How UV Light Touches the Brain and Endocrine System Through Skin, and Why

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This randomized, placebo-controlled, clinical trial showed that the SOLIUS system effectively increased 25(OH)D concentrations in individuals with vitamin D deficiency or insufficiency, offering a safe solution for improving vitamin D status. The treatment group showed a significant increase in serum 25(OH)D concentrations, with an average increase of 10.2 ng/ml (p<0.01), compared to a decrease of 2.3 ng/ml in the control group.

A 2019 clinical pilot study published in Frontiers in Microbiology examined whether skin exposure to narrowband UVB light could influence the human gut microbiome. In healthy adults, three controlled UVB exposures over one week increased serum vitamin D levels and led to measurable changes in gut microbiota composition, particularly in participants who were vitamin D insufficient at baseline. Researchers observed increased microbial diversity and enrichment of bacterial families commonly associated with gut health, supporting the existence of a skin–gut axis through which UVB light exposure may influence intestinal homeostasis. These findings suggest that targeted UVB exposure can have systemic biological effects beyond the skin, potentially linking light exposure, vitamin D status, and microbiome health.