Diet and Fitness Expert Dr. Melina Jampolis Physician Nutrition Specialist

Expert answer

HI Max. I'm glad you asked this question, as more and more research is being done lately evaluating the role of vitamin D in health and disease prevention. Vitamin D is a fat-soluble vitamin that occurs naturally in a limited number of foods, so fortified foods including milk, margarines and breakfast cereals and dietary supplements are the major source for most Americans. In addition, vitamin D can be produced by the skin with sunlight exposure, but this ability decreases with age and varies considerably by geography, time of day or year, skin melanin content (darker skin produces less vitamin D) and sunscreen use.

Vitamin D is essential for promoting calcium absorption in the gut. When combined with adequate daily intakes of calcium, vitamin D helps build and preserve bone, helps prevent osteoporosis and helps decrease fracture risk. In addition, emerging research shows a possible role for vitamin D in the prevention or treatment of type 1 diabetes, some cancers, and autoimmune diseases including multiple sclerosis. Vitamin D also appears to play a role in healthy immune function and reducing inflammation, a risk factor for numerous diseases including cancer, heart disease and diabetes.

A simple blood test is available to determine whether you are deficient in vitamin D, but optimal levels have not yet been established. Groups at risk of vitamin D deficiency include breastfed infants, adults age 50 and older, people with limited sun exposure, people with milk allergy or lactose intolerance, people with fat malabsorption, people with dark skin, and people who are obese. (Because it is fat soluble, larger pools of body fat may act as a reservoir for vitamin D.) If you are or feel that you may be deficient in vitamin D, look for foods or supplements containing the D3 form (cholecalciferol), which is significantly more potent than the D2 form (ergocalciferol).

The current Dietary Reference Intakes (DRIs) developed by the Institute of Medicine's Food and Nutrition Board for healthy Americans is available in Table 2 here. If you are deficient in vitamin D or have osteoporosis, you may need higher doses, but talk to your doctor first.

The current tolerable upper limit of vitamin D intake is 50 mcg (2,000 IU) per day. Taking too much vitamin D can cause nausea, vomiting, decreased appetite, constipation, weakness, and can raise blood levels of calcium, which can lead to irregular heart rhythms.