The Truth About Vitamin D Supplements: Are They Healthy Or Unhealthy?

As with any nutrient, mega-dosing is likely to do more harm than good.

In 2012, 20 percent of American adults reported taking vitamin D supplements, a fourfold increase over about a decade earlier, according to a 2016 report published in the Journal of the American Medical Association. This increase in consumption isn't surprising, given the generally low dietary intake of vitamin D and studies widely publicized during that time that suggested it could prevent a plethora of chronic diseases. But in light of more recent research, consumers are now questioning whether they should be taking it at all.

Questions about the usefulness of vitamin D supplements are based on two recently published studies. The first is a meta-analysis published in the Lancet Diabetes & Endocrinology, which suggests vitamin D supplements neither lead to clinically significant improvement in bone mineral density nor reduce the risk of falls and fractures.

The second is the Vitamin D and Omega-3 Trial, known as VITAL, a randomized control trial of more than 25,000 Americans. The study aimed to answer whether taking 2,000 International Units (IU) of vitamin D a day for men 50 and older and women 55 and older would prevent cancer and cardiovascular disease over an average of 5.3 years.

The report concluded that "supplementation with vitamin D did not result in a lower incidence of invasive cancer or cardiovascular events than placebo." But that doesn't mean you should toss out your vitamin D supplements.

Vitamin D is a hormone known for its bone health benefits: It's necessary for the absorption of calcium and phosphorus into bones and teeth. Without enough vitamin D, calcium leaches out of the bones, resulting in loss of bone strength. This can lead to rickets in children and osteomalacia (softening of the bones) or osteoporosis in adults.

Dan Newton, a molecular biologist and research scientist at the Medical University of South Carolina, says vitamin D is a regulator of inflammation and plays an important role in the immune system. "Numerous studies within the last 20 years have linked vitamin D deficiency or insufficiency to various types of cancer, Alzheimer's disease, Type 2 diabetes, heart disease, pregnancy complications and more," he says.

So why has vitamin D gotten a bad rap?

James Fleet, professor in the nutrition science department at Purdue University, says, "We are starting to see some studies show that vitamin D doesn't have a benefit, even for bone health. However, these studies are in people who have generally good vitamin D blood levels. It's not surprising that giving them more of a nutrient they already have enough of doesn't lead to better health outcomes." This is true of the Lancet study. Only 6 percent of the 81 trials included in the analysis were in people with vitamin D deficiency.

JoAnn Manson, the lead author of the VITAL study, says concluding that vitamin D supplements aren't helpful is an oversimplification of the trial results. "Overall, the clinical trial research has shown that there are some benefits of vitamin D supplementation, but they are more limited than what was originally proposed," says Manson, the chief of the Division of Preventive Medicine at Brigham and Women's Hospital and a professor at Harvard Medical School. "A few years ago, people believed that vitamin D was a panacea that could prevent every major chronic disease."

According to Manson, there may be reasons the VITAL trial didn't clearly link vitamin D supplementation to a reduction in heart disease or stroke risk. "This may be because the amount of vitamin D needed for heart health is relatively modest to moderate, similar to the amount needed for bone health and already achieved by many without taking supplements," she says. "For cancer reduction, there may be benefits with larger doses."

Manson and colleagues recently published a meta-analysis of vitamin D supplements and various health outcomes in the Annals of Oncology. "We found a strongly significant reduction in cancer death of 13 percent across the five trials, which included VITAL," Manson says. The vitamin D dosages in these studies ranged from 833 to 2,000 IU per day.

Although the development of cancer wasn't statistically different for the vitamin D group vs. the control group in the meta-analysis, Manson points out that cancer develops over several decades, and many of the studies had less than five years of follow-up.

The National Academy of Medicine has set the recommended amount of vitamin D to get from your diet and supplements based on bone health research. They recommend 400 IU of vitamin D for infants, 600 IU for men and women ages 1 to 70 years, and 800 IU for adults 71 or older. If you eat three ounces of salmon twice a week and have a cup of milk and two eggs every day, you're still only getting about 330 IU of vitamin D a day.

It's not surprising then that vitamin D is listed as an under-consumed nutrient of public health concern in the 2015-2020 U.S. Dietary Guidelines. Getting more vitamin D from nutritious foods, such as oily fish, fortified milk products or fortified milk alternatives, egg yolks and vitamin D-enhanced mushrooms, is helpful advice.

What about the sun, you might wonder. Although our bodies can also synthesize vitamin D with sun exposure, the Dietary Reference Intakes for vitamin D assume minimal sun exposure, factoring in the likelihood of spending time indoors, being at northern latitudes or using sunscreen. Although you may want to get more vitamin D from the sun, the American Cancer Society still recommends protecting your skin from the sun to reduce the risk of skin cancer.

This makes getting enough vitamin D from your diet and supplements becomes increasingly important. And "enough" may depend on other factors. For example, the Endocrine Society says at least 1,500 to 2,000 IU of vitamin D3 a day may be needed to achieve blood levels of vitamin D linked to bone health and muscle function. For obese adults, two to three times as much vitamin D is needed to get to those blood levels. In Manson's research, 2,000 IU a day seems to be the amount needed to reduce the risk of cancer death.

Due to the variations in how much vitamin D people produce and variability in sun exposure, blood tests of vitamin D levels, specifically the level of 25-hydroxy vitamin D, are the best way to determine whether there is a vitamin D deficiency. This is a simple blood test that requires no special preparation and can be done along with your regular bloodwork, though it may result in an additional out-of-pocket fee. But a doctor will usually order it only in cases where there is reason to believe a person is at risk of vitamin D deficiency, such as anyone with a medical condition that affects vitamin D absorption, people who are housebound or institutionalized, those with osteoporosis, and people who don't eat foods rich in vitamin D.

What the blood levels of vitamin D may mean is a contentious topic. As with the ideal amount of vitamin D we need to get from food and supplements, the optimal level of vitamin D in the blood varies based on whether you're aiming for bone health or other health benefits. For bone health, the National Academy of Medicine says most people should be getting enough vitamin D at blood levels higher than or at 50 nanomoles per liter (nmol/L). For colorectal cancer prevention, blood levels of 75-to-100 nmol/L may be needed. To reduce the risk of cancer death and/or slow cancer growth, 100 nmol/L may be needed. In the VITAL study, this was achieved with a daily vitamin D supplement of 2,000 IU.

The National Academy of Medicine recommends that people ages 9 years and older stay below 4,000 IU of vitamin D a day from food and supplements. This is based on concerns that high doses of vitamin D can cause dangerously high levels of calcium in the blood along with kidney stones and buildup of calcium in the blood vessels and soft tissues. This can cause lead to serious complications such as kidney and heart problems.

Taking a supplement of 2,000 IU a day of vitamin D over an average period of 5.3 years was determined to be safe in the VITAL study. According to Manson, "With very high doses - especially 10,000 IU per day or higher - there may be health risks associated with high calcium levels in the blood and urine."

As with any nutrient, mega-dosing is likely to do more harm than good.

The takeaway on vitamin D is that the research is constantly evolving and incredibly nuanced. Before you toss out your vitamin D supplements, consider how much you're getting from your daily diet and how much unprotected sunlight you're getting each day.

At the very least, using a supplement to get to the levels recommended by the National Academy of Medicine makes sense to protect your bones. For additional health benefits, taking 2,000 IU a day appears to be both sensible and safe for most people.