The sun can indeed shine out of your backside – skin UV exposure reflects in your poop

When we think of sun damage, we think of the visible effects on your skin – sunburn, redness, pigmentation and early signs of ageing. But new research suggests that UV rays can even affect the bacteria in your gut – with positive results.

An analysis published in the journal Frontiers in Microbiology has shown that vitamin D mediates changes that UVB light causes in the gut microbiome. The research could help explain the protective effect of UVB light in inflammatory diseases such multiple sclerosis (MS) and IBD.

How is vitamin D linked to our guts?

According to the news report, our exposure to sunlight, our vitamin D levels and the combination of bacteria in our guts are directly linked to risks of inflammatory diseases such as MS and IBD.

The research states that exposure to UVB in sunlight increases our levels of vitamin D, and recent studies suggest that vitamin D alters the human gut microbiome. It’s important to note that the effect of UV rays on the gut microbiome has only been shown in rats before.

Now a new clinical pilot study recruited healthy female volunteers to test the effect of UVB exposure on the gut microbiome. The volunteers were exposed to full-body UVB light for three one-minute sessions during the course of a week. Stool samples were tested for gut bacteria and their blood was tested for vitamin D levels.

What were the findings?

The gut microbial diversity was significantly increased in the participants after UVB exposure, but only in those who hadn’t taken a vitamin D supplement during the winter.

According to Prof Bruce Vallance who led the study, the women had less diverse and balanced gut microbiome prior to UVB exposure than those who regularly took a vitamin D supplement.

"UVB exposure boosted the richness and evenness of their microbiome to levels indistinguishable from the supplemented group, whose microbiome was not significantly changed," Prof Vallance said.

What does this study mean for our health?

"In this study we show exciting new data that UVB light is able to modulate the composition of the gut microbiome in humans, putatively through the synthesis of vitamin D," stated Prof Vallance in the news release.

"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," suggests Vallance.

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

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