Digging Into Vitamin D: All About the “Sunshine” Vitamin

“We actually need only small to moderate amounts of vitamin D, not mega-doses,” says Dr. JoAnn Manson, a vitamin D expert at Brigham and Women’s Hospital and Harvard University.

Current guidelines recommend adults get 600 to 800 IUs (international units) of vitamin D each day. Those amounts are very important. Not getting enough can lead to serious health issues. Children’s bones can’t develop properly without enough vitamin D. In adults, a long-term deficiency can lead to fragile bones or osteoporosis.

It’s important to make sure you get enough vitamin D. But scientists are finding that more isn’t always better.

Where to Get It

You can get vitamin D from the sun and from your diet. Your body makes vitamin D when your skin is exposed to the sun, which gives off UVB light.

But many people don’t go outside enough to get all the vitamin D they need this way. Other factors, such as clothing and sunscreen, can block how much vitamin D your skin makes when you’re in the sun.

How much melanin you have also plays a role. Melanin is a pigment that gives your skin color. Higher melanin levels cause darker skin complexions. The more melanin you have, the less vitamin D you can make from sunlight. This may put you at potential risk for vitamin D deficiency.

Sunlight exposure isn’t the only way to get vitamin D. Vitamin D is found naturally in some foods, like fatty fish. It’s also added to many dairy products and other fortified foods.

With so many potential sources, most people in the U.S. aren’t at risk for vitamin D deficiency, but that does not mean that everyone has sufficient levels. Additionally, getting enough vitamin D from foods can be difficult for some. These can include breastfed infants and people with certain gut problems that limit how nutrients are absorbed.

Older adults can be at risk of vitamin D deficiency, too. “As we age, our ability to make vitamin D in the skin declines,” says Dr. Sarah Booth, a nutrition researcher at Tufts University. Older adults may also be less likely to get outdoors.

Experts don’t recommend screening healthy people for vitamin D. But if you’re in a high-risk group, talk with a healthcare professional. Vitamin D levels can be measured with a blood test. Vitamin D supplements are sometimes recommended for very low levels.

Is More Better?

Although most people get enough vitamin D to avoid deficiencies, researchers have long wondered if adding extra vitamin D could be good for overall health. Many studies have linked higher levels of vitamin D in the blood with healthier outcomes.

Manson and her team conducted a large clinical trial, called VITAL, to see whether vitamin D supplements could lower the risk for some health problems. They compared health outcomes for over 25,000 people in the U.S. Participants were randomly assigned to two groups: half were given vitamin D supplements and half were given a placebo (an inactive pill that looked similar).

After five years, both groups had the same risk for most of the health problems studied. These problems included heart disease, cancer, depression, and bone fractures.

“So, the vast majority of healthy people did not benefit from vitamin D supplements,” Manson says. “But we didn’t find any risk from the 2000 IUs per day that we tested.”

Other studies have also shown that taking moderate doses daily is safe over the long term. “However, at higher doses you’re going to have to start to worry about risks,” Manson warns. The upper daily limit for vitamin D is 4,000 IUs daily. Consuming more can lead to side effects like kidney stones, nausea, vomiting, and muscle weakness.

Too much vitamin D is almost always a result of taking too many supplements. But for people who can’t get enough vitamin D from the sun or their diet, vitamin D dietary supplements in moderation can help prevent a deficiency.

Finding New Uses

Scientists continue to study how vitamin D can help people since it plays a role in many of the body’s functions.

Manson’s team is following up on their findings in VITAL that suggest taking vitamin D supplements may lower the risk of developing autoimmune disease or advanced cancer. They’re also testing whether vitamin D can reduce the risk of COVID-19 infection, severe COVID symptoms, and Long COVID.

Booth and her team recently found that older adults with more vitamin D in their brains had a lower risk of dementia. But the study couldn’t tell whether vitamin D caused the lower risk. Her team is now doing more research to better understand how vitamin D affects brain health.

She thinks the answer is likely to be complicated. “Vitamin D is important,” Booth says. “But there’s no evidence that a single nutrient will slow cognitive decline or prevent Alzheimer’s disease.”

Another research team, led by Dr. Sushil Jain at Louisiana State University, is investigating the connection between diabetes, vitamin D, and a molecule called glutathione. Glutathione helps the body use vitamin D efficiently.

Black Americans have a relatively high risk of both vitamin D deficiency and diabetes. They’re also more likely to have low levels of glutathione. Jain’s team is testing if boosting both glutathione and vitamin D levels can help prevent diabetes in Black study participants.

For now, most people concerned about vitamin D would get the greatest benefit from living a healthy lifestyle, Manson explains. This includes getting outside, being physically active, not smoking, and eating a variety of healthy foods rich in vitamin D.

“A dietary supplement will never be a substitute for a healthy diet or a healthy lifestyle,” she says.

Get More Vitamin D:

Vitamin D can be found in many foods and beverages:

• Fatty fish. Trout, salmon, tuna, mackerel, sardine, and fish liver oils naturally contain high amounts of vitamin D.
• Dairy milk. Almost all cow’s milk in the U.S. is fortified with vitamin D. But be sure to check the label.
• Some plant-based milk. Some brands of soy, almond, oat, or other milk alternatives are fortified with vitamin D. See the labels for how much they include.
• Many brands of breakfast cereals, orange juice, yogurt, and other foods also contain added vitamin D.
• Egg yolks, cheese, and mushrooms. These foods naturally contain a small amount of vitamin D.