In reply,

We thank Chang-Martinez for the letter in response to our systematic review article The Impact of Migration on Body Weight: A Review. We could not agree more with Chang-Martinez’s view on the relevance of ethnic factors in obesity and acculturation, as we state in our conclusions. In fact, several ethnic and sociocultural factors are essential for understanding the impact of acculturation on obesity levels for multiple reasons, including: genetic pool variations, political environment during the migration process, cultural identity, body image perceptions, amongst others. As we have stated, research about migration impact on health tends to confuse geographic origin with such ethnic and sociocultural determinants. They should not be seen as equivalent. On the other hand, racial factors are referred to by the Chang-Martinez but even though the term is commonly used in the literature, we believe it has a negative connotation and should not be used in this context.

Any systematic review of the literature demands a clear definition of goals and criteria of inclusion and exclusion of the sample units (in this case, scientific articles), which makes it inherently, and necessarily, limited and restricted. Moreover, it is particularly difficult to include very specific ethnic subgroups due to a lack of literature on the subject and frequent omission of (or imprecise) ethnicity definitions. As a matter of fact, the concept of ethnicity is commonly used in medical literature, but it is complex, hard to define and, as a result, inconsistent. It is also a multidimensional concept that can involve aspects such as: shared origins or social background, shared distinctive culture or tradition maintained between generations and promoting a sense of identity and group, and common language and religious traditions ¹1 . Senior P, Bhopal R. Ethnicity as a variable in epidemiological research. BMJ 1994; 309:327-9.. Additionally, self-classification of ethnicity is commonly used (where people are asked to indicate to which ethnic group they feel they belong). Even though this self-defining approach has its advantages, the main drawback is its changeable nature – the self-perception of ethnicity may change over time and according to contexts. Better definitions and terminology are urgently needed to allow for scientific progress ²2 . Agyemang C, Bhopal R, Bruijnzeels M. Negro, Black, Black African, African Caribbean, African American or what? Labelling African origin populations in the health arena in the 21st century. J Epidemiol Community Health 2005; 59:1014-8.. A few journal editors have also set standards by publishing explicit guidelines for the use of ethnicity ²2 . Agyemang C, Bhopal R, Bruijnzeels M. Negro, Black, Black African, African Caribbean, African American or what? Labelling African origin populations in the health arena in the 21st century. J Epidemiol Community Health 2005; 59:1014-8. ^{, 3}3 . McKenzie K, Crowcroft NS. Describing race, ethnicity, and culture in medical research. BMJ 1996; 312:1054..

One of the main suggestions Chang-Martinez makes in her letter is the inclusion of ethnicities in all studies on this matter and not just the geographic region of birth of the immigrants, as well as a more clear definition of the ethnicities included. Without these changes, it is hard to compare studies and to take appropriate conclusions from the literature on this topic. A further problem is the lack of agreement on how to define those different ethnic groups amongst experts on the field. The definitions vary according to the studies and over time ⁴4 . Gatineau M, Mathrani S. Obesity and ethnicity. Oxford: National Obesity Observatory; 2011., but without the terms of ethnicity explicitly defined, it is impossible to make local or international comparisons ²2 . Agyemang C, Bhopal R, Bruijnzeels M. Negro, Black, Black African, African Caribbean, African American or what? Labelling African origin populations in the health arena in the 21st century. J Epidemiol Community Health 2005; 59:1014-8..

Different groups of Hispanic immigrants can have immensely diverse health trajectories, as pointed out by Chang-Martinez. We agree that the presentation of stratified results for these subgroups would be ideal. It is important to keep in mind that historical and socio-political factors that influence migration (from both origin and host countries) vary across different Latino/Hispanic groups. In fact, this applies to any flow of migration: different momenta, reasons for migration, type of social ethnical subgroups and even their path can have an effect on future migrants’ health in the host country. Societal contexts that promote and inhibit health must be studied in order to better understand the health differences amongst different ethnicities and subgroups within the same ethnicity ⁵5 . Abraído-Lanza AF, Armbrister AN, Flórez KR, Aguirre AN. Toward a theory-driven model of acculturation in public health research. Am J Public Health 2006; 96:1342-6.. Abraído-Lanza et al. ⁵5 . Abraído-Lanza AF, Armbrister AN, Flórez KR, Aguirre AN. Toward a theory-driven model of acculturation in public health research. Am J Public Health 2006; 96:1342-6. give, as an example, the conditions in which Cuban immigrants entered the USA and how such conditions positively affected their health status. The impact that historical and political factors have on acculturation processes and health outcomes is likely to be immense but it is very rarely studied. Behavioural and social measures may not be enough to explain the difference between Hispanic subgroups, but there are obvious disparities among Cuban and South/Central American immigrant groups in the US, more closely comparable with the situation of other subgroups such as Puerto Ricans and Mexican immigrants in the same country ⁶6 . Albrecht S, Gordon-Larsen P. Ethnic differences in body mass index trajectories from adolescence to adulthood: a focus on Hispanic and Asian subgroups in the United States. PLoS One 2013; 8:e72983..

Afro-Caribbean immigrants are a specific group of immigrants, usually included in studies conducted in the United Kingdom. For that reason, there’s little information available on this subgroup that could further inform our systematic review article. However, it is important to make the distinction between this subgroup of African immigrants and others since they are different in terms of beliefs, risk factors and disease experiences ²2 . Agyemang C, Bhopal R, Bruijnzeels M. Negro, Black, Black African, African Caribbean, African American or what? Labelling African origin populations in the health arena in the 21st century. J Epidemiol Community Health 2005; 59:1014-8.. The term Afro-Caribbean usually refers to people with African ancestral origins who migrated via the Caribbean islands ²2 . Agyemang C, Bhopal R, Bruijnzeels M. Negro, Black, Black African, African Caribbean, African American or what? Labelling African origin populations in the health arena in the 21st century. J Epidemiol Community Health 2005; 59:1014-8. but it is frequently used inconsistently ⁷7 . Rait G. Counting heads may mask cultural and social factors. BMJ 1999; 318:305-6. leading to confusion when applied and masking eventual differences between diverse subgroups coming from different islands. Mixing Afro-Caribbean with African populations from Africa can lead to even more variances being ignored. In the United Kingdom, a higher obesity prevalence has been reported for Black Caribbean women compared with women from the general population. However, men are less likely to have a raised waist-to-hip ratio compared with the general population ⁴4 . Gatineau M, Mathrani S. Obesity and ethnicity. Oxford: National Obesity Observatory; 2011.. One possible contributing factor is how obesity is perceived in their home country, as healthy and overweight women are viewed as preferential compared with thin women ⁸8 . Bramble J, Cornelius L, Simpson G. Eating as a cultural expression of caring among Afro-Caribbean and African American women: understanding the cultural dimensions of obesity. J Health Care Poor Underserved 2009; 20:53-68. . On the other hand, the health of 2^nd and 3^rd generations of African Caribbean groups might be quite different from their parents and grandparents ²2 . Agyemang C, Bhopal R, Bruijnzeels M. Negro, Black, Black African, African Caribbean, African American or what? Labelling African origin populations in the health arena in the 21st century. J Epidemiol Community Health 2005; 59:1014-8.. Upward intergenerational social mobility of the 2^nd and 3^rd generation seems to play a protective role ⁹9 . Smith N, Kelly Y, Nazroo J. The effects of acculturation on obesity rates in ethnic minorities in England: evidence from the Health Survey for England. Eur J Public Health 2012; 22:508-13..

In conclusion, we believe Chang-Martinez’s views are quite relevant and that ethnicity-based analyses, amongst other factors, are essential when presenting studies about migration and its impact on health. However, the lack of consensus on ethnic definitions, as well as the complexity of the concept itself, and the fact that it is frequently obtained by self-classification present a challenge. There is need for further clarification in this field that might allow researchers to study these phenomena in greater depth and in more heuristic and insightful ways, namely through accurate and informative systematic reviews or meta-analysis.