A lot of our days are spent indoors, whether at school, work, or home. Especially in the winter, it can be hard to find the motivation to go outside. But exposing your skin to the sun’s rays helps your body produce much-needed vitamin D. Vitamin D is often referred to as “the sunshine vitamin” because the UV rays from the sun activate vitamin D synthesis in the body, as noted by the National Institutes of Health (NIH).
Vitamin D has long been lauded for maintaining strong bones, but this essential vitamin also shines in your gut, helping to strengthen gut cells and the immune system.
Chris Damman, MD, clinical associate professor of gastroenterology at the University of Washington in Seattle, says to think of the gut like pipes made of cells that are held together with junctions. Vitamin D helps strengthen the junctions, or connections between cells, so the pipes don’t leak.
“There are vitamin D receptors in the lining of the gut that communicate with the tight junctions that stitch the gut cells together,” says Dr. Damman.
Gut health and your immune system are extremely interlinked, and vitamin D is one of the many factors that helps keep the microbiome healthy. According to UCLA Health, 70 percent of the immune system is found in the gut.
A cross-sectional study published in 2017 in Metabolism looked at 150 healthy adults and stratified them based on their vitamin D intake. The analysis showed that higher vitamin D levels correlated with an abundance of certain gut microbes that are associated with gut health, indicating a potential relationship between the two.
Margherita T. Cantorna, PhD, professor of molecular immunology and nutrition at Pennsylvania State University in Univesity Park, says vitamin D also helps your body regulate itself following infection. When your immune system is working properly, your immune response turns on to fight off infection.
“After the infection goes away, it's really important to be able to get rid of that immune response so you don't have inflammation all the time,” says Dr. Cantorna. “Vitamin D helps control that. It doesn't necessarily help you get rid of the infection, but once the infection is gone, it helps you get back to normal.”
Where to Get Vitamin D
“The best source of vitamin D is from sunlight — at least 30 minutes outdoors,” says Damman. “The next best source of vitamin D is whole foods, especially fatty fish like salmon.”
Oily fish, like trout, naturally contain high levels of vitamin D.
Mushrooms contain a variation of vitamin D, and some brands increase the amount of vitamin D in their mushrooms by exposing them to UV lights.
Milk and nondairy milk in the United States are enriched with vitamin D.
Other foods are fortified with vitamin D, too, like some yogurts, orange juice brands, and breakfast cereals.
Some People Need Help Getting Enough Vitamin D
Most people need 600 international units of vitamin D daily, with the exception of infants, who need less, and adults 71 and older, who need more. It’s hard to have too much vitamin D; as long as you stay below 4,000 international units of vitamin D — more than 6 times the recommended daily amount, as noted by Mayo Clinic — you should be safe.
The Nutrition Source at Harvard identifies some groups of people as having more trouble getting and absorbing enough vitamin D, such as:
- Older adults, because your body’s ability to turn sunlight into vitamin D gets worse
- People with darker skin tones, because deeper skin absorbs less vitamin D from the sun
- People who wear clothes that cover most of their skin, because less direct sun exposure on skin means less vitamin D synthesis
- People with Crohn’s disease, ulcerative colitis, or celiac disease, because these conditions alter how your body absorbs fat, which in turn makes it more difficult to absorb vitamin D
- People who have had gastric bypass surgery, because you may need more vitamin D than average
- People who live far north (for example, Boston) or far south (for example, Buenos Aires) or in cloudy climates, because it’s difficult to get enough vitamin D from the sun during winter.
- People who spend a lot of time inside, because if you’re not out in the sun, how could you absorb that light?
If you fall in any of these groups, talk to your doctor about starting a vitamin D supplement. However, Damman says most doctors don’t object to patients taking vitamin D. “It's one of those supplements that is a little bit more equal opportunity,” says Damman. But checking with your doctor is best; taking too much vitamin D could badly interact with other medications or cause your body to absorb too much calcium, leading to conditions like kidney stones. If you take a vitamin or supplement containing vitamin D, do so with a meal or snack that has some fat in it for best absorption.
