Thinking About Popping a Vitamin D? Read This First

Vitamin D is critical throughout the body—essential for bone-building, cognitive function, joint health, immune function, blood sugar control, positive mood, and more. It can help prevent everything from osteoporosis to autoimmune disorders to cardiovascular disease and cancer. Some studies suggest that the lower your vitamin D level is, the higher your risk of mortality from any cause. That said, it’s not a great idea to just grab a supplement off the drugstore shelf and start taking it.

Since many of us are deficient in vitamin D, also known as the “sunshine vitamin,” doctors will often prescribe high levels of vitamin D supplements, with doses ranging from 2000 to 10,000 IU (International Units) per day, up to 50,000 IU per week, and sometimes more. But like all good things, it’s possible to get too much. “Mega doses of vitamin D can cause a buildup of calcium in the blood, causing nausea, vomiting, poor appetite, constipation, weakness, and weight loss,” says Madeline Basler MS, RDN, CDN, of Real You Nutrition. What’s more, many supplements can have up to 22 percent more vitamin D3 than the amount listed on their label, according to an analysis of 21 vitamin D best-selling supplements by labdoor.

Here’s the bigger issue: Since vitamin D is fat soluble, any excess can get stored in the body. “A build-up of stored vitamin D can cause avoidable problems,” says Marra Francis, MD, Medical Director at EverlyWell, including excess calcium absorption and conditions associated with hypercalcemia such as kidney stones and GI upset. Levels that are too high can even cause vitamin D toxicity. (The Mayo Clinic has noted toxic reactions after several months of consumption of 50,000 IU vitamin D daily.) “People should not take anything above the daily recommended doses of Vitamin D unless instructed to do so by their doctor,” Dr. Francis says.

The average daily recommended amounts vary based on your age. In 2010, the National Institute of Health set the Recommended Dietary Allowance for vitamin D at 600 IUs daily for infants, children, and adults up to 70 years of age, which is an increase from their previous recommendation of 200 IUs daily. Adults 71 and older need 800 IU, since the ability to absorb vitamin D declines with age. According to a study in the American Journal of Clinical Nutrition, 70-year-olds have a 75 percent reduced capacity to manufacture vitamin D in their skin compared to people in their twenties. People who are obese may also need more vitamin D, because the fat cells take it up, making it less available for use by the body. In the winter, when you’re getting less sun exposure, some experts say it’s safe to take daily doses of around 1,000 to 2,000 IUs without any risk of complications. Here are clear signs you might not be getting enough vitamin D.

Two other considerations when choosing a vitamin D supplement: First, you’ll want to make sure that it uses vitamin D3 (cholecalciferol) instead of vitamin D2 (ergocalciferol), as vitamin D3 is the form of vitamin D that is already synthesized in the human body, so it’s expected be more effective. Second, you’ll want to consider the other nutrients coming along for the ride. Magnesium, for example, gets a lot of attention. “Magnesium is not needed for ‘absorption’ of Vitamin D, but it is needed for vitamin D to convert to its active form in the blood stream,” says Dr. Francis. “And as there are as many people with magnesium deficiencies and Vitamin D deficiencies, these are often suggested to be taken together.”

Though you’ll get a bit of vitamin D from foods—including fatty fish, beef liver, cheese, egg yolks and mushrooms, plus fortified foods like milk, orange juice, and cereal—the amounts are so small that there’s little need to worry that they’ll put you over the daily dose.

The upshot? Talk to your doctor about your vitamin D levels—whether you should have them checked—and if you should supplement. From there, you can discuss the dosage that would be right for you.