High-dose supplements are no more helpful for the heart than moderate doses for most adults, but researchers say there's more to know about the value of this essential vitamin

It's a key nutrient that strengthens bones, supports immune function, helps the heart and provides energy to the body in many other ways. But the results of recent studies on vitamin D are clear: For most adults, high doses do not improve heart and circulatory health any more than moderate doses.

Only small to moderate amounts of vitamin D are needed for optimal cardiovascular function, and not more is better. Adults who took moderate or high doses (at least 1,000 IU) of vitamin D supplements daily did not have a reduced risk of heart disease, stroke or cardiovascular-related death compared to adults who took a placebo without vitamin D.

Vitamin D supplements are not the panacea for a range of health problems that many people believe they are. For example, VITAL and other randomized trials have found that higher intakes do not prevent cancer, fractures or falls, nor do they relieve conditions such as knee pain, cognitive decline or atrial fibrillation.

That's why researchers are beginning to turn their attention to other, more subtle issues with vitamins. For example, why do some people benefit more from vitamins than others? Are supplements beneficial for specific populations (such as those at increased risk for heart disease)? Given that VITAL has shown that high-dose vitamin D supplements reduce the risk of autoimmune disease, do they also help reduce the severity of COVID-19?

Vitamin D: Get enough, but not too much

While researchers have sorted through the issues, guidelines that have been in use for years still apply.

The National Academy of Medicine recommends 600 IU of vitamin D per day (primarily from food) for people ages 1-70 and 800 IU per day for adults ages 71 and older. however, Manson notes that adults concerned about inadequate vitamin D intake during an outbreak should supplement with 1, 000 ~ 2,000 IU is reasonable. She cautions against taking more vitamin D. Intake above 4,000 IU per day, the upper daily limit, is considered a high dose and may lead to adverse effects, including high calcium levels in the blood or kidney stones.

So, what's the best way to get vitamin D?

Occasional sun exposure, such as being outdoors, or eating foods rich in vitamin D, including fatty fish, fortified dairy products and cereals, and certain mushrooms, such as those exposed to ultraviolet light, are all good ways to get vitamin D. Reading nutrition labels can also help people assess how much vitamin D they are consuming through food.

Supplements can be helpful for people who are concerned about getting too little.  But if you want to protect your heart by consuming vitamin D, it's important to be careful that your intake is moderate and precise.

Understanding vitamin D and heart health

The idea that higher vitamin D intake could improve heart health emerged years ago when observational studies found that people with higher blood levels of vitamin D were less likely to develop cardiovascular disease.

To examine whether vitamin D drove this effect or was simply a marker of risk, researchers conducted randomized controlled trials including VITAL, in which more than 25,000 adults participated from 2011 to 2013, which found that high-dose vitamin D supplementation did not prevent cardiovascular events. And as VITAL's study director, Manson conducted a meta-analysis on the topic. After reviewing 21 randomized trials related to vitamin D and cardiovascular disease, she found that "none of the trials showed a significant benefit of vitamin D supplementation in preventing heart disease or stroke."

"In observational studies, correlation does not prove causation," she explained, emphasizing the need for randomized controlled trials.

Manson said multiple factors could explain why adults with higher vitamin D levels were less likely to develop cardiovascular disease in the observational study. It's a workout. Those who spend more time outdoors doing physical activities that benefit heart and blood vessel health may have higher vitamin D levels due to occasional sun exposure. Diet is another. Fish and other nutrient-rich foods contribute to heart health and tend to have higher levels of vitamin D.

Inflammation is a third reason, she says. Inflammation levels can act as a signal of disease. Because vitamin D can bind to a protein that is more likely to be depleted by inflammation, lower vitamin D levels may be a marker for chronic diseases such as heart disease rather than a causal factor. Once adults have adequate levels of vitamin D, the benefits stabilize. Higher vitamin D intake or blood levels do not further reduce the risk of cardiovascular disease.