The vitamin D status of Turkish, Moroccan, Indian, and sub-Sahara African immigrant populations in Europe was poor compared to the indigenous European populations. The vitamin D states of studied populations in Turkey, Morocco, and India varied between concentrations similar to the immigrant populations in Europe (low) and concentrations similar to or higher than the European indigenous populations (high). Determinants of the serum 25(OH)D concentration included both sources of vitamin D: exposure to sunlight and intake of vitamin D.
Gender and age were each associated with serum 25(OH)D concentration in both directions. Differences according to gender and age group could be the result of biological differences but might also reflect behavioral differences; dress style (e.g., wearing a veil) is often mentioned as a reason for a higher prevalence of vitamin D deficiency among women than men. A lower serum 25(OH)D concentration among older participants can partly be the result of the lower capacity of the skin to produce vitamin D after exposure to sunlight. The study that found lower serum 25(OH)D concentrations at younger ages [33
] might have had a study population that was too young to find an effect of a lower skin capacity (their mean age was below 40 years).
As the described studies were observational, not all determinants could be studied due to a lack of variation in the determinants. For instance, Sahu et al. described the dietary intake in rural India as remarkably monotonous from meal to meal, with a low consumption of dairy and foods containing reasonable amounts of vitamin D [36
]. As a consequence, it is difficult to find an association between dietary intake and serum 25(OH)D.
The darker skin types of the immigrant populations are a suitable protection against the intensity and amount of sunlight in their countries of origin, while they are a risk factor for vitamin D deficiency in northerly European countries. The serum 25(OH)D concentrations of the populations in the country of origin may, therefore, indicate normal or reference concentrations. However, those populations may themselves be deficient or suffer from insufficient concentrations as a whole. Given that until recently, mankind lived and worked outside, the serum 25(OH)D concentrations of groups who currently spend much of their time outdoors might, therefore, be considered “normal” [47
]. Serum 25(OH)D concentrations of rural populations, who are expected to have a greater exposure to sunlight as a result of their agricultural occupation than urban populations [20
], might be a more suitable indicator of normal concentrations than those of total populations.
The high (>100 nmol/l) serum 25(OH)D concentrations in subgroups of two Turkish studies, which were performed at the end of the summer, suggest a large impact of sunlight. As sun exposure does not lead to toxic vitamin D concentrations due to a feedback mechanism, these serum 25(OH)D concentrations are expected to be within the normal or reference range, which is an additional argument that the low serum 25(OH)D concentrations (found in immigrant populations) can be interpreted as a deficiency. Of course, assay differences might also explain part of the difference with other studies.
Symptomatic vitamin D deficiency is also suggested by the prevalence of rickets in Turkey, India, and some African countries [48
]. The incidence of rickets in Eastern Turkey declined from 6.09% to 0.099% after a nationwide free vitamin D supplementation program [54
]. Within European countries, rickets is not highly prevalent, but immigrant populations are groups at risk [55
]. Additionally, although most nonwestern immigrant populations are younger than the indigenous European populations, cases of osteomalacia in nonwestern immigrants have been observed [58
]. Finch et al. found all but one case of osteomalacia within the vegetarian Asian group in England, the group with lowest vitamin D concentrations in their study [32
]. Furthermore, osteoporotic and peripheral fractures were found in the vitamin-D-deficient subgroup in Morocco [17
]. Erkal et al. found that 61% of the Turkish group (in Turkey) and 55% of the Turkish immigrant group in Germany complained of bone pain and/or nonspecific generalized muscle aches and pain, while it was 15% within the German group with higher serum 25(OH)D concentrations [2
]. However, one should keep in mind that serum 25(OH)D is not the sole determinant of rickets; calcium intake is also important [48
The comparison of serum 25(OH)D concentrations of the various populations in this article has some limitations.
First, several studies present the prevalence of vitamin D deficiency but have excluded individuals using drugs or medication known to affect bone metabolism, those recently treated for vitamin D deficiency, or those who used vitamin D supplements [1
]. Medications that affect bone metabolism include, among others, vitamin D and calcium. One can argue whether the presented values represent the real prevalence in the respective populations when these individuals are excluded. However, we expect the number of excluded individuals to be small and, therefore, not of great influence on the prevalence. Furthermore, it implies that the prevalence is applicable for an apparently healthy population.
Second, the season of blood sampling varies, and this might account for a part of the observed differences between studies, because the intensity of sunlight and the amount of sunlight per day varies between seasons. These differences may be larger when studies in European countries are part of the comparison, because seasonal differences in sunlight are expected to be higher in countries at higher latitudes. For that reason, the time of year was mentioned in the tables.
Third, the comparison is hampered because the serum 25(OH)D assessment methods differ, which may influence differences between groups [62
]. In addition, the level of accuracy of studies within Europe and in the country of origin might differ. However, although we could not adjust for this type of bias, we presume that the differences will not be systematic or large enough to substantially alter the conclusions.
Finally, in comparing the various populations, it is important to realize that the social conditions of the immigrants might not be the same as those of the original populations. The cultural habits (skin-covering clothes, sun exposure, diet) might also change after immigration, particularly among the second generation.
Serum 25(OH)D concentrations of nonwestern immigrants in Europe and of subgroups of Turkish, Moroccan, Indian, and sub-Saharan countries are low. Ways to increase the serum 25(OH)D concentration include increased exposure to sunlight and increased intake of products that contain vitamin D. The strategy to effectuate these increases will differ in the various countries and populations and should be the subject of further study. These studies should ideally include measures of health to support the need for this increase in serum 25(OH)D.