If you follow health news online, you’re likely to see an endless barrage of contradictory information on this crucial vitamin. One day, it’s the key to preventing cardiovascular disease, but the next day, a study finds that it plays no role at all. To find out how to interpret these contradictory studies, Bottom Line Health spoke with Michael F. Holick, MD, PhD, a leading expert in vitamin D research.

Are you deficient?

Your body naturally makes vitamin D when your skin is exposed to the sun. That may work well for people who spend a lot of time outdoors in warm climates, but most people north of Atlanta simply can’t get enough sun from November through March to attain optimal levels. In a study that looked at 2.3 million blood samples, the average level was only 29 nanograms per milliliter (ng/mL) at the end of summer, when levels are highest. In the winter, the average level plummeted to 10 to 22 ng/mL. Certain medications can destroy vitamin D and cause a deficiency, including steroids, like prednisone, as well as AIDS and anti-seizure drugs.

The outward symptoms of deficiency are subtle. You may have joint stiffness, muscle aches, and fatigue. But under the surface, more serious problems may be percolating:

  • Long-term deficiency is associated with a higher risk of autoimmune disorders, such as type 1 diabetes and multiple sclerosis.
  • The Framingham heart health study found that people with levels under 15 ng/mL had a 15 percent higher risk of having a heart attack.
  • Data suggests, but does not prove, that deficiency may increase the risk of cognitive changes and Alzheimer’s disease.
  • Young and middle-aged women who are deficient have a 40 percent higher risk of developing breast cancer, according to the Nurses’ Health Study.

A recent study looked at the vitamin D status of 191,000 people with COVID. It found that people with sufficient levels (at least 30 ng/mL) were 54 percent less likely to get the virus, and those who did get it had fewer complications and were less likely to die.

It’s important to note that these are all what’s called association studies. That means that the study can clearly see that vitamin D deficiency and a specific condition appear together, but it doesn’t prove that the deficiency caused the other condition. An autoimmune disorder, for example, could cause the vitamin D deficiency, not the other way around. Or a third factor may cause both.

Testing your levels

You can find your vitamin D levels through a simple blood test called the 25-hydroxyvitamin D test. (Some doctors may include the test in your annual bloodwork, but you may have to ask for it to be added.) Ideally, you should have a level of at least 30 ng/mL. If your levels are 20 to 30 ng/mL, you have a vitamin D insufficiency. Under 20 ng/mL is a deficiency—and it’s very common. Remember that levels are highest in the summer, so the time of year affects your results. Fortunately, vitamin D deficiency is reversible. Spending more time outside on clear sunny days can help, but the amount of vitamin D you make is affected by many factors, such as time of day, time of year, skin color, and even air pollution. Midday exposure on a large part of your body (like your back) on a sunny day is best. A light box that supplies 10,000 lux of full-spectrum light can also stimulate vitamin D production. But most people should consider taking a supplement.

Even if you don’t have your levels tested, it’s reasonable and safe to take a daily supplement of 2,000 to 5,000 international units (IU) daily. If you’re obese, you need two to three times more supplemental vitamin D to boost your blood levels. Vitamin D is remarkably safe. If you forget a dose one day, you can double up the next. Dr. Holick participated in a study that found that taking large doses of vitamin D every two weeks—instead of smaller doses daily—caused no safety concerns after six years. People who are deficient may take 5,000 IUs a day, and the Endocrine Society says it’s safe to go up to 10,000 IUs.

The VITAL Trial

While most studies that report on vitamin D have been association studies, the Vitamin D and Omega-3 Trial (VITAL) was different. Researchers gave one group of participants supplemental vitamin D and fish oil and compared them with a control group that did not take the supplements. The investigators wanted to see if the supplements reduced the occurrence of cancer or cardiovascular disease. They reported in the New England Journal of Medicine that neither of those conditions were affected, and the mainstream media quickly picked up the message that vitamin D doesn’t help your heart.

We asked Dr. Holick if that was a fair interpretation of the results. He explained that about half of the nearly 26,000 people in the study had adequate levels of vitamin D from the start, so the study didn’t examine the effects specifically on people with deficiencies. Supplementation may play a protective role only in people who are deficient, but those results could be masked by the way the data were analyzed, he said. So the study didn’t show a benefit or prove a lack of benefit.

Interestingly, there was a benefit that was clear: Among people who developed colorectal cancer, those who were taking 2,000 IU of vitamin D had a 25 percent lower risk of death—even if they started off with healthy levels of vitamin D. That suggests, Dr. Holick says, that it might be worth taking vitamin D even if your levels are sufficient.

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