Vitamin D is necessary in maintaining appropriate calcium and phosphate homeostasis in the body (classical function) and ensuring appropriate functioning of many tissues, organs and cells, unrelated to mineral economy (non-classical function). Vitamin D deficiency in adults may cause osteomalacia, increase fracture risk in osteoporosis, induce cardiovascular diseases, diabetes type 1 and 2, multiple sclerosis, Lesniowski-Crohn disease, and cancer, including colon, breast, and prostate cancer. Possible causes of vitamin D deficiency in a healthy population include decreased cutaneous synthesis and an inadequate intake of vitamin D, both in food and in supplements. Vitamin D deficiency level (25(OH) D. <20 ng/mL), is fairly widespread, being found in a substantial percentage of healthy subjects around the world, regardless of race, gender and age. Daily vitamin D dose, as determined by the Food and Nutrition Board in 1997, is now rather insufficient, the biggest problem being associated with maximal vitamin D levels (50 μg/day) in actually available food supplements. Nowadays, it is recommended that adults need a minimum of 800-1,000 U/day when their exposure to the sun is inadequate (in Poland from October to April). This dosage should be provided to all subjects who avoid sunlight, as well as to those aged over 65 because of their slower skin synthesis of vitamin D and for its proven anti-fracture and anti-fall effects.