Global high prevalence of vitamin D insufficiency and re-emergence of rickets and the growing scientific evidence linking low circulating 25-hydroxyvitamin D to increased risk of osteoporosis, diabetes, cancer, and autoimmune disorders have stimulated recommendations to increase sunlight (UVB) exposure as a source of vitamin D. However, concern over increased risk of melanoma with unprotected UVB exposure has led to the alternative recommendation that sufficient vitamin D should be supplied through dietary sources alone. Here, we examine the adequacy of vitamin D intake worldwide and evaluate the ability of current fortification policies and supplement use practices among various countries to meet this recommendation. It is evident from our review that vitamin D intake is often too low to sustain healthy circulating levels of 25-hydroxyvitamin D in countries without mandatory staple food fortification, such as with milk and margarine. Even in countries that do fortify, vitamin D intakes are low in some groups due to their unique dietary patterns, such as low milk consumption, vegetarian diet, limited use of dietary supplements, or loss of traditional high fish intakes. Our global review indicates that dietary supplement use may contribute 6-47% of the average vitamin D intake in some countries. Recent studies demonstrate safety and efficacy of community-based vitamin D supplementation trials and food staple fortification introduced in countries without fortification policies. Reliance on the world food supply as an alternative to UVB exposure will necessitate greater availability of fortified food staples, dietary supplement use, and/or change in dietary patterns to consume more fish.
Most circulating 25-hydroxyvitamin D originates from exposure to sunlight; nevertheless, many factors can impair this process, necessitating periodic reliance on dietary sources to maintain adequate serum concentrations. The US and Canadian populations are largely dependent on fortified foods and dietary supplements to meet these needs, because foods naturally rich in vitamin D are limited. Fluid milk and breakfast cereals are the predominant vehicles for vitamin D in the United States, whereas Canada fortifies fluid milk and margarine. Reports of a high prevalence of hypovitaminosis D and its association with increased risks of chronic diseases have raised concerns regarding the adequacy of current intake levels and the safest and most effective way to increase vitamin D intake in the general population and in vulnerable groups. The usual daily intakes of vitamin D from food alone and from food and supplements combined, as estimated from the US third National Health and Nutrition Examination Survey, 1988-1994, show median values above the adequate intake of 5 microg/d for children 6-11 y of age; however, median intakes are generally below the adequate intake for female subjects > 12 y of age and men > 50 y. In Canada, there are no national survey data for estimation of intake. Cross-sectional studies suggest that current US/Canadian fortification practices are not effective in preventing hypovitaminosis D, particularly among vulnerable populations during the winter, whereas supplement use shows more promise. Recent prospective intervention studies with higher vitamin D concentrations provided evidence of safety and efficacy for fortification of specific foods and use of supplements.
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