Overall, low concentrations of 25(OH)D were frequent, especially in immigrants. Reported sun holiday and ethnic Swedish status were the factors that positively, significantly and independently affected 25(OH)D concentrations in plasma, with sun holiday ranking highest. The non-significant effect of vitamin D intake from food and supplements as well as wearing of veil, was unexpected. Regarding vitamin D intake, it may reflect the overall low dietary intake of vitamin D.
Plasma concentrations of 25(OH)D are believed to reflect the vitamin D status in terms of production, absorption, and storage, with a half-life of 1-2 months . In spite of this assumption, our results indicate a prolonged effect of a sun holiday. Whether this is due to the body being able to access stored vitamin D or whether other mechanisms provide an explanation is not known.
From a global point of view, vitamin D from sun exposure is said to be of most importance, but at latitudes around 60°N, where vitamin D is produced in the skin from sun irradiation only from April to September, it is not clear how large this contribution is [8, 9]. In the present study, vitamin D intake from food and supplements was not always sufficient to avoid low plasma concentrations of 25(OH)D. In order to raise vitamin D in plasma addition of vitamin D through food, supplements or via increased production in the skin would be particularly important for women living at high latitudes.
Previous studies report that 25(OH)D concentrations <50 nmol/L are common in women living in the Norwegian arctic region (65-70ºN) during January and February . The results of the present study with measurements between January and March are consistent with these observations, although our study is performed at more southern latitude (60ºN). Our data also concurs with results from epidemiological studies on immigrant populations in Norway and Denmark [9–13]. The importance of a sun holiday found in the present study accords with a study on elderly Swedish women from the general population , although our study population included younger patients, and from a primary health care setting [12, 13].
Regular sun holidays seemed in this study to contribute more to high 25(OH)D concentrations than vitamin D through food and supplements. The findings suggest that sun holidays and immigrant status explain more variability in vitamin D levels than does food. Immigrant status might be anticipated to be confounded with socioeconomic factors such as income level, smoking habits and educational level. However, there was no difference in the number of sun holidays between immigrants and Swedes. As to vitamin D intake through food, since foods containing high amounts of vitamin D, such as egg yolk, fortified milk and margarines are relatively cheap, the choice of food is more likely to be dependent on culture, traditions and knowledge than to differences of income. There was no difference in the frequency of tobacco smoking between the two groups. Recommendations for sunbathing are also controversial, owing to the risk of skin cancer. However, the latter, may be less relevant for people with darker skin.
Frequently cited studies state that 15 minutes of sun irradiation per day is enough to obtain sufficient vitamin D . However, these studies are based on individuals living further south and these results cannot be translated to apply to the situation at more northerly latitudes around 60°, where no vitamin D is produced in the skin from October to March, irrespective of hours of sunshine .
According to Webb and Engelsen , a rough calculation of the vitamin D production following sun exposure for three hours a day for seven days at latitude 40° leads to a total production of 4,500 μg vitamin D. Considering the fact that the half-life of vitamin D in the body is 2 months, and that the average time since the women had been on a sun holiday was six months, this would correspond to 500 μg being left at the time of the study. This circumstance would imply a substantial addition of vitamin D in the body.
Increasing interest in and knowledge about vitamin D deficiency in relation to multiple conditions [1, 15, 16], has led many physicians to test their patients’ vitamin D status. The high frequency of low concentrations of 25(OH)D in immigrant women in our study indicates that vitamin D deficiency may be common in female patients of immigrant origin in primary care. Vitamin D deficiency and insufficiency have by some researchers previously been considered to be uncommon in the female Swedish population , but these views are based on studies of postmenopausal women with osteoporosis. This observation might not be relevant for all women, and especially not for immigrant subpopulations.
For good bone health, blood concentrations of 50 nmol/L 25(OH)D are needed . For other aspects of general health, a concentration of 75 nmol/L has been suggested . Fifty percent of the ethnic Swedish women in our study had plasma 25(OH)D concentrations below 50 nmol/L. For immigrant women, the majority should probably be advised to take greater supplements of vitamin D than the present recommendations, and perhaps even without testing for vitamin D concentrations in the blood, especially if their intake from food is low. The latter could easily be assessed with a few questions since most vitamin D consumed in food is present in only a few food items.
The strengths of the study was that women with limited knowledge of the Swedish language were not excluded and that the individual interviews enabled inclusion of illiterate women, since they are a vulnerable population group with an increased risk of health problems . Moreover, the study appointments took place between January and March in order to obtain blood samples that reflected the lowest values during the year in Sweden .
A limitation was that the study was too small to reliably rule out non-significant results and limited the generalizability. Also, the concept of sun holiday was difficult to standardize. In addition, the FFQ questionnaire has not been validated, but the results were compared with the results from a nationwide Swedish survey  and the intake of vitamin D was estimated to be similar (5.1 μg/d respectively 4.9 μg/d). Another limitation of this study may have been the choice of method for analysing plasma-25(OH)D. Other assays are available, and their comparability is uncertain. The HPLC method seems to be more accurate and reliable than other methods. In our study we found that two thirds of the immigrants had deficient concentrations of plasma-25(OH)D (<25 nmol/L), but if we had used another method, e.g. HPLC, probably fewer women would have had deficient concentrations. However, the difference in plasma-25(OH)D between our two groups would still remain.
In future research, the need of treatment for low 25(OH)D values in immigrants who have moved from southern to northern latitudes should be further investigated. Culturally adjusted measures should be considered for this vulnerable group.