How to Get the Right Amount of Vitamin D

Vitamin D is an essential nutrient for maintaining bone health and strength. It also contributes to nerve, muscle and immune function, and helps to moderate inflammation. But ensuring that you get enough vitamin D is complicated by much public misinformation about it.

There has been a lot of discussion lately about testing (well … overtesting) for levels of vitamin D, and whether people who have been diagnosed with a vitamin D shortage are truly deficient or just have doctors engaging in a trendy form of overtreatment. You can read about these issues here and here.

Preliminary studies show that the vitamin might have a role in cancer risk, diabetes, high blood pressure, multiple sclerosis and other disorders. But the science is young, and at this point, only interesting, not particularly helpful in a medical sense.

But there’s no question that you should pay attention to how the body receives and uses vitamin D, and whether your situation makes that possible. A recent report from Harvard Health Publications provides a helpful guideline.

The body makes vitamin D from absorbing sunlight, and the liver and kidneys also play a role in vitamin D production. Food also supplies the vitamin, not because a lot of what you eat naturally contains it (it doesn’t), but because manufacturers supplement lots of foods with it, especially milk, yogurt, breakfast cereals and orange juice. Some people take vitamin D supplements, but doing so without knowing if you’re not getting enough through other means probably isn’t a good idea.

Lots of things can affect your vitamin D level. They are:

Where you live. As the Harvard report explains, the farther away from the equator you live, the less vitamin D-producing light reaches the earth’s surface during winter. So if you live in a more northern region, your body probably isn’t making much vitamin D during winter, and because you’re covering up your body when it’s cold, even when it’s sunny the rays aren’t reaching the skin. In addition to shorter days during the winter, according to the National Institutes of Health (NIH), thick cloud cover also can impair your ability to get enough sunshine any time of year.

Air quality. The carbon particles produced when fossil fuels (cars, factories), wood and other materials are burned absorb the sun’s UVB rays that the skin needs to make vitamin D. Pollution that destroys the ozone layer, however, could promote vitamin D production because ozone absorbs UVB radiation.

Sunscreen use. If you regularly apply sufficient amounts of sunscreen you’re getting less vitamin D because sunscreen blocks UVB light. But as the Harvard report makes clear, very few people use enough sunscreen, or use it often enough, to block all UVB light.

Skin color. How dark your skin is depends on how much melanin it contains. As the Harvard writers put it, melanin “competes” for UVB with the substances in the skin that signal the body to produce vitamin D, so dark-skinned people are more likely to make less vitamin D from sunlight than light-skinned people who get the same sun exposure.

Weight. Body fat absorbs vitamin D, so some people have theorized that fat might store the nutrient so that it can be used when its host doesn’t eat or produce enough vitamin D. But studies have shown that obesity is associated with low levels of the vitamin, and that too much weight might interfere with its bioavailability.

Age. The older you are, the less you have of the substance in skin required to convert UVB light into vitamin D-helper. Some evidence has indicated that older people produce vitamin D less efficiently in all ways than younger folks.

In addition to low vitamin synthesis resulting from these factors, you might be at risk of a deficiency, according to the NIH, if you have inflammatory bowel disease or another condition that prevents you from absorbing dietary fat properly, or if you have undergone a gastric bypass procedure. (Such procedures routinely require nutritional supplementation.)

Some medicines can interfere with vitamin D absorption, including steroids and those for weight loss, to lower cholesterol and to prevent and control epileptic seizures. If you take any of these drugs, discuss with your doctor if you should take a vitamin D supplement.

But you also can get too much vitamin D, primarily through supplementation. The NIH says vitamin D toxicity can cause anorexia, heart arrhythmias and other serious problems. It can raise blood levels of calcium to dangerous levels, leading to heart, blood vessels and kidney damage.

If you take vitamin D supplements and experience vomiting, nausea, poor appetite, weakness, constipation, dehydration and/or frequent urination, call your doctor immediately.