Researchers in Canada have shown how exposure to sunlight/UVB light can change the human gut microbiome, particularly in people who are vitamin D-deficient. Findings from the study in healthy human volunteers could help to explain the protective effect of UVB against inflammatory diseases such as multiple sclerosis (MS) or inflammatory bowel disease (IBD). The results, while not exactly demonstrating that the sun can shine out of your backside, are the first to show that skin exposure to UVB light alters the gut microbiome in humans, and so affects what comes out in our poop. They indicate that vitamin D mediates UVB-modulated changes in gut microbiota, uncovering the existence of a skin-gut axis that could provide new insights into approaches to managing and improving intestinal health.

The research was headed by Bruce Vallance, PhD, investigator at the BC Children’s Hospital Research Institute, University of British Columbia. Vallance and colleagues describe their work in Frontiers in Microbiology, in a paper titled, “Skin Exposure to Narrow Band Ultraviolet (UVB) Light Modulates the Human Intestinal Microbiome.”

The recent worldwide rise in idiopathic immune and inflammatory diseases such as MS and IBD has been linked to Western society-based changes in lifestyle and environment, the authors explained. “These include decreased exposure to sunlight/UVB light and subsequent impairment in the production of vitamin D, as well as dysbiotic changes in the makeup of the gut microbiome.” Exposure to UVB in sunlight is known to drive vitamin D production in the skin, and recent studies have also suggested that vitamin D alters the human gut microbiome. In humans, 80% of vitamin D requirements have to be met through exposure to UVB light from sunlight, and there are high rates of vitamin D insufficiency or deficiency worldwide, especially in locations with long winter seasons, where the lack of UVB from sunlight during the winter means that people don’t produce enough of the vitamin. And as the authors pointed out, “Limited UVB exposure is one of the most important environmental factors linked to the onset of immune mediated chronic inflammatory diseases, like IBD and MS.”

Vitamin D is also known to promote intestinal health, the team continued. “Vitamin D deficiency has been shown to promote an inflammatory environment which leads to dysbiosis of the gut microbiota, even in clinically healthy individuals. Oral vitamin D supplementation is known to be beneficial for individuals who suffer from chronic inflammatory diseases.” However, the researchers continued, at present it’s still unclear if there is a direct association between UVB light and human intestinal microbiota. To date the link between UVB exposure and changes to gut microbiome changes, via vitamin D production, has only been shown only in rodents. “…the potential for UVB light to affect the intestinal mucosal immune system and the gut microbiota has received little attention. Moreover, it is unclear whether such changes would be dependent or independent of the effects of UVB light on systemic levels of vitamin D.”

To investigate this link in humans, the researchers carried out a trial, involving 21 healthy female human volunteers, to see whether increasing vitamin D levels by exposing the skin to UVB light would also change the makeup of intestinal microbiota. The volunteers were each given three one-minute sessions of full-body UVB exposure over the course of a week. At set timepoints before and after treatment, stool samples were taken for analysis of gut bacteria, and blood samples were taken to measure vitamin D levels. Nine of the participants reported that they had been taking vitamin D supplements during the three months before the start of the trial (VDS+), but the other participants had not (VDS-). Tests showed that most of these VDS+ group participants were vitamin D-sufficient at the start of the trial, whereas most of the VDS- group were vitamin D-insufficient before the trial started.

The results of the study confirmed that skin UVB exposure significantly increased gut microbial diversity, but only in the 12 subjects who were not taking vitamin D supplementation. “… participants in the VDS- group started the study with a significantly lower microbial alpha diversity as compared to the VDS+ group, with UVB exposures increasing their diversity to the same level as the VDS+ group,” the investigators stated. “A diverse microbiome is thought to be more resilient against stressors and is seen as a hallmark of health.”

“Prior to UVB exposure, these women had a less diverse and balanced gut microbiome than those taking regular vitamin D supplements,” stated Vallance. “UVB exposure boosted the richness and evenness of their microbiome to levels indistinguishable from the supplemented group, whose microbiome was not significantly changed.”

Interestingly, the types of bacteria that were differentially abundant in the VSD- group were commensal bacteria that are generally associated with a healthy microbiome, the investigators noted. The largest effect in the VSD- group seen as a result of UVB exposure was an increase in the relative abundance of at least one “healthy” family of bacteria that are less abundant in people with inflammatory diseases. “Several of the enriched genera are within the bacterial family Lachnospiraceae, and have been previously reported to be associated with an improved health status as compared to the microbiome of those individuals suffering from diverse immune-mediated inflammatory diseases.”

“Previous studies have linked Lachnospiraceae abundance to host vitamin D status,” added Vallance. “We too found a correlation with blood vitamin D levels, which increased following UVB exposure.” This indicated that vitamin D at least partly mediates UVB-induced gut microbiome changes. “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,” Vallance concluded. The overall results are also in some agreement with prior mouse UVB exposure studies, such as an increase in Firmicutes and decrease in Bacteroidetes in the gut following exposure.

“To our knowledge, this is the first study that reports changes in the human gut microbiota in response to UVB light,” the authors commented. “Our observations support findings that humans display seasonal fluctuations in their microbiome composition, potentially coinciding with fluctuations in serum vitamin D levels throughout the year … While seasonal variation of the microbiome might not have overt effects on healthy individuals, it could be of greater importance for people with immune dysfunction. Several chronic inflammatory diseases display seasonal patterns in the severity of disease. Specifically, the relapsing and remitting nature of IBD and MS are strongly associated with vitamin D levels.”

The authors acknowledge there are still questions to be answered. Their study wasn’t designed to identify the exact mechanism by which microbiome changes occur, although both UVB and vitamin D are known to influence the immune system. “… innate and adaptive immune cells are influenced by vitamin D, with the vitamin suppressing pro-inflammatory responses,” the scientists wrote.

“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 favorable the intestinal environment is for the different bacteria,” suggested Vallance. “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.”

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