Dysbiotic changes in the composition of the gut microbiome form one of the factors implicated in the rise of chronic immune and inflammatory diseases like multiple sclerosis (MS) and inflammatory bowel disease (IBD). Among the environmental and lifestyle factors implicated is reduced exposure to sunlight, which adversely affects the production of Vitamin D.

Why Vitamin D is vital for gut health in addition to bone integrity

Vitamin D is an all-important vitamin. Its deficiency, at whatever age, can lead to health issues. Vit D helps our body absorb dietary calcium to keep our bones strong. Vit D is produced by the human body when the sun’s ultraviolet (UV) rays come in contact with our skin.

It is highly recommended to eat foods containing Vit D, such as fish, eggs, Vit D-fortified dairy products, and/or take supplements in case of greater deficiency.

Vitamin D undergoes a lot of processing within our body. Within the liver, it transforms into its major circulating form known as 25-hydroxyvitamin D (a.k.a. calcidiol). This is the compound that is detected in the “25-OH Vit D test” designed to monitor Vit D levels in our blood. The total serum 25(OH)D level is the current laboratory standard to detect the presence of Vit D in our body, gained from sun exposure and nutritional intake.

The skin-gut axis

Vit D is important to gut health, as it is thought to promote intestinal barrier integrity. Also, it influences innate and adaptive immune cells, suppressing pro-inflammatory responses. A deficiency of Vit D promotes an inflammatory environment conducive to gut dysbiosis, even in clinically healthy persons.

The availability of Vit D from dietary sources is limited. Humans have to meet 80% of their Vit D needs through exposure to Ultraviolet B (UVB) light. UVB light converts a cholesterol derivative named 7-dehydrocholesterol to increase serum levels of 25(OH)D.

A recent study explored whether exposing human skin to Narrow Band UVB (NB-UVB) light to increase serum Vit D levels would affect the gut microbial composition. Participants belonged to two groups: Those who took Vit D supplements throughout the winter before the start of the study, and those who did not.

The researchers found a significant effect on gut microbiota composition after repeated NB-UVB exposures, particularly in the subjects that had not been taking Vit D supplements. Both the alpha and beta diversity of their microbiota improved in response to UVB light exposure.

This is the first study that demonstrates that the gut microbiome of individuals with Vit D insufficiency, which manifests as low 25(OH)D serum levels, can change in response to NB-UVB exposure. The corresponding increase in 25(OH)D serum levels also suggests the presence of a novel skin-gut axis. This finding opens up new avenues to explore in the effort to promote intestinal homeostasis and overall health and well-being in individuals suffering from chronic illnesses.