A lack of sun exposure/UVB radiation has been linked to a decrease in vitamin D synthesis, which has been hypothesized to be one of the environmental factors related to the current rise in immune-related conditions through involving the gut microbiota. However, mouse and small human observational studies that showed associations between vitamin D and the gut microbiome do not allow for establishing a direct relationship between UVB light and the gut microbiome.
A new intervention study, led by Dr. Bruce A. Vallance from the University of British Columbia and the Women’s Health Research Institute in Vancouver (Canada), has found that exposure to UVB light may lead to specific beneficial changes in the human gut microbiome, which depend on a subject’s preceding serum levels of vitamin D.
The researchers assigned a small group of 21 healthy adult females to two groups. The first group (n=9) took vitamin D supplements throughout the winter at doses ranging from 500 IU to 3500 IU per day over the 3 months prior to the study, whereas the second group did not (n=12). While most participants from the group taking vitamin D supplements before the study showed sufficient vitamin D serum levels, most of the subjects who had not taken them displayed insufficient vitamin D serum levels.
The participants in each group were subsequently exposed to three full-body narrow band UVB light sessions. The research team measured 25-hydroxy vitamin D [25(OH)D] serum levels—while metabolically inactive, this metabolite is the major circulating form of vitamin D—at the beginning of the study and after the third exposure to UVB light.
The research was carried out in Vancouver (Canada) during winter, which means participants were not exposed to ambient UVB light during the study. Serum 25(OH)D levels increased on average 7.3 nmol/L (approximately 10%) in participants in both groups within a week of the UVB exposures. However, a negative association was found between the initial serum 25(OH)D concentrations and their rise across the study (i.e. the lower the initial 25(OH)D levels, the greater the increase).
The authors also found that repeated skin exposure to UVB light increased alpha (within-subject) and beta diversity (between-subjects) in the gut microbiota in subjects who reported they had not taken vitamin D supplements before the study. Such changes comprised an enrichment in several genera from the Lachnospiraceae, Ruminococcus, and Clostridiaeae families that have been associated with a healthy microbiota composition.
UVB exposures over a one-week period increased the gut microbiota diversity of the group that started the study with vitamin D insufficiency to the same level as that of the group that started the study with sufficient serum vitamin D levels. The exposure to UVB also had a selective influence on gut microbiota composition, as positive associations were found between Lachnospira and Fusicatenibacter and serum 25(OH)D levels.
That gut microbiota diversity only improved with UVB light exposure in participants who were not taking vitamin D supplements before the study—most of whom suffered from vitamin D insufficiency—highlights how changes in the gut microbiota may depend on systemic levels of vitamin D measured by serum 25(OH)D concentrations.
This is the first study that reports changes in the human gut microbiota in response to UVB light. As decreased exposure to UVB light and subsequent decreased vitamin D production has been suggested to take part in the current rise in chronic inflammatory diseases, the fact that gut microbiota composition could also be a piece in this puzzle opens new avenues for research that needs to be validated in larger cohorts.
Bosman ES, Albert AY, Lui H, et al. Skin exposure to narrow band ultraviolet (UVB) light modulates the human intestinal microbiome. Front Microbiol. 2019; 10:2410. doi: 10.3389/fmicb.2019.02410.