When most people think vitamin D they think of bone health, but this vitamin does a lot more and it may even help the body fight off upper respiratory infections, like Covid-19. 

Why vitamin D helps build strong bones

Vitamin D is a fat-soluble vitamin, meaning it's stored in fat tissue in the body. One of its main functions is that it enables the digestive tract to absorb calcium and phosphorus from food, which helps keep bones strong. 

In fact, without vitamin D, experts estimate that you'd only absorb about 10-15% of calcium and 60% of phosphorus from the foods you eat. Without enough calcium, your body produces what are called osteoclasts, which are cells that literally dissolve the minerals in your bones, making them weak and brittle.

This helps explain why people who are vitamin D deficient are at greater risk for osteoporosis, a condition that leads to weak bones.

How vitamin D and depression are linked

Vitamin D deficiency is associated with an increased risk of depression. Moreover, people with seasonal affective disorder may not produce enough vitamin D, which could explain their symptoms. 

However, these associations and links do not mean that enough vitamin D can help treat depression. More research is needed to understand if vitamin D deficiency is the cause or if there's another reason for the association between it and depression.

How vitamin D may reduce cancer deaths

In the last 20 years, observational studies have linked a number of diseases to low vitamin D levels, like type 2 diabetes, cardiovascular disease, and cancer. But so far clinical trials have not revealed significant relationships between low levels of the vitamin and these conditions. 

The most convincing evidence for vitamin D benefits beyond bone health came from a massive studypublished in 2019 that examined the impact of vitamin D supplementation on cancer and cardiovascular disease through a randomized trial of over 25,000 people. It's the largest randomized clinical trial on this topic to date. 

JoAnn Manson, the study's lead investigator who is a professor at Harvard and chief of preventive medicine at Brigham and Women's Hospital, says that she and her colleagues found no significant reduction in the incidence of cancer or cardiovascular disease. 

However, they "did see vitamin D supplementation was associated with a signal for a reduction in cancer deaths," Manson says. Meaning that people with cancer who took vitamin D supplements for two years, or more, had a 25% reduced risk of dying from cancer than a control group who took a placebo, instead.

How vitamin D and Covid-19 are linked

"Avoiding vitamin D deficiency is important for countless reasons, and now it is even more important for its potential role in improving immune function during this Covid pandemic," says Manson. 

Preliminary observational research recently found a link between vitamin D deficiency and Covid-19. Researchers compared severe Covid-19 cases in the US, UK, Germany, Italy, and other heavy-hit countries against vitamin D deficiency. 

They report that "the risk of severe Covid-19 cases among patients with severe Vit D deficiency is 17.3% while the equivalent figure for patients with normal Vit D levels is 14.6%."

The paper has not been published yet. However, it suggests that, "Vitamin D may actually have a favorable effect on the immune system to fend off the virus and also to tamp down the inflammatory response to it," Manson says, which could protect against severe infection. 

This isn't the first study to find a link between vitamin D and the immune system. A meta-analysis — published in 2017 — of 10,933 participants in 25 randomized trials found that vitamin D supplementation was associated with a lower risk of acute respiratory infection. The protective benefits were greatest when patients took vitamin D supplements daily or weekly, instead of in one large dose. And that those who were vitamin D deficient benefited most.

How to get enough vitamin D 

People get some of their vitamin D from sunlight because the body synthesizes the vitamin when UV light hits the skin. Manson says that while UV exposure is important to get vitamin D, she discourages people from going in the sun for a long period with unprotected skin because it can lead to sunburn and an increased risk of skin cancer. 

There are not many foods that are naturally rich in vitamin D. Fatty fish like salmon and tuna are some of the best naturally occurring sources of vitamin D. One serving of cod liver oil provides 170% of the daily value of the vitamin. In the US, cow milk, plant milk, cereals, and some other foods are typically fortified with vitamin D. It has been added to milk since the 1930s to prevent rickets, a bone disease in children caused by vitamin D deficiency.

Studies have shown older adults are more at risk of vitamin D deficiency because the ability to synthesize vitamin D decreases with age. It also doesn't help that the elderly also spend more time indoors. 

Most people do not have a true vitamin D deficiency, but could still have less than the optimal amount in their blood. If your sun exposure is limited and you are worried you aren't getting enough vitamin D, you can consider a supplement to be sure you are getting the recommended daily allowance.

But be sure to address your symptoms with your doctor and be careful not to go above 4,000 IUs, which is considered the upper limit, without speaking to a doctor. Too much vitamin D can trigger nausea, vomiting, and frequent urination.

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