Throughout the pandemic, many unproven miracle COVID-19 “cures” emerged, and vitamin D claims have been one of the most persistent. This is not new for the vitamin. It’s been touted in recent decades as a way to “boost” the immune system, improve overall health, prevent a host of diseases, and allegedly even substitute for vaccines.
But as with many internet-popular health “remedies,” the reality is far less flashy and far more nuanced.
What is vitamin D, and why is it important?
Vitamin D is a nutrient that helps the body absorb calcium, which is essential for bone health. In the sunlight, your skin naturally produces vitamin D that is then stored in fat cells until it is used.
The skin pigment melanin absorbs the UV rays necessary for vitamin D production, meaning that more highly pigmented or darker skin produces less vitamin D than lighter skin with the same amount of sun exposure. Thus, people with darker skin are at higher risk of vitamin D deficiency.
Most of our vitamin D comes from the sun. An additional 10 percent to 20 percent of our vitamin D comes from foods like fatty fish (such as salmon), eggs, and mushrooms. Vitamin D supplements are another source of the nutrient for people who are unable to get enough from sun exposure and diet.
Vitamin D deficiency is real, but there’s no epidemic
Some people who promote vitamin D supplements claim that vitamin D deficiency is an epidemic causing widespread health issues. There is little evidence to support this claim. A 2022 analysis of 2001-2018 data found that 2.6 percent of people in the U.S. had severe vitamin D deficiency.
Severe vitamin D deficiency can cause serious health issues, such as muscle weakness, bone loss in adults, and rickets (weak bones) in children. Some people are at higher risk for the deficiency, including individuals with certain disorders that prevent the body from absorbing or processing vitamin D or those with a family history of vitamin D deficiency.
Black Americans have the highest rates of severe vitamin D deficiency at nearly 12 percent. Severe vitamin D deficiency is also slightly higher in the U.S. during the winter when people get less sun exposure. Rates of moderate vitamin D deficiency are higher at 22 percent overall and are highest among Black Americans (49 percent) and Mexican Americans (35 percent).
Although severe vitamin D deficiency exists in the U.S., it is far from common. Most tellingly, conditions that are directly linked to vitamin D deficiency are not widespread. There is no epidemic of rickets, for example, or bone loss in adults.
There’s little evidence that vitamin D supplements improve overall health
Vitamin D supplements have clear, proven positive effects for people with vitamin D deficiency. Other health benefits of vitamin D supplements are less certain.
There is some evidence that the supplement may reduce the risk of fracture in adults with osteoporosis, a condition that causes weak, fragile bones. However, the benefit appears to be limited to people who have low vitamin D levels. In adults with normal vitamin D levels, supplements have no effect on fracture risk.
The largest randomized controlled trial of vitamin D, called VITAL, investigated the effects of vitamin D supplementation in people without an existing deficiency. The study found that vitamin D supplements had no effects on the risk of cancer, diabetes, or cardiovascular disease, including heart attack and stroke. The study concluded that more research is necessary to determine who may benefit from vitamin D supplements.
Independent analyses found that vitamin D supplementation may be associated with a long-term decrease in cancer mortality, but results are mixed and also require more investigation.
A 2021 analysis of past vitamin D trials found no overall health benefits from vitamin D supplements in people with normal vitamin D levels. Most large-scale studies have found no link between vitamin D supplements and lower all-cause mortality (deaths from any cause), except in older adults and those with vitamin D deficiency.
Vitamin D provides modest protection against respiratory infections
Vitamin D is important for immune function, but this is often misconstrued as vitamin D “boosting” the immune system.
Some people falsely believe that taking vitamin D supplements will keep them healthy and prevent infections like the flu or COVID-19. In reality, clinical trials and large-scale studies of vitamin D have found only minimal protective effects against respiratory infections.
A 2021 analysis of 46 trials found that 61.3 percent of participants who took daily vitamin D supplements got respiratory infections during the study periods—compared to 62.3 percent of people who did not take the supplements. A 2024 meta-analysis of 43 trials found no overall protective effect against respiratory infections, but it detected a slight decrease in risk among people who took specific doses daily.
In young children, there is some evidence that vitamin D supplementation may reduce the length of respiratory infections. However, it does not affect the number or severity of infections that children have.
Despite claims that taking vitamin D can protect against COVID-19, two clinical trials found that taking daily vitamin D supplements did not reduce the risk or severity of COVID-19 infections, even at high doses.
Context is key when considering vitamin D’s benefits
None of these studies contradict the well-established evidence that people with vitamin D deficiency benefit from vitamin D supplements. But it’s important to remember that many of the most popular health claims about vitamin D’s benefits are based on research in people with vitamin D deficiency.
Research in vitamin D-deficient populations is important, but it tells us little about how vitamin D will affect people with normal or close to normal vitamin D levels. A closer look at vitamin D research in people without low levels reveals little evidence to support the idea that the general population benefits from taking vitamin D supplements.
For more information, or to learn about your vitamin D levels, talk to your health care provider.