What’s Vitamin D Good For? The Evidence vs. The Hype

When you hit the physician’s office for your annual check-up and bloodwork, does your doctor test your vitamin D levels?

If a “quick check” of vitamin D is a routine part of your health care, you aren’t alone. According to numbers I recently found, in 2016 alone, tests for vitamin D deficiency were ordered for Medicare patients alone no fewer than 10 million times—and apparently, many of those tests had no medical indication, meaning the roughly $365 million price tag for those tests paid for information that may not have been useful.


The issue is this: We’re still gathering evidence for and against the role of vitamin D for specific health conditions. Its potential role in staving off maladies has been touted based on preliminary evidence for many common conditions—everything from heart disease and depression and cancer to weight gain and fatigue.

As with many nutritional vitamins, there’s no reason to overhype a hormone that’s already important. The National Institutes of Health is clear: Vitamin D is critical for the formation and maintenance of strong, healthy bones, because it’s critical for calcium absorption. In this way, it helps to protect older adults from osteoporosis—a disease that causes the thinning and breakage of bones as people age. Nerves and muscles also require adequate levels of vitamin D, as does the immune system.

But other claims that have emerged in recent years about the role of vitamin D have not held up under scientific scrutiny.

In January 2019, the New England Journal of Medicine published findings of a study tracking more than 25,000 patients taking vitamin D and Omega-3 supplements (the VITAL Trial). With a vast dataset from patients who were assigned to groups taking either 2,000 IU of vitamin D a day or a placebo, it was finally possible to evaluate the outcomes of these patients, over a period of 5.3 years.

The results for both groups, overall, were not statistically significant. While vitamin D was thought to reduce the risk of invasive prostate, breast, and colorectal cancers, the VITAL trial evidence overall did not bear this out. Neither did the supplement reduce the risk of cardiovascular events, such as heart attacks or stroke. (One separate finding in the demographic cross-tabs considered worthy of further study did note that black participants in the vitamin D group experienced an overall 23% reduction of cancer risk.)

Vitamin D supplementation is generally safe at levels under 4,000 IU per day among adults, and the nutrient does play an important role in bone, nerve, and overall health. It is possible to take too much vitamin D; the consequence is a condition in which too much calcium can build up in the blood, causing a variety of symptoms. Supplementation of milk and breakfast cereals actually began in the United States in the 1930s, when rickets caused by widespread vitamin D deficiency was a severe public health problem; nobody wants to return to those days.

But as with many health claims associated with nutritional supplements, the evidence is still accumulating. No nutritional supplement is a magic bullet. No pill from the corner drug store “prevents cancer.”