The main finding of the study was the high rate of vitamin D deficiency and insufficiency among patients born outside of Europe compared to patients born in Europe, where half of the patients born outside of Europe had a vitamin D deficiency and only a tenth had adequate vitamin D levels.
The finding is consistent with a review of data from the Nordic countries presenting extremely low serum levels of vitamin D and a high prevalence of deficiency in immigrants from outside Europe and North America, specifically a very high prevalence of vitamin D deficiency among immigrant women from Arab countries, Pakistan, and Somalia, and among immigrant men from Pakistan (Wandell 2013). What our study adds is that non-European men showed lower vitamin D-levels and higher risk of vitamin D deficiency, which partly can be attributed to a lower intake of vitamin D supplements.
The primary health care center of the study is situated in an area with a high rate of foreign-born individuals (37% vs 14% in Sweden overall) and a rather low socio-economic status (the mean income is 90% of the Swedish mean), including a high unemployment rate, and the population in the catchment area could not be claimed to be representative for the Swedish population, but rather for suburbs with a high proportion of apartment buildings situated around the larger cities of Sweden, in this case Stockholm city. From a clinical point of view, the included patients did not differ from the main population of the patient list, although we have no exact data on this.
In this study, the levels of vitamin D were lower among men, in contrast to earlier reports (Wandell 2013). One possible explanatory factor could be that women seek care more often than men, who also tend to be more ill before seeking health care, a clinical experience from primary care. This pattern, with women having more symptoms and seeking care more often, while men have a shorter life expectancy and get their cardiac diseases around 5 years earlier than women (Forslund et al. 2014; Zarrinkoub et al. 2013), is referred to as the gender paradox, or the male–female mortality paradox (Oksuzyan et al. 2008). Otherwise, vitamin D deficiency is especially serious among pregnant women, since vitamin deficiency is associated with a higher risk of pregnancy and delivery complications (Grant et al. 2011), and also could affect fetal growth (Brunvand et al. 1996) and contribute to rickets among children (Elidrissy et al. 1984).
The study was performed during the winter, when the height of the sun in Sweden is too low to induce vitamin D production in the skin, and the serum levels of vitamin D depend on dietary intake and storage only. Future studies are needed to investigate if the difference in vitamin D levels and prevalence of vitamin D deficiency between patients born outside versus in Europe is present also during the summer.
Patients who reported taking vitamin D supplementation had higher levels of vitamin D than patients that did not, and of the 15 that took supplementation, only two had a vitamin D deficiency and half had adequate vitamin D levels. Although sun exposure was not found to be a significant factor for vitamin D levels in the present study, this may be due to the low number of exposed patients.
In a review of Turkish, Moroccan, Indian, and sub-Sahara African populations in Europe and their countries of origin, the vitamin D status was concluded to be low compared to indigenous European populations (van der Meer et al. 2011). A study comparing Tamil populations in Norway vs in Sri Lanka found a lower vitamin D level in Norway (Meyer et al. 2008). The most important factor for vitamin D in the body is through sun exposure, which is why higher vitamin D levels might be expected in sunnier countries, but in fact the levels do vary a lot (van der Meer et al. 2011), which could be influenced by sun exposure but also dietary intake. The dietary intake of vitamin D is higher in the Nordic countries than in other regions of the world (Lips 2007).
Limitations of the present study were the limited sample size, the lack of nutritional data, e.g., especially on intake of fish, lack of information with regard to educational level (Holvik et al. 2005), and lack of data concerning doses of vitamin D supplementation. The strengths included the fact that consecutive patients were included, during a limited time interval in the winter.
In conclusion, vitamin D deficiency and insufficient vitamin D levels were common at the primary care centre overall, and especially for patients born outside Europe.