Elsevier

Nutrition

Volume 32, Issue 1, January 2016, Pages 21-32
Nutrition

Meta-analysis
The prevalence of vitamin D deficiency among dark-skinned populations according to their stage of migration and region of birth: A meta-analysis

https://doi.org/10.1016/j.nut.2015.07.007Get rights and content

Highlights

  • Meta-analysis suggests increased prevalence of vitamin D deficiency (VDD) in dark-skinned migrants.

  • VDD is associated with increased time in host country.

  • VDD is increased in extended Middle East compared with Sub-Saharan Africa.

Abstract

Objectives

The prevalence of vitamin D deficiency (VDD) varies among migrants from different geographic regions. The aim of this study was to estimate the pooled prevalence of VDD among dark-skinned migrants.

Method

A meta-analysis using meta-regression was undertaken to determine the prevalence of VDD in dark-skinned migrant populations. Prevalence also was determined by study characteristics including study methodology, age of populations examined, and length of time in migrated country.

Results

Thirty-six studies were identified in nonpregnant populations. Of 13 974 individuals in the studies, 9562 were vitamin D deficient. Pooled prevalence in dark-skinned migrants, adjusted for latitude of study country was estimated at 77% (95% confidence interval [CI], 70%–84%). Examination of studies in which migrants from both Sub-Saharan Africa and the extended Middle East were examined (N = 7) showed immigrants from the extended Middle East had a higher prevalence of VDD (65%; 95% CI, 45%–94%) compared with those from Sub-Saharan Africa (56%; 95% CI, 34%–77%). Seven studies were identified in pregnant dark-skinned migrant women. This group tended to have much higher prevalence of VDD compared with native-born pregnant women.

Conclusion

Immigrants with dark skin, and in particular those from the extended Middle East region, have high prevalence of VDD. Migrants who are at high risk for VDD should be educated, screened, and monitored for VDD.

Introduction

Vitamin D deficiency (VDD) is a worldwide health problem affecting more than 1 billion individuals [1] and accounting for a significant burden of disease. For example, consequences of VDD include skeletal disorders such as rickets in children and osteomalacia in adults [2]; nonskeletal disorders such as multiple sclerosis, cardiovascular diseases (CVD), and diabetes; and all-cause mortality [3]. Other consequences of VDD include mood disturbances and impaired neuropsychiatric conditions [3].

Although there is a debate over the definition of VDD, most experts agree that a serum 25-hydroxyvitamin D [25(OH)D] level <50 nmol/L is an indication of VDD [4]. Studies have found poor vitamin D status among non-Western migrant subpopulations in European countries, the United States, and Australia. In Belgium 76.8% of Congolese and 90% of Moroccans were found to have serum 25(OH)D levels <50 nmol/L [5]. In the United States, 73.3% of immigrants and refugees from East Arica and South East Asia were reported to have VDD [6]. In Australia, the prevalence of VDD has been reported in Asians, East, West, and Central Africans [7], with a prevalence of VDD estimated at 87% among East African children [8] and 92% among African migrant adults [9]. Risk factors for VDD among migrants include dark-skin pigmentation, cultural practices such as veiling, insufficient exposure to sunlight [10], and longer duration of residence in the host country [9]. VDD is especially common among dark-skinned individuals who migrate to high northern (>37°N) or low southern (<37°S) latitudes [11], [12].

VDD is associated with adverse health in dark-skinned individuals who have migrated to countries at higher latitudes. Somali women who have migrated to Sweden have increased muscle weakness compared with ethnic Swedish women [13] and low bone mineral density compared with white American and African American reference populations [14]. In a recent study, premigration latitude also was found to be an important factor for VDD. That is, the lower the premigration latitude, the higher the risk for VDD postmigration. The study also found that the premigration latitude predicted future risk for CVD in migrants [15].

Sun is the main source of vitamin D and is produced endogenously when skin is exposed to solar ultraviolet radiation [2]. Synthesis of vitamin D depends on the amount and type of melanin present in the skin and the darker the skin the higher the amount of sunlight required to produce vitamin D [16].

VDD is unequally distributed across different geographic regions and among different ethnic groups in the world. For example, African descent populations who live in a temperate climate are vitamin D deficient, whereas Africans living near the equator are not [17]. However, the variability in the frequency of low vitamin D by ethnicity is likely to be related to the country of origin, pigmentation of skin, proximity to the Equator, and cultural practices relating to sun exposure [18]. However, the effect of these factors is likely to vary across migrant subpopulations, requiring a customised approach to prevention. Studies examining how different dark-skinned populations are affected are lacking.

Understanding the patterns of VDD among the dark-skinned migrants is a first step in combating vitamin D–related health issue among these groups Therefore, the aim of this study was to estimate the pooled prevalence of VDD among dark-skinned migrants relocating to industrialized countries and to examine whether coming from regions associated with increased clothing practices (decreased skin exposure to sunlight) matters. Our pooled estimation of VDD prevalence will provide a basis for improving vitamin D status among the dark-skinned migrants in their host countries.

Section snippets

Search strategy

As assays for vitamin D analysis were introduced in the early 1970s, the initial search period was from January 1, 1970 to October 30, 2014. After the first round of reviewers' comments, the search was updated in May 2015. PubMed, EMBASE, CINAHL, Web of Science, and Cochrane library search engines were searched using the following MeSH, subject heading and key word search terms and their variants: [“Vitamin D” or “25-Hydroxyvitamin D2” or “25-Hydroxyvitamin D3” or “cholecalciferol” or

Results

The search identified 5259 potential papers. A review of study title and abstract enabled nonrelevant studies, reviews and editorials, qualitative studies, studies in disease states associated with VDD, and studies that examined rickets and osteomalacia to be removed. This left 138 papers to be closely examined for eligibility (Fig. 1). An examination of references added another 15 papers to be examined for eligibility. Of these 153 papers, 44 were found to meet the eligibility criteria.

Discussion

This meta-analysis of the literature suggests a high prevalence of VDD in dark-skinned migrants. Univariate regression suggested that length of time in migrated country and age of population studied affects prevalence.

Although people with dark skin require more exposure to sunlight than white-skinned people to maintain adequate serum vitamin D levels [63], we found that prevalence of VDD was lower among migrants from SSA than for migrants from the EME. Studies in dark-skinned native traditional

Conclusion

Immigrants with dark skin have high prevalence of VDD and it appears that VDD prevalence increases with time spent in migrated country. Immigrants from the EME appear to be at greater risk for VDD, which may be due to cultural practice such as the wearing of concealed clothing.

Future research studies should focus on whether routine screening of VDD should be undertaken or whether dark-skinned migrants, including pregnant women, should be automatically supplemented with vitamin D to see which is

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    CM and AR designed and conducted the study. CM analysed the data. CM and UG drafted the article and AR made substantial contributions in the intellectual input and critically reviewed the manuscript. All authors approved the submission of the article. The authors have no conflicts of interest to declare.

    AR is supported by an Australian Research Council Future Fellowship (FT110100345).

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