Skin-gut axis: Sun exposure influences gut bacteria by boosting vitamin D production

24 Oct 2019 --- Skin exposure to sunlight – specifically type B ultraviolet light (UVB) – can alter the gut microbiome in humans, according to a new study from the University of British Columbia (UBC), in Canada. Published in Frontiers in Microbiology, the research suggests that vitamin D, which the body naturally produces when exposed to sunlight, mediates a change in the microbiome. This could help explain the protective effect of UVB light in inflammatory diseases like multiple sclerosis (MS) and inflammatory bowel disease (IBD). The researchers note that the findings could point to novel ways to treat inflammatory gut diseases via phototherapy and vitamin D supplementation, but more research is warranted.

“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,” says Professor Bruce Vallance, who led the UBC study.

The fact that UVB light causes gut microbiome changes via vitamin D production has so far been shown only in rodent studies.

“We found that vitamin D production was the main driver of the shift in the microbiome. It is well known that UVB light produces vitamin D and we now are starting to understand that vitamin D is important to maintaining a healthy gut. Although those facts were known separately, this is the first study that links them in humans. The results were important as there was a strong efLeft to right: Dr. Jan Dutz, Else Bosman, Dr. Bruce Vallance from the Universiry of British Columbia.fect visible within the span of a week,” Else Bosman, a PhD student at UBC tells NutritionInsight. 

Bosman also says that a gut-skin axis has been previously reported by research. The gut and the skin are relatively similar in structure and function of protecting the body from the outside environment. These studies describe how breaching the skin can provoke an immune reaction in the intestine. In this case, there could be vitamin D dependent and independent factors that are made by the skin, which can communicate to the gut. “The concept of a skin-gut axis is new and much more research has to be done to understand how skin and gut communicate with each other,” she adds. 

UVB exposure increases microbial diversity
In the clinical pilot study, the researchers tested the effect of skin UVB exposure on the human gut microbiome. Twenty-one, healthy, female volunteers were given three one-minute sessions of full-body UVB exposure in a single week. Before and after treatment, stool samples were taken for analysis of gut bacteria – as well as blood samples for vitamin D levels. 

Skin UVB exposure significantly increased gut microbial diversity, but only in 12 of the subjects, who were not taking vitamin D supplements during the winter study. 

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

The largest effect was an increase in the relative abundance of Lachnospiraceae bacteria after the UVB light exposures. “Previous studies have linked Lachnospiraceae abundance to host vitamin D status,” adds Vallance. “We too found a correlation with blood vitamin D levels, which increased following UVB exposure.”

This indicates that vitamin D at least partly mediates UVB-induced gut microbiome changes, the researchers note. The results also showed some agreement with mouse studies using UVB light, such as an increase in Firmicutes –the phylum of bacteria – and a decrease in Bacteroidetes in the gut following exposure.

The study is not designed to show the exact mechanism by which the microbiome changes occur, but both UVB and vitamin D are known to influence the immune system. “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,” notes Vallance.

The study identifies a novel skin-gut axis linking UVB light exposure to the treatment of chronic inflammatory diseases.“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 chronic inflammatory diseases like MS and IBD,” he says.

Bosman says that the researchers believe that vitamin D is creating a favorable environment in the intestine for a healthy and diverse microbiome, rather than altering the microbes directly. As the study showed, people who are taking vitamin D supplements were able to maintain the healthy microbiome composition throughout the winter, when there was no UVB light from the ambient sun. This shows that people need a minimum vitamin D level to maintain a healthy microbiome. 

“Unfortunately, it is really hard to obtain enough vitamin D from dietary sources, despite the fact that many dairy products are fortified in Canada. For many people, vitamin D supplementation is the only option between October and April,” Bosman adds.

In this context, Irish researchers previously called for vitamin D fortification and supplementation to battle high deficiency rates in populations living at northern latitudes. 

Could phototherapy boost the gut?
To date, photo-therapy is used for skin conditions like psoriasis and eczema. The effect of UVB light exposure on the skin and skin immunity is very well defined, but not much is known about the effect on the rest of the body. “Once the results from the study are confirmed with a larger study group and in people with intestinal issues, there will be the possibility to try out if gut-related issues can benefit from phototherapy,” Bosman says.

She also explains that more research is warranted. “This study made use of a very selective group of participants, being healthy, female, and of pale skin. It would be very interesting to repeat the study with participants that have a lot more variety in ages and with bigger groups to confirm the results. It would be great if we can test if phototherapy is useful for people with intestinal inflammation to promote their gut health in the future,” she concludes.

By Kristiana Lalou