High latitude and marine diet: vitamin D status in elderly Faroese

Abstract

Human subjects obtain their vitamin D from the diet, especially from marine food, and from endogenous synthesis following cutaneous sun exposure. The risk of an insufficient vitamin D synthesis is increased in northern populations, but it may be counteracted by a high intake of marine food in fishing populations, e.g. at the Faroe Islands. We examined the vitamin D status and its statistical determinants in a cross-sectional study of 713 elderly Faroese aged 70-74 years, about two-thirds of all the eligible residents in this age group. Clinical examination included measurement of body weight and height, and marine food intake was estimated using a questionnaire. We measured serum 25-hydroxyvitamin D3 (S-25(OH)D3) by LC-MS/MS in 669 of the 713 subjects in whom sufficient serum was available. Of the population, 19% had S-25(OH)D3 concentrations < 25 nmol/l, and only 10.3% of the population had S-25(OH)D3 concentrations >80 nmol/l. In a logistic regression analysis, BMI < 30 kg/m2, blood sampling in summer season, eating pilot whale blubber more than once per month and female sex were positively associated with vitamin D levels >80 nmol/l. The high prevalence of low vitamin D levels among the elderly Faroese population reflects the low skin synthesis during most months of the year, which is caused by the limited sun exposure and insufficient benefits from marine diet. Thus, even in a population with a high intake of marine food, the northern latitude causes a low vitamin D status. Efforts to improve vitamin D status in this population are warranted.

Fig. 1

Fluctuation of serum 25-hydroxyvitamin D₃ (25(OH)D₃) concentrations according to the month in which blood sampling was done. Boxes represent lower and upper quartiles, and the centre line represents the median value. The grey circle represents an outlier observation. The median values for July, August and September (summer months) are significantly different from those for the other months (P < 0·0001; see Table 2).