Objectives: The purpose of this theoretical research paper is to collect and integrate data from the latest literature on complex functions of vitamin D, with reference to physiology and pathophysiology of this vitamin and its relation to various diseases, anemia, multiple sclerosis, psoriasis, endocrine disorders and others. The primary goal is to, on the basis of a review of various studies, find out whether additional intake of vitamin D is needed, whether it is sufficient and effective for healthy people and people with chronic diseases, and what should be done to further utilize the benefits of vitamin D supplementation in prevention and treatment. Material and Methods: For this theory-specialist paper, professional medical literature, as well as scientific research involving human subjects were used. For search, the results of electronic network cell and bibliographic database were used. Results: People with multiple sclerosis have lower average vitamin 25(OH)D blood levels compared to those in a controlled group, making them more likely to develop osteoporosis and bone fractures and earlier transition from the relapse and remission phase to the secondary progression phase. Use of vitamin D in treatment of psoriasis shows a significant reduction in skin lesions. High-dose vitamin D significantly decreases the length of hospital stay of intensive care unit (ICU) patients. A study that investigated the effects of intermittent application of vitamin D3 on muscle strength and metabolic parameters in menopausal women with diabetes mellitus (DM) showed significant improvement in muscle strength in the group of women with diabetes mellitus type 2 in menopausal period, who were taking vitamin D3. Most epidemiological studies have demonstrated a positive association between higher concentrations of vitamin D and its higher dosages and lower incidence of colon cancer. Conclusion: The results of recent clinical studies presented in this paper indicate a very high prevalence of vitamin D deficiency in different age groups around the world. It is now generally accepted that vitamin D deficiency is a global health problem. The 2000 to 4000 IU supplement that is the most commonly advised today is too low to utilize the potential that vitamin D might provide in prevention and treatment of many chronic diseases.