Discussion: Skin Exposure to Narrow Band Ultraviolet (UVB) Light Modulates the Human Intestinal Microbiome
I will start out by saying that this article should leave the scientific community with more questions than answers. I have thought about these results a lot and it’s almost impossible to reach any firm conclusions about what this means for human health and its link to the intestinal environment and the broader environment. Studies such as these give us the best indication that we are genuinely a product of our surroundings. This particular study also strongly disrupts the paradigm that has led many to believe the only way they can affect their intestinal microbiome is to eat specific foods and take specific probiotics. This is clearly not the case and in my opinion food plays a much smaller role in the sculpting of the microbiome than most believe.
The article was inspired by the rise in idiopathic immune and inflammatory diseases in the western world and how they appear to be inexorably linked to environmental changes. Multiple Sclerosis is one of these diseases mentioned by the authors as its distribution is significantly more pronounced further away from the equator . Type 1 diabetes (T1D) is another disease that seems to be directly affected by latitude . UVB radiation also appears to be the main factor involved in reducing the incidence of T1D closer to the equator as vitamin D plays a huge role in its pathogenesis . Irritable bowel disease (IBD) is another disease mentioned by the authors that exhibit an inverse correlation between latitude and disease prevalence/severity . As many of these modern diseases exhibit a concomitant dysbiotic gut microbiota, the authors hypothesised a potential effect of UVB radiation on the microbial composition of the gut.
“The…etiology of most chronic inflammatory diseases…likely reflects maladaptive interactions between genetic predisposition and environmental factors that drive disease pathogenesis. These environmental factors include limited exposure to sunlight (UVB) resulting in reduced serum 25-hydroxy vitamin D (25(OH)D) levels”
The study was conducted in Canada with a small (n=21), all female cohort (aged between 19 and 40) as to minimise gender differences in gut composition. The participants were divided up into 2 groups; one that supplemented with vitamin D over the winter (VDS+), and one that didn’t (VDS-). It must be said that those included in the VDS+ group were not supplementing judiciously, with many of them taking 1000IU per day (well below the endocrine society’s recommendations for maintaining adequate 25(OH)D levels). The VDS+ group (n=9) used an average of 1389IU of vitamin D per day with most of them having a serum 25(OH)D level >75 nmol/L, which the authors stated is typically considered sufficient.
I do disagree at least in part with this assertion that 75nmol/L is sufficient. I believe a stronger measure should be used to delineate between adequacy and inadequacy with a level of roughly 100nmol/L as a measure of sufficiency. Although it’s slightly controversial and there is definitely nuance to this lab, it is important to keep in mind that even 100nmol/L is considered by some experts in the field to be the minimum level to confer the whole host of benefits associated with vitamin D.
In the VDS- group (n=12), the average serum 25(OH)D level was in the insufficient range between 25 and 75 nmol/L. Faecal samples were collected 3 days prior to the exposure for analysis. Participants were then exposed to three full-body exposures to narrow band UVB light (peak emission at 311nm). The first dose given was at 70% minimal erythemal dose dependent on their skin type. The two following doses were increased by 20% except in patients who exhibited adverse skin reactions (redness or itchiness). Then, three days after the intervention, faecal samples were again collected.
Microbiota analysis after the UVB exposures revealed enhanced diversity and richness of faecal microbiome composition in the VDS- group, but not the VDS+ group. I emailed the author who had their correspondence on the article about why exactly that was, but I never did hear back from him. I was interested in whether the group thought that the action seen here was mediated by an increase into a more normal vitamin D range for the VDS- group, or whether there were other non-vitamin D influences involved.
The researchers examined various aspects of the microbiota including alpha diversity and relative abundance of Firmicutes, Bacteroidetes, Verrucomicrobia and Proteobacteria. These were compared with the samples taken three days prior to the UVB exposure. Statistically significant changes were only seen in the VDS- group. It is difficult to examine why this may have been in such a small sample group. I believe the authors just chalked these changes in the VDS- group only up to the replenishment of the circulating 25(OH)D levels closer to those seen in the VDS+ group. I wonder if there is something unique about UVB light that works via the skin that might be blocked in some way by exogenous supplementation of vitamin D. I am increasingly suspicious of vitamin D supplementation when comparing it to sunlight due to the other photoproducts and sulphation cycles that that are involved in full spectrum irradiation by the sun. However, the authors hypothesise that the mechanism by which vitamin D production is altering the microbiome may be through vitamin D’s ability to alter immune cells. It is well known that vitamin D is capable of modulating the immune system . Clark and Mach showed in 2016 that UVB light exposure leads to local changes in the innate and adaptive immune cells . On these grounds, it is possible that these immune cells are shuttled to sites around the body, including the gut, and alter its microbial composition via release of mediator molecules. These molecules subsequently affect the localised environment where our microbes reside.
The idea that alpha diversity can be shifted just through radiation with UVB light on the skin should make a lot of clinicians and practitioners wonder about seasonal variation as it applies to human health. The diversity of the microbiome has been linked with health and resilience by many of the leading researchers within the field . The idea that vitamin D adequacy may play a vital role in forging and maintaining a healthy and diverse microbiota is one that is seemingly completely overlooked. This could be the most simple, effective and cheap way to enhance gut health: expose yourself to UV light in the context of full spectrum sunlight.
As a side note, when most people think of the microbes that inhabit the gut, they think of probiotics. Probiotics have yielded mixed results in their ability to mediate gastrointestinal conditions  and prevent antibiotic associated reduction in diversity . It seems as though the reason probiotics have been so inconsistent in having positive effects on health is a result of our lack of understanding with regard to the gut and it’s billions of inhabitants. Microbial diversity is complex beyond what we can understand. The ability to track and understand the countless interactions of host microbes, our biology and the environment is one I believe will never be wholly realised. This is why such silver bullet approaches to mending a troublesome microbiota will always fail. Single strains, regardless of the amount, will never usher in microbial diversity or gut health. A reliance on using the environment we too often shy away from may be the best way to build a more diverse microbiome. The only source of probiotics that may be appropriate and capable of having positive effects, I believe, are wild ferments. These types of microbe rich foods have been used for many thousands of years and comprise of a mixture of synergistic bacteria we couldn’t replicate.
Maintaining vitamin D levels can be difficult depending on the latitude at which you live. However, given the broad array of effects vitamin D can exert from reducing the risk of cardiovascular disease , preventing chronic diseases , enhancing intestinal integrity , and strengthening muscle function  (just to name a few), it is vitally important that vitamin D status is followed closely. Vitamin D supplementation has a fairly remarkable record in the published literature, however, I believe this is merely a reflection of how poor human health currently is. I will go out on a limb here and suggest that supplementation with vitamin D confers benefits only in that replenishing a crucial hormone in the body is going to bring about changes at a metabolic and epigenetic level. I do think that this is a simplistic answer to a very difficult question. It seems very likely to me that seasonal variation in light cycles and access to foods are key in bringing about a natural physiological ebb and flow that may bring about optimal human functioning in ways we do not yet understand.