Human skin produces vitamin D-3 with exposure to sunlight, according to Mayo Clinic. As little as 10 minutes of sun exposure provides enough of the vitamin to prevent a vitamin D deficiency. Vitamin D is present in foods such as eggs, fish and fortified milk.
Another name for vitamin D-3 is cholecalciferol, according to Drugs.com. Vitamin D helps with the absorption of calcium from the stomach and with the function of calcium in the body. For deficiency, adults take 400 to 1,000 units daily, and for insufficiency, they take 1,000 units. Geriatric patients take 800 units daily to prevent falls and 100,000 units every four months to prevent fractures. Infants and children need 200 units daily. Animal sources of the vitamin appear to be more potent than vegetable sources, according to the National Institutes of Health.