 2008; 67:1300-10. reported that the BMI of immigrants living in the host country for less than 5 years was 1.6% higher than that of immigrants who had been living in the country for 15 years or more. However, the authors also found that the BMI of white immigrants living in United States for 15 years was higher than the BMI of those who had lived in the country for 10 to 14 years. These results contradict the findings of a cross-sectional population-based study carried out in the United States by Oza-Frank et al. ³¹31. Oza-Frank R, Narayan KMV. Effect of length of residence on overweight by region of birth and age at arrival among US immigrants. Public Health Nutr 2009; 13:868-75. which shows that long-term Caucasian immigrants (resident for ≥ 15 years) are three-times more likely to be overweight than recent immigrants (resident for < 5 years) from the same ethnic background. Newly-arrived Caucasian immigrants in Canada are 15% less likely to be overweight but the rate of overweight progressively increases reaching host country levels after around 30 years of residence ³⁰30. McDonald JT, Kennedy S. Is migration to Canada associated with unhealthy weight gain? Overweight and obesity among Canada's immigrants. Soc Sci Med 2005; 61:2469-81..

• Africans

Two studies included North Africans (Moroccans). The host countries were the Netherlands and Italy and sample sizes were 339 and 1,384. Both studies found a significant positive association between the two variables of interest. However, the study in the Netherlands, carried out by Dijkshoorn et al. ⁴²42. Dijkshoorn H, Nierkens V, Nicolaou M. Risk groups for overweight and obesity among Turkish and Moroccan migrants in The Netherlands. Public Health 2008; 122:625-30., found that this association was only statistically significant among women (cross-sectional study with data derived from a sample taken from the Amsterdam population register).

The sample sizes of Sub-Saharan African immigrants varied between 208 and 163,797 and covered the following host countries: United States (6), Canada (1), Israel (1); and Norway (1). Nine mentions were made in nine different studies, five of which reported a positive and significant association between length of residence and BMI/rate of overweight (55.6%). Only two studies reported the nationality of immigrants (Ethiopia and Somalia). A significant increase in the rate of overweight with increasing length of residence was found among Somali immigrants living in Oslo (Norway) ²³23. Gele A, Mbalilaki AJ. Overweight and obesity among African immigrants in Oslo. BMC Res Notes 2013; 6:119..

Comparison of immigrants with respective home-country residents

Of the six studies found in this category, four were from European countries and two from Australia (Table 3). Sample sizes ranged from 250 ⁴³43. Mahajan D, Bermingham MA. Risk factors for coronary heart disease in two similar Indian population groups, one residing in India, and the other in Sydney, Australia. Eur J Clin Nutr 2004; 58:751-60. to 2,000 ⁴⁴44. Zahid N, Meyer HE, Kumar BN, Claussen B, Hussain A. High levels of cardiovascular risk factors among Pakistanis in Norway compared to Pakistanis in Pakistan. J Obes 2011; 2011:163749. and the papers were published between 1995 ⁴⁵45. Bhatnagar D, Anand IS, Durrington PN, Patel DJ, Wander GS, Mackness MI, et al. Coronary risk factors in people from the Indian subcontinent living in west London and their siblings in India. Lancet 1995; 345:405-9. and 2011 ⁴⁴44. Zahid N, Meyer HE, Kumar BN, Claussen B, Hussain A. High levels of cardiovascular risk factors among Pakistanis in Norway compared to Pakistanis in Pakistan. J Obes 2011; 2011:163749.. All samples included both genders. Two studies ⁴³43. Mahajan D, Bermingham MA. Risk factors for coronary heart disease in two similar Indian population groups, one residing in India, and the other in Sydney, Australia. Eur J Clin Nutr 2004; 58:751-60.^,4545. Bhatnagar D, Anand IS, Durrington PN, Patel DJ, Wander GS, Mackness MI, et al. Coronary risk factors in people from the Indian subcontinent living in west London and their siblings in India. Lancet 1995; 345:405-9. used relatives as controls. Five of the studies ⁴⁵45. Bhatnagar D, Anand IS, Durrington PN, Patel DJ, Wander GS, Mackness MI, et al. Coronary risk factors in people from the Indian subcontinent living in west London and their siblings in India. Lancet 1995; 345:405-9.^,4646. Tennakoon S, Kumar BN, Nugegoda DB, Meyer HE. Comparison of cardiovascular risk factors between Sri Lankans living in Kandy and Oslo. BMC Public Health 2010; 10:654.^,4747. Khan LK, Sobal J, Martorell R. Acculturation, socioeconomic status, and obesity in Mexican Americans, Cuban Americans, and Puerto Ricans. Int J Obes Relat Metab Disord 1997; 21:91-6.^,4848. Hargreaves MK, Schlundt DG, Buchowski MS. Contextual factors influencing the eating behaviours of African American women: a focus group investigation. Ethn Health 2002; 7:133-47.^,4949. Lindström M, Sundquist J. Immigration and leisure-time physical inactivity: a population-based study. Ethn Health 2001; 6:77-85. reported that immigrants had a higher BMI than the control group. A study of Indian immigrants living in Australia ⁴³ 43. Mahajan D, Bermingham MA. Risk factors for coronary heart disease in two similar Indian population groups, one residing in India, and the other in Sydney, Australia. Eur J Clin Nutr 2004; 58:751-60.showed no significant difference in BMI among men. However, the group living in India had higher waist-hip ratios. Two studies ⁴³43. Mahajan D, Bermingham MA. Risk factors for coronary heart disease in two similar Indian population groups, one residing in India, and the other in Sydney, Australia. Eur J Clin Nutr 2004; 58:751-60.^,4646. Tennakoon S, Kumar BN, Nugegoda DB, Meyer HE. Comparison of cardiovascular risk factors between Sri Lankans living in Kandy and Oslo. BMC Public Health 2010; 10:654. showed that the cardiovascular risk profile was better in immigrants than in the control group.

Five out of the six studies included Asian samples. Two studies ⁴³43. Mahajan D, Bermingham MA. Risk factors for coronary heart disease in two similar Indian population groups, one residing in India, and the other in Sydney, Australia. Eur J Clin Nutr 2004; 58:751-60.^,46 46. Tennakoon S, Kumar BN, Nugegoda DB, Meyer HE. Comparison of cardiovascular risk factors between Sri Lankans living in Kandy and Oslo. BMC Public Health 2010; 10:654.of Indian and Sri Lankan immigrants reported that the cardiovascular risk profile was worse in nonimmigrants, despite the fact that BMI was lower or equal in this group. Mahajan & Bermingham ⁴³43. Mahajan D, Bermingham MA. Risk factors for coronary heart disease in two similar Indian population groups, one residing in India, and the other in Sydney, Australia. Eur J Clin Nutr 2004; 58:751-60. found that the level of abdominal obesity was lower (waist-hip ratio) among Indian immigrants than among men and women living in the country of origin. Tennakoon et al. ⁴⁶46. Tennakoon S, Kumar BN, Nugegoda DB, Meyer HE. Comparison of cardiovascular risk factors between Sri Lankans living in Kandy and Oslo. BMC Public Health 2010; 10:654. found that female Sri Lankan immigrants had higher BMI and higher waist circumference, but lower triglyceride and blood pressure levels than women living in Sri Lanka, while Bhatnagar et al. ⁴⁵45. Bhatnagar D, Anand IS, Durrington PN, Patel DJ, Wander GS, Mackness MI, et al. Coronary risk factors in people from the Indian subcontinent living in west London and their siblings in India. Lancet 1995; 345:405-9. reported worse cardiovascular profiles, higher BMI values, and higher total cholesterol levels among Indians living in England when compared to their counterparts living in India.

Kouris-Blazo et al. ⁵⁰50. Kouris-Blazos A, Wahlqvist ML, Trichopoulou A, Polychronopoulos E, Trichopoulos D. Health and nutritional status of elderly Greek migrants to Melbourne, Australia. Age Ageing 1996; 25:177-89. reported that the rate of obesity among elderly Greek immigrants living in Australia was higher than among their counterparts in Greece. These immigrants apparently had healthier food habits but a worse health status.

Discussion

The association between length of residence in the host country and increase in BMI seems to vary according to the ethnic background of immigrants. This section therefore takes a more detailed look at the association by ethnic group.

Hispanics

The risk of developing obesity is greater among Hispanic immigrants than among people from other ethnic groups living in the United States ²⁹29. Sanchez-Vaznaugh EV, Kawachi I, Subramanian SV, Sánchez BN, Acevedo-Garcia D. Differential effect of birthplace and length of residence on body mass index (BMI) by education, gender and race/ethnicity. Soc Sci Med 2008; 67:1300-10.. The results of linear regression show a clear increase in mean BMI across generations of Hispanic immigrants ¹⁰10. Misra A, Ganda OP. Migration and its impact on adiposity and type 2 diabetes. Nutrition 2007; 23:696-708.. This increase is greater among female immigrants ³⁸38. Barcenas CH, Wilkinson AV, Strom SS, Cao Y, Saunders KC, Mahabir S, et al. Birthplace, years of residence in the United States, and obesity among Mexican-American adults. Obesity (Silver Spring) 2007; 15:1043-52., showing that women are more vulnerable to the acculturation process ²⁸28. Koya DL, Egede LE. Association between length of residence and cardiovascular disease risk factors among an ethnically diverse group of United States immigrants. J Gen Intern Med 2007; 22: 841-6..

High rates of prevalence of obesity and type 2 diabetes among Hispanics may be explained by the genetic heritage received from American Indians ¹⁰10. Misra A, Ganda OP. Migration and its impact on adiposity and type 2 diabetes. Nutrition 2007; 23:696-708.. Sanchez-Vaznaugh et al. ²⁹29. Sanchez-Vaznaugh EV, Kawachi I, Subramanian SV, Sánchez BN, Acevedo-Garcia D. Differential effect of birthplace and length of residence on body mass index (BMI) by education, gender and race/ethnicity. Soc Sci Med 2008; 67:1300-10. explain that newly-arrived Hispanic immigrants have lower educational attainment and are less likely to speak English, stalling upward social mobility and constraining life choices and opportunities. Furthermore, higher BMI may be due to cumulative exposure to social and physical environments and neighborhoods that lack access to nutritious food, promote the consumption of high-density foods and constrain physical activity. This is a relevant aspect considering that Hispanics tend to be from more socioeconomically disadvantaged backgrounds than other migrant groups ²⁰20. Park Y, Neckerman KM, Quinn J, Weiss C, Rundle A. Place of birth, duration of residence, neighborhood immigrant composition and body mass index in New York City. Int J Behav Nutr Phys Act 2008; 5:1-11.. The effect of the neighborhood factor can be so significant that some studies recommend it as a better measure of the socioeconomic status of Hispanic immigrants ⁴⁷47. Khan LK, Sobal J, Martorell R. Acculturation, socioeconomic status, and obesity in Mexican Americans, Cuban Americans, and Puerto Ricans. Int J Obes Relat Metab Disord 1997; 21:91-6..

Cuban and Puerto Rican long-term immigrants living in United States were the only groups where the risk of being overweight did not increase ¹³13. Singh GK, Siahpush M, Hiatt R, Timsina LR. Dramatic increases in obesity and overweight prevalence and body mass index among ethnic-immigrant and social class groups in the United States, 1976-2008. J Community Health 2011; 36:94-110.. Albrecht et al. ¹⁸18. Albrecht SS, Diez Roux AV, Kandula NR, Osypuk TL, Ni H, Shrager S. Immigrant assimilation and BMI and waist size: a longitudinal examination among Hispanic and Chinese participants in the multi-ethnic study of atherosclerosis. Obesity (Silver Spring) 2013; 21:1695-703. suggest that the impact of acculturation varies within the Hispanic subgroup. However, their study was the only longitudinal study found in our review and found no significant increase in BMI in Hispanic subgroups over a five-year follow up period, except in Mexicans where significant increases in BMI and waist circumference were observed. This increase is not significantly different from the one observed in Mexican-Americans born in the United States. Hispanics of Mexican origin often adopt negative health behaviors and are likely to be influenced by behaviors established in the country of origin. The study sample was relatively small and the cohort was relatively older and healthier and was sampled from selected regions, which may compromise the generalizability of the results. A study carried out by Park et al. ¹⁹19. Park J, Myers D, Kao D, Min S. Immigrant obesity and unhealthy assimilation: alternative estimates of convergence or divergence, 1995-2005. Soc Sci Med 2009; 69:1625-33. revealed that the rate of obesity among Hispanic immigrants in the United States increases with increasing length of residence, but does not reach the levels of the population born in the United States, and, according to Park et al. ¹⁹19. Park J, Myers D, Kao D, Min S. Immigrant obesity and unhealthy assimilation: alternative estimates of convergence or divergence, 1995-2005. Soc Sci Med 2009; 69:1625-33., obesity rates among Hispanic immigrants increase at a slower rate than in the native population. Park et al. ¹⁹19. Park J, Myers D, Kao D, Min S. Immigrant obesity and unhealthy assimilation: alternative estimates of convergence or divergence, 1995-2005. Soc Sci Med 2009; 69:1625-33. highlight the following reasons behind the misleading conclusions about obesity among immigrants: differences in aging between immigrants and the native population, the growing duration of the immigration trend and historical trends in obesity. All studies of Hispanic immigrants were conducted in the United States and Canada. Therefore, there is no evidence to suggest that the same increases in BMI and rates of overweight with increasing length of residence would be experienced by Hispanic immigrants in other host countries.

Asians

The analysis of this group is more challenging since the association between length of residence in the host country and BMI varies greatly according to the region of birth, which was not always described in the papers. Asian immigrants living in the United States seem to be the ethnic group that is least affected by acculturation ¹⁷17. Roshania R, Narayan KM, Oza-Frank R. Age at arrival and risk of obesity among US immigrants. Obesity (Silver Spring) 2008; 16:2669-75.^,3434. Goel MS, McCarthy EP, Phillips RS, Wee CC. Obesity among US immigrant subgroups by duration of residence. JAMA 2004; 292:2860-7.. Researchers have suggested that this may be due to the fact that Asians are more likely to have higher educational attainment than Hispanics and are more likely to live in better neighborhoods, which in turn influences access to nutritious food ²⁹29. Sanchez-Vaznaugh EV, Kawachi I, Subramanian SV, Sánchez BN, Acevedo-Garcia D. Differential effect of birthplace and length of residence on body mass index (BMI) by education, gender and race/ethnicity. Soc Sci Med 2008; 67:1300-10.. The evidence related to Middle East or West Asian immigrants is too scarce to make any firm conclusions. There is a positive association between BMI and rates of overweight with increasing length of residence among Vietnamese immigrants living in the United States, but not among those living in Norway. This suggests that the host country may be an important factor influencing the direction and strength of the association. Furthermore, the BMI of immigrants fluent in Norwegian is lower than in the native population, suggesting that greater acculturation is a protective factor for weight increase in this group ²⁶26. Iversen T, Ma C-TA, Meyer HE. Immigrants' acculturation and changes in body mass index. Econ Hum Biol 2013; 11:1-7.. Comparative studies with samples of Asian immigrants showed mixed results. While Mahajan & Bermingham ⁴³43. Mahajan D, Bermingham MA. Risk factors for coronary heart disease in two similar Indian population groups, one residing in India, and the other in Sydney, Australia. Eur J Clin Nutr 2004; 58:751-60. and Tennakoon et al. ⁴⁶46. Tennakoon S, Kumar BN, Nugegoda DB, Meyer HE. Comparison of cardiovascular risk factors between Sri Lankans living in Kandy and Oslo. BMC Public Health 2010; 10:654. reported a better cardiovascular risk profile in immigrants living in Sydney (Australia) and Oslo, respectively, Bhatnagar et al. ⁴⁵45. Bhatnagar D, Anand IS, Durrington PN, Patel DJ, Wander GS, Mackness MI, et al. Coronary risk factors in people from the Indian subcontinent living in west London and their siblings in India. Lancet 1995; 345:405-9. found the opposite in immigrants living in England. The first study had several limitations. The sample was composed of a small number of qualified professionals (n = 125), which may have influenced results. All the same, these differences may indicate a more pronounced change in food habits, caloric intake and, potentially, saturated fat intake in England. Tennakoon et al. ⁴⁶46. Tennakoon S, Kumar BN, Nugegoda DB, Meyer HE. Comparison of cardiovascular risk factors between Sri Lankans living in Kandy and Oslo. BMC Public Health 2010; 10:654. conducted their study with Sri Lankan immigrants living in Oslo and, according to the authors, the higher BMI in women living in Sri Lanka may be explained by the lack of availability of specific foods in Norway, where, despite increasing BMI among the native population, cardiovascular diseases have decreased with improvements in diet quality from the last 30 years. However, methodological limitations of this study included a gap of three years between the measurements of each sample group and the use of different methodologies to measure blood pressure, and a low response rate conditioned by educational level.

Europeans

The studies of European immigrants lack information regarding the country of origin, making it difficult to interpret the results. Sanchez-Vaznaugh et al. ²⁹29. Sanchez-Vaznaugh EV, Kawachi I, Subramanian SV, Sánchez BN, Acevedo-Garcia D. Differential effect of birthplace and length of residence on body mass index (BMI) by education, gender and race/ethnicity. Soc Sci Med 2008; 67:1300-10. reported a decrease in BMI with increasing duration of residence of immigrants in the United States which, according to the authors, may be explained by the fact that Caucasian immigrants are often more likely to speak English and to live in better neighbourhoods than Hispanic immigrants, which in turn leads to greater access to healthy food and facilitates transition into the host country. In Canada, rates of obesity are more likely to reach the same levels as the native population among Caucasian immigrants than in non-Caucasian immigrants. It has been suggested that ethnic minority immigrants acculturate more slowly than those from educated urban settings with similar physical characteristics (such as skin color) ⁵¹51. Setia MS, Quesnel-Vallee A, Abrahamowicz M, Tousignant P, Lynch J. Convergence of body mass index of immigrants to the Canadian-born population: evidence from the National Population Health Survey (1994-2006). Eur J Epidemiol 2009; 24:611-23.. Also, Caucasian immigrants are more likely to speak English, therefore suggesting that linguistic isolation is not a predictor of BMI ⁵¹51. Setia MS, Quesnel-Vallee A, Abrahamowicz M, Tousignant P, Lynch J. Convergence of body mass index of immigrants to the Canadian-born population: evidence from the National Population Health Survey (1994-2006). Eur J Epidemiol 2009; 24:611-23.. An increase in the proportion immigrants in the neighborhood was associated with lower BMI, suggesting that living in a predominantly immigrant neighborhood is a protective factor for this group ²⁰20. Park Y, Neckerman KM, Quinn J, Weiss C, Rundle A. Place of birth, duration of residence, neighborhood immigrant composition and body mass index in New York City. Int J Behav Nutr Phys Act 2008; 5:1-11.. One comparative study reported that obesity rates were higher among Greek immigrants than among their counterparts living in their home country. Greek immigrants living in Australia in the 1980s seemed to be living in a "morbidity-mortality paradox": despite high levels of obesity and physical inactivity, data from the time shows that this population group had the second highest average life expectancy, even greater than that of their counterparts in Greece. This paradox has been explained by the adherence to the traditional Mediterranean diet ⁵²52. Kouris-Blazos A. Morbidity mortality paradox of 1st generation Greek Australians. Asia Pac J Clin Nutr 2002; 11 Suppl 3:S569-75.. However Kouris-Blazo et al. ⁵⁰50. Kouris-Blazos A, Wahlqvist ML, Trichopoulou A, Polychronopoulos E, Trichopoulos D. Health and nutritional status of elderly Greek migrants to Melbourne, Australia. Age Ageing 1996; 25:177-89. did not confirm this protective effect, possibly due to changes in the original Mediterranean diet or greater exposure to the Australian diet.

North Africans

Studies concerning North African immigrants are scarce but unanimous. Toselli et al. ⁵³53. Toselli S, Galletti L, Pazzaglia S, Gualdi-Russo E. Two-stage study (1990-2002) of North African immigrants in Italy. Homo 2008; 59:439-52. and Dijkshoorn et al. ⁴²42. Dijkshoorn H, Nierkens V, Nicolaou M. Risk groups for overweight and obesity among Turkish and Moroccan migrants in The Netherlands. Public Health 2008; 122:625-30. showed an increase in BMI in immigrants living in Italy for over 12 years. These studies also reported a significant increase in stress-related factors and the desire to return home among North Africans living in Italy ⁵³53. Toselli S, Galletti L, Pazzaglia S, Gualdi-Russo E. Two-stage study (1990-2002) of North African immigrants in Italy. Homo 2008; 59:439-52.. Acculturative stress has been observed by other studies and can lead to a lowered mental health status (particularly depression and anxiety) that is a known risk factor for obesity. Moroccan families generally fall into lower income categories because of their level of education and professional training ⁵⁴54. Brussard JH, van Erp-Baart MA, Brants HA, Hulshof KF, Löwik MR. Nutrition and health among migrants in The Netherlands. Public Health Nutr 2001; 4:659-64., which is often associated with higher rates of obesity ⁵⁵55. Dinsa GD, Goryakin Y, Fumagalli E, Suhrcke M. Obesity and socioeconomic status in developing countries: a systematic review. Obes Rev 2012; 13:1067-79.. In the Netherlands, macronutrient intake among Turkish and Moroccan immigrants was satisfactory, while micronutrient intake was inadequate, leading to differences in weight and health status ⁵⁴54. Brussard JH, van Erp-Baart MA, Brants HA, Hulshof KF, Löwik MR. Nutrition and health among migrants in The Netherlands. Public Health Nutr 2001; 4:659-64..

Sub-Saharan Africans

Black immigrants often gain weight during the acculturation process. However, not all the studies observed a significant association between the two variables ²⁰20. Park Y, Neckerman KM, Quinn J, Weiss C, Rundle A. Place of birth, duration of residence, neighborhood immigrant composition and body mass index in New York City. Int J Behav Nutr Phys Act 2008; 5:1-11.^,3434. Goel MS, McCarthy EP, Phillips RS, Wee CC. Obesity among US immigrant subgroups by duration of residence. JAMA 2004; 292:2860-7.. There was a dramatic increase in rates of overweight with length of residence in the host country among Sub-Saharan African immigrants living in Canada ¹²12. Regev-Tobias H, Reifen R, Endevelt R, Havkin O, Cohen E, Stern G, et al. Dietary acculturation and increasing rates of obesity in Ethiopian women living in Israel. Nutrition 2012; 28:30-4.^,3030. McDonald JT, Kennedy S. Is migration to Canada associated with unhealthy weight gain? Overweight and obesity among Canada's immigrants. Soc Sci Med 2005; 61:2469-81. and Israel. This ethnic group is also susceptible to diet-related illnesses such as cardiovascular disease, hypertension, diabetes mellitus, obesity, and certain cancers ⁵⁶56. Beagan BL, Chapman GE. Meanings of food, eating and health among African Nova Scotians: "certain things aren't meant for Black folk". Ethn Health 2012; 17:13-29.. BMI and obesity and insulin resistance prevalence rates tend to be higher among Africans living in the United States than in those living in their home country despite similar genetic origin, showing that differences in chronic disease prevalence are probably due to environmental factors ¹⁰10. Misra A, Ganda OP. Migration and its impact on adiposity and type 2 diabetes. Nutrition 2007; 23:696-708.. Often, newly-arrived African immigrants are malnourished and have health problems ⁵⁷57. Okafor M-TC, Carter-Pokras OD, Picot SJ, Zhan M. The relationship of language acculturation (English proficiency) to current self-rated health among African immigrant adults. J Immigr Minor Health 2013; 15:499-509. which can influence their understanding of food and weight. Studies have shown racial differences on weight misperception. For example, overweight and obese African men are more likely to misperceive their own weight status than men from other ethnic backgrounds ⁵⁸58. Godino JG, Lepore SJ, Rassnick S. Relation of misperception of healthy weight to obesity in urban black men. Obesity (Silver Spring) 2010; 18:1318-22.. Unhealthy practices in African-American communities have been attributed to limited neighborhood healthy food availability and the high cost of healthy food, black consumer-oriented marketing, food preferences and eating patterns ⁵⁶56. Beagan BL, Chapman GE. Meanings of food, eating and health among African Nova Scotians: "certain things aren't meant for Black folk". Ethn Health 2012; 17:13-29.. Cultural eating patterns are considered a barrier to healthy eating ⁴⁸48. Hargreaves MK, Schlundt DG, Buchowski MS. Contextual factors influencing the eating behaviours of African American women: a focus group investigation. Ethn Health 2002; 7:133-47. and deviation from mainstream dietary guidelines in United States is greatest among African-Americans ⁵⁹59. Kumanyika S. Nutrition and chronic disease prevention: priorities for US minority groups. Nutr Rev 2006; 64:9-14.. Several studies indicate that the connections made with family and community through food are highly valued by African Americans ⁶⁰60. Ahye BA, Devine CM, Odoms-Young AM. Values expressed through intergenerational family food and nutrition management systems among African American women. Fam Community Health 2006; 29:5-16.. Dietary changes therefore unavoidably entail cultural loss, and resistance towards following healthy eating guidelines may be seen as a form of resistance to racism or cultural dominance ⁵⁶56. Beagan BL, Chapman GE. Meanings of food, eating and health among African Nova Scotians: "certain things aren't meant for Black folk". Ethn Health 2012; 17:13-29..

Immigration involves a number of changes in lifestyle, culture and socioeconomic status (at individual, family, and community level) for both immigrants and host populations. Understanding the links between immigration and health is a major epidemiological and public health challenge. The main goal of this article was to summarise current knowledge regarding the impact of immigration on the evolution of body mass index which is recognized as an important health indicator.

Major methodological differences between the studies reviewed made the direct comparison and interpretation of results very difficult, if not impossible, in some cases. Furthermore, a relatively large proportion of the literature analysed by this study did not provide sufficient information about methodological aspects, particularly sampling procedures, recoding of data and statistical details. A consensual definition of sample selection criteria for studies involving immigrant groups is important to facilitate a better understanding of results and the interpretation and comparison of findings from different studies.

One of the limitations of our study is the non-inclusion of grey literature. Unpublished master's and doctoral theses and reports are a possible further source of detailed information about this issue.

Stratification of data by gender is also important due to different health behaviors adopted by men and women throughout the immigration process ²⁸28. Koya DL, Egede LE. Association between length of residence and cardiovascular disease risk factors among an ethnically diverse group of United States immigrants. J Gen Intern Med 2007; 22: 841-6.. Women are generally considered more vulnerable to the acculturation process than men and consequently are more susceptible to being overweight than men ⁸8. Oza-Frank R, Cunningham S. The weight of US residence among immigrants: a systematic review. Obes Rev 2010; 11:271-80.. It is also important to study morbidity differences according to gender, ethnic background and age ⁶¹61. Madrigal L, Brady J, Raxter M. Obesity, hypertension, and migration: a meta-analysis of populations of the South Asian diaspora. Hum Biol 2011; 83:71-86.^,6262. Delavari M, Farrelly A, Renzaho A, Mellor D, Swinburn B. Experiences of migration and the determinants of obesity among recent Iranian immigrants in Victoria, Australia. Ethn Health 2013; 18:66-82.. Cultural preferences, such as media usage ⁶³63. Yeh M-C, Fahs M, Shelley D, Yerneni R, Parikh NS, Burton D. Body weight and length of residence in the US among Chinese Americans. J Immigr Minor Health 2009; 11:422-7., and other indicators are often used to measure acculturation instead of, or in addition to, length of residence in the host country.

Figure 2 shows several variables that may mediate the impact of immigration on health. The process is layered and variables are interdependent.

Conclusion

We found only five literature reviews of the effect of immigration on weight gain: four specifically addressed the epidemiology of obesity among immigrants living in the United States, and one, conducted by Misra & Ganda ¹⁰10. Misra A, Ganda OP. Migration and its impact on adiposity and type 2 diabetes. Nutrition 2007; 23:696-708. and published in 2007, focused on adiposity and diabetes in ethnic groups. The present study aimed to carry out a worldwide review and clearly focus on comparable research in this field in order to provide a qualitative synthesis of current data and an updated perspective on this topic. The flux of immigrants has changed since 2007 and this is an important factor to consider when analysing and interpreting data. Notwithstanding the methodological differences, the most relevant conclusion of these studies is that there is a significant and positive relationship between body weight and length of residence among U.S. immigrants ⁸8. Oza-Frank R, Cunningham S. The weight of US residence among immigrants: a systematic review. Obes Rev 2010; 11:271-80.. This association varies according to ethnic background and may be weaker or inexistent among immigrants with a higher level of education ⁷⁸78. Kaushal N. Adversities of acculturation? Prevalence of obesity among immigrants. Health Econ 2009; 18:291-303., and BMI tends to converge towards the levels of the host population at a quicker rate among women immigrants ⁷⁹79. Antecol H, Bedard K. Unhealthy assimilation: why do immigrants converge to American health status levels? Demography 2006; 43:337-60..

Studies about the impact of immigration on health present several challenges due to the number of variables involved and constant changes in these factors. First, migrant populations may not comprise a representative sample of their home country's population and may be healthier or unhealthier than their nonimmigrant counterparts ¹⁰10. Misra A, Ganda OP. Migration and its impact on adiposity and type 2 diabetes. Nutrition 2007; 23:696-708.. Secondly, the increased influence of the Western food industry is radically affecting the diet of children and teenagers in several underdeveloped countries and therefore the eating habits commonly seen in first generation immigrants may also have been acquired by their nonimmigrant counterparts in their home country ¹⁰10. Misra A, Ganda OP. Migration and its impact on adiposity and type 2 diabetes. Nutrition 2007; 23:696-708.. All but one of the studies covered by this review are cross-sectional, making it hard to make causal inferences about the length of residence in the host country and the increase in BMI and prevalence of overweight. This is a significant drawback and therefore further research of this association involving longitudinal studies of immigrants from different ethnic backgrounds is necessary. Nevertheless, some important conclusions can be drawn from these studies. Acculturation, often measured as length of residence in the host country, is an important determinant of the health of immigrants whose effect depends on a number of variables which have major implications for nutrition transition. Whereas in some countries immigrants have a lower risk of obesity than the home country population (for example the United States), in other countries the risk of obesity among immigrants is apparently greater (for example Sweden). Therefore, the cultural aspects of the host country seem to play a key role in health protection. Despite these differences, the impact of acculturation is often negative and generally the risk of obesity among immigrants increases as length of residence in the host country increases. Furthermore, nonimmigrants tend to have a lower mean BMI than their counterparts who decided to immigrate. Understanding the culture, beliefs and traditions of the country of origin, as well as food acculturation, may be the key to a better understanding and preventing the negative impacts of this process.

References

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