Vitamin D (vit D) has several roles in the body, including modulation of cell growth, neuromuscular and immune function, and reduction of inflammation. In addition, many genes encoding proteins that regulate cell proliferation, differentiation, and apoptosis are modulated in part by vit D. Serum concentration of 25(OH)D is the best indicator of vit D status. It reflects vit D produced both cutaneously and that obtained from food and supplements and has a fairly long circulating half-life of 15 days. 25(OH)D functions as a biomarker of exposure but do not indicate the amount of vit D stored in body tissues. In contrast to 25(OH)D, circulating 1,25(OH)2D (calcitriol) is generally not a good indicator of vit D status because it has a short half-life of 15 hours and serum concentrations are closely regulated by parathyroid hormone, calcium, and phosphate. Levels of 1,25(OH)2D do not typically decrease until vit D deficiency is severe. Generally is considered that persons are at risk of vit D deficiency at serum 25(OH)D concentrations <30 nmol/L (<12 ng/mL). Some are potentially at risk for insufficiency at levels ranging from 30–50 nmol/L (12–20 ng/mL). Practically all people are sufficient at levels ≥50 nmol/L (≥20 ng/mL); the committee ad hoc stated that 50 nmol/L is the serum 25(OH)D level that covers the needs of 97.5% of the population. Vit D deficiency, among other factors, is considerd a risk factor for autoimmune diseases and cancer. To obtain the optimal serum leves of ≥50 nmol/L (≥20 ng/mL) in healthy subjects is suggested after age of 15 years a dietary allowance of 600-1.000 IU. How much of this allowance is obtainable from foods and how much is vit D produced cutaneously? The daily intake with foods is very limited, no more then 10-20% of the needs in fact very few foods and beverages in nature contain vit D. The flesh of fatty fish (such as salmon (90 grams =566 IU), tuna, and sward fish) and fish liver oils are among the best sources and small amounts are found in beef liver, cheese, and egg yolks. Vit D in these foods is primarily in the form of vit D3 and its metabolite 25(OH)D3 and the average intake levels for males from foods alone ranges from 204 to 288 IU/day depending on life stage group; for females the range was 144 to 276 IU/day. Therefore, 80% of the vit D needs are obtained through exposure to sunlight: ultraviolet (UV) B radiation penetrates uncovered skin and converts cutaneous 7-dehydrocholesterol to previtamin D3, which in turn becomes vit D3. Season, time of day, length of day, cloud cover, smog, skin melanin content, and sunscreen are among the factors that affect UV radiation exposure and vit D synthesis and explain the epidemiological differences observed in different studies. Generally, 1 minimal erythemal dose (MED) of 54 mJ/cm2 exhibited a 50-fold increase in blood concentrations of vit D3 within 8 h and is equivalent to ingesting between 10,000 and 25,000 IU of vit D2 (akready hydroxylated). However, a sunscreen with a sun protection factor (SPF) of 8 reduces the capacity of the skin to produce vit D3 by >95%! Of course, since sun is less available at high latitudes and in winter time, in order to keep vit D serum levels optimal, dietary supplements and fortified foods, are suggested where vit D is available in two forms, D2 (ergocalciferol) and D3 (cholecalciferol) that differ chemically only in their side-chain structure (1,500 -2,000 IU day are suggested in winter).
Holick MF. Nutrition and health: vitamin D. Springer, 2010.
Disclosure of Interest None declared