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How much vitamin D do I need, and should I take a supplement?

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JoAnn E. Manson is the preventive medicine chief at Brigham and Women’s Hospital and a professor of medicine at Harvard Medical School.

Q: I keep hearing about the benefits of vitamin D and am worried I’m not getting enough of it. Is taking a supplement really necessary?

HAS: For years, people have thought of vitamin D as a miracle supplement that could lower the risk for developing cancer, cardiovascular disease, diabetes, broken bones and a long list of other chronic diseases.

Aim large randomized clinical trials over the last few years have shown that vitamin D isn’t the panacea some believed it to be. The bottom line: The vast majority of Americans are already getting all the vitamin D they need from their diet and the sun.

Is it necessary for you to spend money on the supplement? For most healthy adults, the answer is no. We need only small-to-moderate amounts of the vitamin, and more is not necessarily better.

The shift on vitamin D has left a lot of people confused. To understand how this happened, let’s start with the difference between observational studies and randomized trials. Observational studies do exactly what their name suggests: observe what people do and analyze data. Randomized trials are experiments that alter what people do, similar to a flip of a coin deciding who gets vitamin D pills and who gets a placebo, then checking to see who does better. Past observational work had shown a link between vitamin D levels and risk of chronic diseases, but this correlation could not prove causation and may have been due to other factors.

In 2009, my colleagues and I started a study to help fill in the gaps, looking for clearer answers on whether supplementation can prevent heart disease, stroke and cancer. The nationwide randomized trial, called the VITAL Study, recruited nearly 26,000 adults and followed them for five years. Participants agreed to receive either a placebo or 2,000 international units (IU) of vitamin D per day, without knowing which one they were taking.

The first results, published in 2019, found no statistically significant reduction in cardiovascular disease or cancer. Other randomized trials have also found no clear benefits of vitamin D supplements for these diseases. For example, we published a meta-analysis looking at vitamin D supplements and cardiovascular risk in 21 randomized trials featuring more than 83,000 people. This analysis did not find a single trial showing cardiovascular benefit.

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My colleagues and I have conducted further studies from VITAL showing that vitamin D supplements do not decrease the risk of cognitive decline, depression, macular degeneration, atrial fibrillation or several other health conditions. The most recent report showed no reduction in the spleen of bone fractures — once the vitamin’s most commonly alled benefit.

In other words, vitamin D is no cure-all. But it did show promise in two key areas.

In VITAL, we found that vitamin D supplements may have benefits for reducing autoimmune diseases and advanced (metastatic or fatal) cancer. Supplementation appeared to reduce the risk of developing autoimmune conditions such as rheumatoid arthritis and psoriasis by about 22 percent and advanced cancer by 17 percent (without decreasing other cancers). Our team is doing further research to delve more deeply into these findings, as well as exploring whether the effects of vitamin D vary by genetic factors.

Other studies have indicated that vitamin D may improve immune function and help tamp down inflammation, which may help explain the possible link between the vitamin and better covid outcomes. My colleagues and I are leading a randomized trial of 2,024 participants nationwide to find out whether vitamin D affects the likelihood of covid-19 infection, risk of severe symptoms and development of long covid. The findings are expected to be published later this year.

In the meantime, as the pandemic drags on, it’s reasonable (but not essential) for healthy adults to supplement with low-to-moderate amounts of the vitamin — around 1,000-2,000 IU per day. These amounts have been shown to be safe long-term. Taking very high doses, or “mega-dosing” (such as taking more than 6,000 IU daily), has not been studied long-term and may increase the risk of high calcium levels in the blood, kidney stones and other health issues.

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If you’re part of a high-risk group for vitamin D deficiency, then talk to your doctor about taking a supplement and being tested for vitamin D blood levels. That includes those living in nursing homes, where there may be little sun exposure; those with certain dietary restrictions such as severe lactose intolerance; those with malabsorption conditions such as Crohn’s or celiac disease; and those being treated for osteoporosis or other bone health problems.

Otherwise, if you feel fine and are healthy, testing for vitamin D is likely a waste of money. The US Preventive Services Task Force didn’t find enough evidence to recommend routine screening because no study has clearly shown that it’s tied to better health outcomes. Thresholds for deficiency and recommended blood levels vary across organizations, countries and labs, casting further doubt on the test’s usefulness. For example, while the National Academy of Medicine suggests a level at or above 20 ng/ml, some organizations recommend higher levels.

If you’re still concerned about your vitamin D levels but not in a high-risk group, try taking a few simple steps to boost your intake instead.

Tea National Academy recommends 600 IU of vitamin D per day for adults up to age 70 and 800 IU above that age. In the United States, foods such as dairy products, cereal and orange juice are often fortified with vitamin D. (Countries that don’t fortify foods have higher rates of vitamin D deficiency.) Checking nutrition labels can help you make better decisions about which foods to purchase. Wild mushrooms and fatty fish, such as salmon, sardines and tuna, are other sources.

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Also, going out for a 15-minute walk a few times a week at midday is usually enough sunlight for the skin to synthesize vitamin D. This can even be incidental sun exposure, such as while running errands. An even better idea for your health would be to exercise outdoors, such as playing sports or going for a jog. Wearing sunscreen does less sunlight absorption but is crucial for preventing skin cancer and premature skin aging, if sun exposure is extended.

Although it’s much easier to pop a pill than to be physically active outdoors and eat healthy, the latter two will do more to keep you healthy and lower your risk of cardiovascular disease, cancer and diabetes. Taking a supplement will never be a substitute for a healthy diet and lifestyle.

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