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.

How much vitamin D do you need?

The average daily recommended amounts vary based on your age. The National Institutes of Health recommended dietary allowance (RDA) for vitamin D is 600 IUs daily for infants, children, and adults up to 70 years of age. Adults 71 and older need 800 IU, since the ability to absorb vitamin D declines with age.

People of higher weight may also need more vitamin D, because fat cells absorb it, 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. If you have any of these clear signs you might not be getting enough vitamin D, talk to your doctor about getting a blood test to check your levels. You may need a prescription mega-dose to get you back on track.

You can get too much D

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 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, a dietitian in Long Island, NY.

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, an ob-gyn in San Antonio, TX, including excess calcium absorption and conditions associated with hypercalcemia such as kidney stones and GI upset. Levels that are too high for too long can cause vitamin D toxicity. It’s rare, but “people should not take anything above the daily recommended doses of Vitamin D unless instructed to do so by their doctor,” Dr. Francis says.

How to choose a supplement

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). D3 is the form of the vitamin that is already synthesized in the human body, so it’s believed to be more effective as a supplement. Second, you’ll want to consider the other nutrients coming along for the ride. Magnesium, for example. “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 your vitamin D levels checked—and if you should supplement. From there, you can discuss the dosage that would be right for you.