Vitamin D production is increased by sunlight exposure. People with darker skin may have lower levels of vitamin D because they produce less melanin, which acts like a natural sunblock.

Studies investigating the effect of vitamin D supplements on cancer prevention have had mixed results. In the VITAL clinical trial, supplementation with 2,000 IU/day of vitamin D did not decrease rates of cancer in people with adequate baseline serum 25(OH)D levels.

Effectiveness

Vitamin D is made in the skin, absorbed through the digestive system and stored in fat tissue. The vitamin is also found in foods, such as fatty fish and some fortified milk. The vitamin's absorption, however, can be reduced by some medications, especially antacids. Some diseases can also reduce vitamin D levels. For example, chronic inflammation and a high fat diet can lower them. In addition, obesity can reduce vitamin D levels because the body stores fat in fat cells, removing it from the bloodstream.

Observational studies suggest that vitamin D deficiency may be linked to a number of conditions, including multiple sclerosis, heart disease and certain cancers. But clinical trials have shown mixed results. One randomized trial gave 36,282 postmenopausal women daily calcium and vitamin D supplements or a placebo for a median of seven years. The treatment did not affect cancer incidence, but it did cut cancer mortality by 7% [95].

Too much vitamin D can cause too much calcium to build up in the body (hypercalcaemia), which can damage bones and harm the kidneys and heart. But toxicity is rare and usually happens only when large doses of supplements are taken for long periods of time. Experts recommend avoiding supplementation with more than 1,000 IU of vitamin D per day.

Health Benefits

Vitamin D deficiency is associated with a variety of diseases, especially those that affect the bones. It also has a role in cell growth and modulation of immune function. But getting enough of the nutrient can be tricky. For instance, inflammatory conditions can lower vitamin D levels, as can obesity because excess body fat stores the vitamin, preventing it from entering the bloodstream.

Most people can meet their vitamin D needs from sunlight or fortified foods, though some groups might need supplements. These include older adults and people living in the northern hemisphere (because sun exposure is reduced there), who might have more difficulty obtaining adequate blood levels of vitamin D. In addition, some health conditions and medications can interfere with vitamin D absorption.

The US Preventive Services Task Force (USPSTF) recommends taking a daily supplement with calcium and vitamin D to promote bone health. But more research is needed to determine whether a supplement reduces heart attack, stroke, cancer, or other health risks. Some clinical trials have shown an inverse association between 25(OH)D levels and cancer risk. Others have found no benefit. Further study is needed to understand the reasons for these differences. [101]

Usage Tips

Vitamin D is a fat-soluble vitamin, so it's best absorbed when taken with meals. The supplements available include D2 (ergocalciferol) and D3 (cholecalciferol). D3 seems to be twice as effective in raising blood levels of the vitamin than D2.

Your body makes vitamin D when your bare skin is exposed to sunlight. Most people get at least some of their vitamin D this way. But clouds, smog, old age, and dark-colored skin reduce the amount of vitamin D your body makes. Sunscreens that block UVB rays also limit vitamin D production.

Once in your body, vitamin D is converted to calcidiol, or 25(OH)D, in the liver and to calcitriol, or 1,25(OH)2D, mostly in the kidneys. Calcitriol binds to the vitamin D receptor in cells, changing how genes are turned on or off.

Almost one out of four people has blood levels of vitamin D that are too low for bone and overall health. Getting enough vitamin D protects against diseases such as rickets in children and osteomalacia in adults. Vitamin D works with other nutrients, including calcium, magnesium, and vitamins A and K, to promote bone health. However, these other nutrients are not usually included in multivitamin/mineral supplements. Also, some medications and foods interact with vitamin D.