Sunlight might affect gut microbiome diversity

Researchers at the University of British Columbia in Canada have found a significant link between exposure to ultraviolet rays and an increase in gut bacterial diversity. The findings could prove important in managing autoimmune diseases known to be associated with gut bacterial diversity, such as inflammatory bowel disease.

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Ultraviolet (UV) designates a band of the electromagnetic spectrum with wavelengths ranging from 10 nm to 400 nm, shorter than that of visible light but longer than X-rays. Our senses cannot detect UV rays — not until the damage is done — which is why they can be especially dangerous. Too much time in the sun causes sunburns, eye damage, accelerate aging, and skin cancer.

There are 2 main types of UV rays that interact with our skin.

  • UVB, which is responsible for the majority of sunburns, and
  • UVA, which penetrates deep into the skin. It ages the skin, but contributes much less towards sunburn.

Although prolonged exposure to UV radiation can be very harmful, we also require moderate exposure in order to live healthily. Research has shown a link between UV exposure and the synthesis of vitamin D, which promotes the formation and strengthening of bones (a deficiency will cause bone softening diseases, which then causes rickets in children and osteomalacia in adults), strengthens the immune system, and offers protection against some cancers.

In the study, 21 female participants received three 60-second full-body UVB exposure sessions over the course of one week. The researchers drew blood and fecal samples from each subject at the beginning and end of the trial in order to analyze changes in vitamin D and gut bacteria. Half of the participants had taken vitamin D supplements during the prior three winter months.

The subjects who didn’t take vitamin D supplements experienced an increase in alpha and beta gut microbiome diversity. They also experienced a 10% increase in blood serum vitamin D concentration after the week-long UVB exposure. On the other hand, those who took vitamin D supplements did not experience an increase in gut microbiome diversity. This suggests that vitamin D is the mediating factor between UVB exposure and gut microbial activity.

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

Although the researchers haven’t identified a formal mechanism, they think that the initial exposure to UVB light alters the immune system — first at the level of the skin, then more systematically throughout the body, affecting how bacteria interact with the environment inside the intestines. Previously, studies have shown that inflammatory bowel disease symptoms can worsen when the body experiences vitamin D deficiency, which strengthens the idea sunlight exposure and gut health are somewhat connected.

Next, the team plans on performing similar studies on a larger cohort of subjects in order to investigate this link better.

“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,” says Vallance.

The findings appeared in the journal Frontiers in Microbiology.

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