Ultraviolet B light is good for the gut microbiome

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A small study published in the journal Frontiers in Immunology on October 24, 2019, shows that exposure to ultraviolet B (UVB) light, which is commonly achieved through sunlight exposure, can trigger specific beneficial changes in the gut microbiome, which are dependent on the preceding serum levels of vitamin D.

Immune and inflammatory illnesses are on the rise in Western society, due largely to changes in modern lifestyle and environment. Some such conditions include inflammatory bowel disease (IBD). Modern conditions of employment and work may be accompanied by low sunlight exposure, including to ultraviolet B (UVB) rays, and overuse of antibiotics. These two factors drive associated changes in the composition of the gut microbiome which are in turn associated with chronic inflammatory conditions.

Gut bacteria, flora, microbiome. Illustration Credit: Anatomy Insider / Shutterstock
Gut bacteria, flora, microbiome. Illustration Credit: Anatomy Insider / Shutterstock

Vitamin D, UVB, and the gut microbiome, in human health

Vitamin D is a fat-soluble vitamin which strengthens the intestinal mucosal cells that provide a barrier to the entry of bacteria and other microbes, as well as their toxic products, from the gut into the blood and body tissues. The mechanism of its barrier function is by enhancing the expression of genes that increase the production of tight junctions between adjacent epithelial cells in the intestinal mucous lining; produce antimicrobial compounds; and promote autophagy to remove nonfunctional cell components.

Since few natural food sources contain vitamin D, 80% of our requirements are met by UVB skin exposure which leads to the production of vitamin D. Factors which increase the chances of vitamin D deficiency include living far away from the equator, keeping indoors and choosing to keep skin unexposed to the sunlight. Seasonal vitamin D deficiency is exacerbated in such situations because of the lack of UVB in winter.

The deficiency of vitamin D causes inflammation in the environment which promotes gut dysbiosis, and supplementing vitamin D is helpful in cases of chronic inflammation. Oral supplements also seem to increase the number of certain specific types of bacteria in the gut.

Interestingly, limited exposure to UVB is a primary factor that promotes chronic immune-linked inflammatory conditions like IBD. Thus UVB has other effects on health besides the production of vitamin D.

In short, vitamin D levels, UVB exposure, and the gut microbiome are independently related to an increased risk of autoimmune inflammatory conditions like multiple sclerosis and IBD, and this study attempted to link the three in one chain of cause-and-effect.

The study

The current study was aimed at exploring the effect of narrow-band UVB (NB-UVB) exposure on the gut microbiome.

A small group of 21 females with an average age of 28 years were studied in a pilot study. They were assigned to two groups: in one, they had taken vitamin D pills throughout the winter at doses ranging from 500 IU to 3500 IU per day, before the study started, and those who didn’t. Vitamin D levels were measured at the start.

During the study, each of the participants was exposed to three full-body NB-UVB sessions in a phototherapy unit in one week. This was followed by the measurement of serum 25-hydroxy cholecalciferol (25-OH D).

In the next step, RNA sequencing was performed to find out the composition of the fecal microbiota in samples taken 3 days before the first, and 3 days after the last UVB exposure.

The findings

The pre-UVB vitamin D concentration in blood was 75 nmol/L or more in the first group, while in those who were not supplemented, it was in the insufficient range of 25-75 nmol/L. Thus vitamin D supplementation seems to be necessary to keep levels sufficient during seasons with limited UVB exposure.

On average, following UVB exposure, vitamin D levels showed an increase of 7.3 nmol/L, but the lower the initial concentration, the larger was the rise. This corresponds to a rise of about 10% and indicates that UVB exposure can effectively increase vitamin D levels in the blood.

The fecal RNA sequencing results an initial lack of diversity in the gut microbiome of patients with low initial vitamin D levels. After UVB exposure, there was a significant increase in the number of bacterial species in the first group with low initial vitamin D, but no change in the other group.

Some of the species that were increased in concentration included Lachnospiracheae, Rikenellaceae, Desulfobacteraceae, and one group of Clostridiales. The Lachnospiraceae concentrations increased with the serum vitamin D concentration, in agreement with earlier studies. This genus is associated with good health when compared to autoimmune inflammatory conditions. This seems to show that UVB protects against such conditions by increasing vitamin D levels, which in turn normalizes the gut microbiome.

Seasonal fluctuations in the composition of the gut microbiome have been reported and this may, in the light of these findings, be due to the different levels of exposure to UVB light in various seasons. For instance, IBD flare-ups are common in vitamin D deficiency, and this may be due to the gut dysbiosis associated with low vitamin D levels.


This study showed for the first time that skin exposure to UVB changes the composition of the gut microbiome, possibly via an increase in vitamin D levels. The researchers found that such exposure increased vitamin D levels in blood within a week in healthy volunteers.

They also found that a low vitamin D level in human beings is associated with changes in the composition of the gut microbiome, which appear to be corrected following NB-UVB exposure due to the increase in vitamin D levels.

The scientists think it possible that this may be due to the effect of UVB on the immune system. As more immune cells travel to different organs within the body, including the gut, they release chemicals which shape the profile of the gut bacterial population in a manner that favors gut and systemic health.

This points to the fact of there being a novel skin-gut axis hitherto unknown that can enhance gut health via a balanced microbiome. This could help explain how chronic autoimmune inflammatory disorders respond to UVB exposure, because it helps them achieve stable and adequate vitamin D levels.

Journal reference:

Skin exposure to narrow band ultraviolet (UVB) light modulates the human intestinal microbiome. Else S. Bosman, Arianne Y. Albert, Harvey Lui, Jan P. Dutz and Bruce A. Vallance. Frontiers in Microbiology. 24 October 2019 | https://doi.org/10.3389/fmicb.2019.02410. https://www.frontiersin.org/articles/10.3389/fmicb.2019.02410/full

Dr. Liji Thomas

Written by

Dr. Liji Thomas

Dr. Liji Thomas is an OB-GYN, who graduated from the Government Medical College, University of Calicut, Kerala, in 2001. Liji practiced as a full-time consultant in obstetrics/gynecology in a private hospital for a few years following her graduation. She has counseled hundreds of patients facing issues from pregnancy-related problems and infertility, and has been in charge of over 2,000 deliveries, striving always to achieve a normal delivery rather than operative.


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