Scientists show that ultraviolet (UV) light exposure leads to changes in the gut microbiome, but only in volunteers who were deficient in vitamin D.

There is plenty of evidence that links vitamin D, the sunshine vitamin, to health outcomes.

Living at higher latitudes, which means less exposure to UV light and a greater chance of being vitamin D deficient, carries a higher risk of developing diseases such as multiple sclerosis (MS) and inflammatory bowel disease (IBD).

Research into the gut microbiome indicates that our microbial passengers may play a significant part in these conditions.

But what links vitamin D to our intestinal microbiota?

A team of researchers, many from the University of British Columbia, in Vancouver, Canada, set out to answer this question by studying how the gut microbiome responds to UV light.

When volunteers who were deficient in vitamin D received three sessions of UVB exposure, their gut microbiomes changed and bore the same hallmarks as those of study participants who were not vitamin D deficient.

The team published their findings in Frontiers in Microbiology.

UVB light boosts 'richness' of microbiome

The study included nine female volunteers who took vitamin D supplements in the 3 months leading up to the experiments and 12 who did not.

All participants had fair skin, specifically Fitzpatrick skin types 1 to 3.

The volunteers who had taken the supplements had vitamin D blood levels that are classed as adequate, while all but one of those who had not taken the supplements were vitamin D deficient.

All participants then had three sessions of full body exposure to UVB light. The research team saw increases in the vitamin D levels in all of the volunteers, as a result.

They then compared the composition of each participant's gut microbiome before and after the treatments.

The authors found significant changes in the microbial compositions in the group that had been mostly vitamin D insufficient at the start of the experiment.

"Prior to UVB exposure, these women had a less diverse and balanced gut microbiome than those taking regular vitamin D supplements," senior study author Prof. Bruce Vallance notes, summarizing the teams' results. "UVB exposure boosted the richness and evenness of their microbiome to levels indistinguishable from the supplemented group, whose microbiome was not significantly changed."

Specifically, the vitamin D deficient volunteers experienced an increase in Firmicutes and Proteobacteria and a decrease in Bacteroidetes, to bring their levels in line with the microbiomes of participants who had taken vitamin D supplements.

Vitamin D 'main driver' of shift in microbiome

Medical News Today spoke to first study author Else Bosman about the study.

"We found that vitamin D production was the main diver of the shift in the microbiome," she explained. "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."

"Although those facts were known individually, this is the first study linking them up together," Bosman continued. "The results were surprising in the way that there was a strong effect visible within 1 week's time."

When asked how much time we should each be spending in the sunshine to boost our vitamin D levels, Bosman urged caution.

"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," she explained. "From my study, it is hard to conclude how much sun exposure is enough to produce vitamin D."

This is down to our individual skin types and the amount of UV radiation in the environment that we live in.

"Unfortunately, it is really hard to obtain enough vitamin D from diet alone, so it is wise to supplement with vitamin D during the winter," Bosman recommended. "Your body is very efficient in making vitamin D from sunlight in the summer."

The extent to which variations in our microbiomes resulting from fluctuating vitamin D levels affect our health is unclear at this point.

But Prof. Vallance suggests that this may be more important for people with inflammatory diseases, such as MS and IBD.

Larger studies are needed and should include the full spectrum of skin types, as well as male and female participants, the authors suggest in their paper.

"This study made use of a very selective group of participants, e.g., healthy, female, pale skin," Bosman told MNT. "It would be very interesting to repeat the study with participants that have a lot more variety in ages and with bigger study groups to confirm the results. It would also be great if we can test if the phototherapy is useful for people with intestinal inflammation to promote their gut health."

"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."

—Prof. Bruce Vallance