Sunlight Exposure Can Affect Your Gut Bacteria

For the first time, scientists have directly linked sun exposure to changes in the gut microbiome. (Photo: Kanchana P/Shutterstock)

People living at higher latitudes may be safer from the dangers of too much sunlight, but they also face a greater risk of having too little vitamin D in their bodies, a deficiency that affects as many as 1 billion people around the world.

A vitamin D deficiency alone can lead to a variety of health problems. Along with other environmental factors like diet and antibiotic use, it's also seen as one reason why many developed countries have such high rates of chronic inflammatory diseases, including multiple sclerosis and inflammatory bowel disease (IBD).

Research suggests our gut microbiomes — the bacteria and other tiny creatures living in our digestive tracts — might wield some influence over these conditions. And now, in a study published in the journal Frontiers in Microbiology, researchers reveal a potentially important way to boost our gut bacteria with sunlight.

A 'novel skin-gut axis'

Even briefly shining ultraviolet B (UVB) light onto a person's skin can lead to a wider diversity of gut bacteria, the researchers found, as well as higher levels of vitamin D. This was a small study, and the effect was only seen in subjects with lower vitamin D levels. Still, this is the first study to find a direct effect from UVB light on the gut microbiome, the researchers write, suggesting "the existence of a novel skin-gut axis that could be used to promote intestinal homeostasis and health."

There are two basic types of ultraviolet light that reach Earth's surface, known as UVA and UVB. Both can be harmful, but UVB rays are the main rays responsible for sunburns and are thought to cause the most skin cancers, according to the American Cancer Society. At the same time, UVB rays from sunlight trigger our bodies to produce vitamin D. Low UVB exposure has been linked to the onset of inflammatory gut diseases, and since vitamin D can influence our gut bacteria, the researchers hoped to shed new light on the relationship among all three factors.

sun rays through trees and mist
Levels of UV light from the sun tend to be naturally lower at higher latitudes, raising the risk of health problems from a vitamin D deficiency. (Photo: yanikap/Shutterstock)

Led by Else Bosman, a doctoral candidate in experimental medicine at the University of British Columbia, the researchers recruited 21 healthy women to participate in the study. All of the volunteers had fair skin and lived in Canada during winter for the three previous months, and thus were exposed to relatively low levels of UVB light. Before the study began, the researchers tested their vitamin D levels and took stool samples, which revealed the makeup of their gut microbiome.

Nine of the women had been taking vitamin D supplements for three months prior to the study, while 12 hadn't. Of those 12, most had insufficient levels of vitamin D. The researchers gave the volunteers three sessions of UVB light exposure in a week, then tested their vitamin D and stool again. Not only did the women with low vitamin D now have normal levels, but their gut microbiomes had also grown more diverse.

"Healthy people tend to have more diverse microbiomes," Bosman tells NBC News, adding that this discovery could be especially important for people with IBD, who often have difficulty absorbing nutrients through their digestive systems.

More research needed

This study was small, and its findings are still "extremely preliminary," as Leslie Nemo writes for Discover Magazine. More research will be needed before we can draw firm conclusions or take action based on these findings. It's especially important to be careful with sunlight exposure given the potential risks, and the researchers note they didn't use actual sunlight in this study, just an isolated component of it.

"During the study, we made use of specialized UVB lamps that don't cause burning. It was a therapeutically used photobooth in a clinical setting," Bosman tells Medical News Today. "From my study, it is hard to conclude how much sun exposure is enough to produce vitamin D." That largely depends on individual factors, like your skin type and the UV index where you live.

Still, this study fills in some blanks and offers intriguing hints in our quest to understand the human microbiome, showing a shift in gut bacteria that was apparently driven mainly by the body's production of vitamin D.

"It is well known that UVB light produces vitamin D, and we now start to understand that vitamin D is important to maintain a healthy gut," Bosman says. While those facts were known individually, she adds, this is the first study linking them all together. This study should now be repeated with more volunteers, the researchers say, including both sexes and a wider range of skin types.