Solar radiation on skin found to have direct influence on our poo

24 Oct 2019

Image: © Sunny Whale/

Researchers have found a direct link between our skin’s exposure to UVB light and the altering of our gut microbiome.

It’s well known that exposure to sunlight will boost levels of vitamin D, but a new discovery made by a team of researchers from the University of British Columbia has revealed an even deeper importance to the body.

In the first study of its kind, the team showed that exposing skin to UVB radiation through sunlight alters the gut microbiome in humans as seen in human stool samples. The findings, published to Frontiers in Microbiology, suggest that vitamin D is behind this microbiome change and would help explain UVB’s protective effects in inflammatory diseases such as multiple sclerosis (MS) and inflammatory bowel disease (IBD).

Until now, tests that attempted to link gut microbiome changes through vitamin D production in the body have only been proven in mice. However, this new pilot study tested healthy volunteers who were exposed to three one-minute sessions of UVB in one week.

Before and after the trial, stool and blood samples were collected and tested for vitamin D levels. The researchers said that skin UVB exposure significantly increased gut microbial diversity, but with one noticeable caveat.

Altering the immune system

“Prior to UVB exposure, these women had a less diverse and balanced gut microbiome than those taking regular vitamin D supplements,” said Prof Bruce Vallance, who led the study.

“UVB exposure boosted the richness and evenness of their microbiome to levels indistinguishable from the supplemented group, whose microbiome was not significantly changed.”

Those with greater UVB exposure were shown to have an abundance of Lachnospiraceae bacteria, which proved previous work that showed a correlation between it and vitamin D levels. The study also appeared to confirm results seen in the mouse studies with an increase in Firmicutes and decrease in Bacteroidetes in the gut after exposure.

“It is likely that exposure to UVB light somehow alters the immune system in the skin initially, then more systemically, which in turn affects how favourable the intestinal environment is for the different bacteria,” Vallance said.

“The results of this study have implications for people who are undergoing UVB phototherapy, and identifies a novel skin-gut axis that may contribute to the protective role of UVB light exposure in inflammatory diseases like MS and IBD.”

Colm Gorey was a senior journalist with Silicon Republic