ReviewVitamin D, obesity, and obesity-related chronic disease among ethnic minorities: A systematic review
Introduction
Migration from low- to high-income countries has been shown to be associated with an increased risk for obesity and other lifestyle-related chronic diseases such as type 2 diabetes (T2DM) and cardiovascular diseases (CVD) in adults and children [1], [2], [3]. Factors predisposing migrants to obesity and chronic diseases are well documented and include lifestyle changes, environmental factors, cultural perceptions, and level of acculturation [2], [4]. However, there is emerging evidence linking vitamin D insufficiency/deficiency (VDI/VDD) to obesity-related chronic diseases [5].
American, European, and Australian studies have demonstrated that migrants and ethnic minorities with dark skin exhibit low serum 25-hydroxyvitamin D (25[OH]D) concentrations [6], [7]. In Australia, the prevalence of VDI/VDD (25[OH]D <50 nmol/L) [8] in migrant populations of an African background has been estimated to be 87% in children [9] and 92% in adults [6]. High levels of VDI/VDD have also been reported in Hispanic, African American, and Asian migrants in the United States [10], [11], and Turkish, Moroccan, Indian and sub-Sahara African populations in Europe [12]. Data from these studies have suggested that the risk factors for VDI/VDD include being of female gender, longer residency in a host country (>2 y), and being mostly covered while outdoors
For migrants with dark skin relocating to Western countries, not only are their 25(OH)D levels lower than their white counterparts, they tend to be lower than that of native populations in their country of origin [12]. Humans derive vitamin D primarily from exposure to sunlight, and only small amounts are derived from dietary sources, unless dietary supplements are used [5]. There are few natural sources of vitamin D in food and the main source for humans is the conversion of provitamin D (7-dehydrocholesterol) in the skin to previtamin D3 after sun exposure to ultraviolet-B radiation, which is subsequently converted to vitamin D3 through a heat-dependent process [13]. Because most people meet their vitamin D needs through exposure to sunlight [13], the synthesis of vitamin D varies by the color of the skin (determined by the amount and type of melanin). The darker the skin, the higher the amount of sunlight required to produce vitamin D [14]. The application of sunscreen decreases the synthesis of vitamin D3 [15], and complete cloud-cover and shade can decrease ultraviolet energy by 50% and 60%, respectively [16]. Similarly, the skin is unable to make vitamin D from the sun at latitudes above 37°N and below 37°S [17], [18]. Thus, when migrants of skin color relocate to Western and cooler climate countries, with high northern (e.g., European) or low southern (e.g., Australian) latitudes, their risk for VDI/VDD increases exponentially.
It is worth noting that the increased risk for obesity, T2DM, and CVD in migrants with dark skin relocating to Western and cooler climate countries [1] occurs simultaneously with the increased risk of VDI/VDD. Data from epidemiologic and experimental studies in non-migrant populations, especially those living at higher latitudes, have found that low levels of 25(OH)D are associated with the risk of hypertension, CVDs, T2DM, cancers, and cancer-related mortality [5], [19].
Several factors have been proposed to explain the VDI/VDD association with obesity [20], [21], [22]: 1) obese people may not get enough sun exposure due to limited mobility or clothing habits; 2) their bodies cannot be easily release vitamin D because it is stored in the body fat compartments; 3) the obese have an increased need for vitamin D for stronger bones to support their greater weight but are unable to meet such needs due to a decreased bioavailability of 25(OH)D; and 4) increased levels of the active vitamin D metabolite decreases serum 25(OH)D by exerting negative feedback control on the hepatic synthesis of 25(OH)D. However, the inverse relation between body mass index (BMI) and 25(OH)D levels is not consistent across ethnic groups. For example, Nesby-O’Dell et al. [23] examined determinants of hypovitaminosis D in 1546 African American women and 1426 white women 15 to 49 y old using data from the Third National Health and Nutrition Examination Survey. They found that VDI/VDD was significantly associated with a BMI of at least 30.0 kg/m2 in white women, but not in African American women. Similarly, in New Zealand Scragg et al. [24] examined the relation between vitamin D status and major cardiovascular risk factors in 390 New Zealand residents 40 to 64 y old (95 Pacific Islanders, 74 Maori, and 221 others mostly of European descent). The investigators found that in residents of mixed ethnicity, serum 25(OH)D concentrations were unrelated to BMI. There is emerging evidence that vitamin D status is only modestly associated with parameters of disturbed glucose and lipid metabolism [25]; hence, the impact of VDI/VDD on this type of cardiovascular risk factors cannot be evaluated with certainty. Furthermore, Wu et al. [26] found no associations between lower 25(OH)D levels and typical cardiovascular risk factors when respective correlations were adjusted for BMI.
Notwithstanding these findings, among migrant populations the links between vitamin D and obesity and obesity-related chronic disease are less clear and not yet completely understood. Understanding this link will help health planners to decrease health inequalities and the burden of obesity and chronic diseases in migrant populations in receiving countries. The aim of this systematic review was to assess the association between vitamin D status as measured by 25(OH)D and CVDs, obesity, and T2DM in migrant and ethnic minority population groups.
Section snippets
Search strategy
A search for relevant publications was conducted using CINHAL with full text, Global Health, MEDLINE with full text, and PsycINFO. Additional articles were located from the reference lists of relevant papers. The search strategy can be viewed in Table 1.
Selection and data extraction
All potentially relevant studies had their abstracts screened by one of the authors (J. A. H.) for eligibility using the following criteria: 1) the study included targeted immigrants from low- to high-income countries or skin-colored populations
Characteristics of studies
In total 20 studies were included in this review, of which 18 were conducted in the United States and two were conducted in New Zealand (Table 2). Sixty-five percent of the studies (n = 13) were in adults only (age ≥18 y), 25% (n = 5) included adults and children/adolescents (age <18 y), and 10% (n = 2) were in children/adolescents. All reviewed studies included multiethnic groups including migrants and/or ethnic minorities. Seventeen of the reviewed studies were cross-sectional and three were
Discussion
This is the first study to examine the relation between 25(OH)D and obesity and obesity-related chronic diseases in migrants and ethnic minorities. Twenty articles met our criteria for inclusion. Most of the studies included in this systematic review were cross-sectional and thus are placed low within the hierarchy of evidence provided by study designs compared with the “gold standard” randomized controlled trial. For this reason, this review could not comment on the potential causality of VDD
Limitations
This study has a number of limitations. First, due to the longitudinal and cross-sectional study design of the included studies, they cannot be used to establish causality. Rather, this review has reported the combined existing evidence, highlighting similarities, differences, and gaps within the current evidence. Second, although we set out to examine migrant and ethnic minority groups, all samples focused on ethnic minorities and included majority population groups and not all studies
Conclusion
Notwithstanding the limitations of the reviewed studies, the emerging evidence is that low vitamin D levels are associated with obesity, T2MD, CVD, and the metabolic syndrome depending on which index was used to measure these diseases, and ethnic minorities are the most affected. This review provides further support toward the links between ethnicity and 25(OH)D status, although the links with migration are still unclear because of a paucity of evidence specific to migrant populations. Further
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