DEAR MAYO CLINIC: It seems that vitamin D is always in the news. Why is it so important, and does the average person need a vitamin D supplement?
Vitamin D is an essential nutrient that your body requires primarily to build strong bones. It does this by helping your body absorb and maintain adequate levels of two other nutrients important to bone health — calcium and phosphate.
You get most of your vitamin D from sunlight. When ultraviolet (UV) rays hit your skin — particularly midday — it triggers production of vitamin D. People in climates with more sunlight tend to get more exposure than do those in climates with less sunlight.
Certain foods — fortified foods, such as milk and cereal, and fatty fish, such as salmon, tuna and mackerel — also provide vitamin D. Chemical reactions in your liver and kidneys transform vitamin D into forms that your body can use.
In general, adults should consume 600 international units of vitamin D a day. That goes up to 800 international units a day for those over 70. National survey data indicate that most Americans don’t get enough vitamin D through their diets. However, the data also indicate that average blood levels of vitamin D are above what’s considered necessary for good bone health for most people. This implies that most American adults get enough vitamin D — most likely through sun exposure.
Severe and prolonged vitamin D deficiency is known to cause bone mineralization disorders such as rickets in children and osteomalacia in adults. Conditions such as these can lead to soft bones, aching muscles, painful movement and fractures. Vitamin D deficiency also may contribute to osteoporosis.
Although numerous studies have reported results associating vitamin D deficiency with various other diseases and conditions — such as fatigue, depression, chronic pain, heart disease, autoimmune disorders, infections, metabolic issues and cancer — clinical trials of vitamin D supplements in people with these conditions generally have failed to show benefit. This implies that a lack of vitamin D probably isn’t causing these conditions. Some experts argue that rather than being a cause of these kinds of illnesses, vitamin D deficiency may be a biological marker for them, signaling the presence of inflammatory processes related to the disease or condition.
Adults who may not get enough vitamin D generally fail to do so due to one or more of these reasons:
Conditions that affect your absorption or processing of vitamin D can affect circulating levels of the vitamin. For example, having conditions such as inflammatory bowel disease or celiac disease, or having had bariatric surgery can affect your intestine’s ability to absorb vitamin D. Obesity appears to drive down, or perhaps dilute, levels of circulating vitamin D. Chronic kidney or liver problems can interfere with the conversion of vitamin D into its active circulating forms, as can certain drugs, such as anti-convulsants and glucocorticoids.
–Reduced skin synthesis
People with darker skin are at greater risk of vitamin D deficiency because greater amounts of melanin in the skin reduce the skin’s ability to produce vitamin D from sunlight. Aging also decreases the efficiency of vitamin D synthesis. Sunscreen, clothing and other UV protective measures that block skin’s exposure to the sun not only help prevent skin cancer, but also reduce production of vitamin D.
–Limited sun exposure
People who spend most of their time indoors generally have low levels of vitamin D. The amount of sun exposure needed for adequate vitamin D production is uncertain, but most estimates are no more than 15 minutes a day between 10 a.m. and 3 p.m., three times a week. However, this sun exposure must be balanced against your risk of skin cancer.
Most healthy American adults have adequate levels of vitamin D. But if you fall into a risk category, talk to your health care provider about whether you need a supplement. Generally, a blood test isn’t necessary because taking the recommended amount of vitamin D as a supplement will ensure adequate levels in most people. Even 600 international units a day will correct a deficiency fairly quickly. However, taking too much vitamin D can overly increase your absorption of calcium, leading to problems such as kidney stones and damage to your heart and blood vessels. The National Academy of Medicine recommends an upper limit of 4,000 international units a day to be safe. (Adapted from Mayo Clinic Health Letter) — Sundeep Khosla, M.D., Endocrinology, Mayo Clinic, Rochester.