Among adults in Portugal, the prevalence of overweight was higher for natives than immigrants, which is in line with the “Healthy Immigrant Effect” (HIE), a phenomenon in which immigrants are on average healthier than the native born [2]. Some explanations for the HIE include health screening by immigration officers, relatively healthier behaviors of new immigrants prior to migration, and immigrant self-selection whereby the healthiest and wealthiest individuals are the people most likely to migrate [2]. This finding is in line with other research conducted in Portugal, showing an immigrants’ health advantage in terms of chronic conditions for some groups of immigrants [35] and a lower prevalence of pre-obesity and obesity among Brazilian and African immigrants, compared with the Portuguese population [36]. Immigrants from this study were an active, educated, and younger population, which could explain the lower levels of overweight. It is reasonable to assume that they are in a better position to take advantage of educational initiatives and health information than their less active, less educated, and older counterparts. Data and participants were selected from accommodations units, which constitute a limitation of the NHS that may explain these results. Therefore, it is possible that the sample did not consider vulnerable groups, namely refuges or irregular immigrants, with different socioeconomic characteristics and health outcomes. The report on health of migrants in Portugal reinforces this effect of selection: the description of health determinants revealed higher food insecurity for Portuguese population; among immigrants, the higher frequency of food restriction due to economic difficulties was reported by recent immigrants (≤5 years) [37].
In Portugal, for long-term immigrants (≥15 years), the odds of being overweight increased. This loss of health advantage with length of residence in the host country is attributed in the literature to lifestyle changes, including patterns of physical activity and dietary habits, that describe the phenomena of nutrition transition and dietary acculturation [10, 38]. The concept of nutrition transition focuses on large shifts in diet and physical activity patterns, due to globalization and urbanization, and influences nutritional outcomes, such as changes in average stature, body composition, and morbidity [39]. Dietary acculturation refers to the process that occurs when members of a migrating group adopt the eating patterns/food choices of their new environment [40]. Looking beyond the overall tendencies, the relationship between overweight and acculturation is complex. Acculturation is a multidimensional and dynamic process, but not necessarily deleterious. Immigrants may retain traditional foods, exclude others, and find new ways to use traditional foods or adopt the diet pattern of the host country. Despite this, in general dietary acculturation has been found to have detrimental effects on diets of immigrants and racial/ethnics minorities [41]. Length of residence may not fully capture the acculturation process, but it is considered a reasonable surrogate measure and has been used in many studies [10, 11, 42].
Nutrition transition is accelerated by migration [43] but promotion of the consumption of traditional foods and retention of cultural eating patterns are known to partially prevent this transition [44]. Food culture arises out of the place of a people’s origin and is shaped by resources (climate, land, soil, water, and fuel), belief and information (religion, education and literacy and communication), ethnicity, technology, colonization, health status, and health care [44]. In the new food environment, socioeconomic and demographic characteristics (e.g. age, gender, education, employment status, language, religion, household composition, income, food availability and accessibility, and place of residence), ethno-cultural norms, political-economic process, and exposure to the host culture (e.g. access to media, peers, and access to traditional supermarkets) may influence dietary choices [41, 45]. Because migrant populations are heterogeneous and move through different phases of the health transition during their life course, researchers studying migrant health should consider risks and exposures not only in the host country, but also during the migration process and in the country of origin [46]. Nutrition transition level in the country of origin is important because of the accelerated pace of changes in diet and physical activity patterns observed in low and middle-income countries [39, 47, 48]. Other important factors include genetic background, health behavior like nutrition, physical activity, and alcohol and tobacco consumption [46]. In line with this, knowledge of traditional food habits and study of health determinants, before and after migration, may be necessary to understand how dietary acculturation could potentially reverse current trends in overweight and to plan intervention policies to improve the health of migrants and natives.
Most studies that confirm a positive association between length of residence and overweight report a threshold effect of duration on weight gain after 10–15 years of residence [14]. This study found the strongest positive association after 15 years in the host country. These results may provide some evidence that immigrants in Portugal maintain their food habits (probably less obesogenic) for a few years, until they adopt (especially males) a more obesogenic behavior (diet and physical activity patterns). To confirm this theory, further research will have to address the characteristics of dietary pattern, upon arrival in the host country and over time. Dietary pattern analysis has emerged as an alternative and complementary approach to examining the relationship between diet and the risk of chronic diseases [49]. Instead of looking at individual nutrients or foods, pattern analysis examines the effects of overall diet. Conceptually, dietary patterns represent a broader picture of food and nutrient consumption, and may thus be more predictive of disease risk than individual foods or nutrients.
The overweight prevalence (60.1%) found by Goulão et al. [36], among Brazilian and African immigrants settled in Portugal for ≥15 years, was higher than the prevalence found in this study (48.8%) for the same time of residence. One possible explanation is that Goulão et al. included irregular immigrants living in enclaves, which may have different socioeconomics characteristics and thus, poorer dietary patterns and health outcomes. In the present study, among immigrant population the prevalence of overweight was higher for Africans (49.4%). One possible explanation is that Africans were living in Portugal for a long time (28 years as median of length of residence). More surprisingly was the high prevalence of overweight within American/Asian (39.9%), notwithstanding that half of this population were in Portugal for only 5 years. According to the Portuguese Immigration Services, the Brazilian community remains the main foreign resident community in Portugal [50]. Therefore, investigating the nutrition transition of Brazil may be important to understand how time of exposure to the host environment and dietary acculturation process may influence overweight.
According to Popkin (2010), belonging to the lower socioeconomic group has conferred strong protection against obesity in low-income countries, but it is a systematic risk factor in upper- to middle-income countries. In Brazil, there is solid evidence that the burden of obesity is shifting toward the poor and, among women is increasing among lower income groups and falling among those in higher income groups [51]. A decrease in the incidence of obesity in Brazilian women was observed from 1989 to 2003, and was seen as related to an increase in income or educational achievement, particularly in the urban setting [52]. Therefore, it is possible that the wave of immigrants from Brazil that arrived in Portugal, because of the intermediate stage of nutrition transition of country of origin, were already overweight, and that the prevalence of this disease is not explained by acculturation. Frequency of American/Asian immigrants that were in Portugal for <4 years (54.8%) may support this statement. These results highlight the importance of other health and nutritional determinants in longitudinal immigrant studies.
In the present study, compared to men immigrant women are less likely to be overweight, which is not consistent with other studies, in which women are found to be more susceptible to being overweight than men, because of their vulnerability to the acculturation process [14, 53]. The explanation for these differences may lie in social factors like gender roles and relations. Among immigrants in Portugal, married males, less educated and 45–54 years old, may constitute a risk group for overweight.
In developed countries men tend to have a higher prevalence of overweight and obesity than women whereas the opposite is more frequently in developing countries [7]. In this study, aggregating countries into only three regions may dissimulate important differences between and within countries. Knowledge about country of origin could give some insight on the subject.
The present study has some limitations. Data from NHS were collected in 2005/2006 and the situation may change at this time. Nevertheless, it was the first to include the entire resident population, regardless origin country, so these results may serve as comparison with future studies. Another limitation is that data were aggregate in just three regions, one of which, America/Asia, has the highest and the lowest overweight prevalence among all WHO regions in 2008 [8]. Factors related to diet or physical pattern were not adequately considered and no questions were included in the NHS to address dietary acculturation. The cross-sectional nature prevents us from making any inferences regarding causality and height and weight were self-reported, with all biases associated with self-reported data.
The main strength of this study is to highlight the importance of nutritional and dietary assessment of immigrants upon arrival in the host country and over time. Future studies on immigrants’ health should consider dietary patterns analysis, information about country of origin nutrition transition level, and measures that could identify degrees of acculturation across several domains, especially dietary acculturation. Further research of the association between migration and weight status should involve longitudinal studies of immigrants from different ethnic backgrounds, as well as instruments that more accurately measure the various steps in the process of dietary acculturation.
