Vitamin D is the nutrient that has captured the attention of the scientific community, regulatory agencies, the food industry and the public alike over the past 15 years. While there are many controversial issues in relation to vitamin D requirements and the impact of vitamin D intake and status on human health, there is a general agreement that prevention of vitamin D deficiency is a public health priority. The major source of vitamin D in humans is sunshine; ultraviolet blue (UVB) radiation stimulates cutaneous synthesis of cholecalciferol, which is stored in adipose tissue or undergoes hydroxylation in the liver to 25-hydroxyvitamin D, (25OHD), the biomarker of vitamin D status, and further hydroxylation in the kidney to 1,25-dihydroxyvitamin D, (1,25(OH)₂D), the biologically active metabolite.
Several environmental factors, such as latitude and prevailing weather conditions, determine whether sunshine of sufficient strength is available to stimulate the conversion of 7-dehydrocholesterol in the skin to pre-cholecalciferol. Personal attributes, such as skin pigmentation, age, attire, sunscreen, working environment, physical activity and sun exposure behaviour can also prevent or impede vitamin D synthesis. Vitamin D occurs in the diet, both naturally and as a fortificant as cholecalciferol (D₃) and ergocalciferol (D₂) and in nutritional supplements. Vitamin D intakes are typically low as it occurs naturally in few foods and in low concentrations.
Substantial proportions of the European population, including all who reside at latitudes greater than around 40°N, rely on body stores and dietary vitamin D to maintain a healthy vitamin D status all year round. Given that body stores are dependent on sun exposure, the importance of food in maintaining vitamin D status is a corollary of sunshine deficit. In Europe in particular, there is increasing evidence that the dietary supply is currently unable to offset the seasonal sunshine deficit, which increases with latitude and the duration of winter. While nutritional supplements contribute a high proportion to total vitamin D intake among users, supplement intake is voluntary, and tends to be highest among infants and elderly adults and lowest among children, adolescents and young adults, who are also at risk of deficiency.