The Gut Bacteria and Sunshine Connection

There were several big news stories last week in the biome world.   One I posted on the Biome Buzz’ Facebook page last Thursday (the 24th) is about how experiments (in a rodent model) demonstrate that the gut bacteria play a significant role in controlling our brains’ fear and anxiety responses.  The 2nd was a really interesting study done by Canadian researchers on the relationship of sunlight exposure to the gut microbiome.  As you know, sun exposure is our primary means of creating vitamin D (it’s virtually impossible to get enough through diet alone, unless you use supplements), which has tremendous impact on our immune system’s functioning.

The study had 21 female subjects, who were given three 60-second full body exposures to ultraviolet light band B (UVB) in the course of a week.  The women donated blood and fecal samples at the beginning and the end of the study which the researchers use to measure both vitamin D levels and gut bacterial diversity.  9 of the subjects had taken vitamin D for 3 months, during the winter; the other 12 had not.  20 of the 21 women had “adequate” vitamin D levels prior to the UVB exposure.

In those women who had not taken vitamin D supplements, there were both a 10% increase in blood vitamin D levels, but also, significantly, marked increases in gut bacterial diversity.

Said the senior researcher on this study:  “Prior to UVB exposure, these women had a less diverse and balanced gut microbiome than those taking regular vitamin D supplements…UVB exposure boosted the richness and evenness of their microbiome to levels indistinguishable from the supplemented group, whose microbiome was not significantly changed.”[i]

The mechanism of action is, as yet, unknown, but this scientist goes on to say that,  “It is likely that exposure to UVB light somehow alters the immune system in the skin initially, then more systemically, which in turn affects how favorable the intestinal environment is for the different bacteria…”[ii]

3 thoughts went quickly through my head, as I read the article and various summations of it:

  1. Firstly, it’s been long known that inflammatory diseases are seen at much higher levels in colder places, where the populations’ vitamin D levels tend to be lower due to lack of sunlight exposure much of the year.  “Several chronic inflammatory diseases display seasonal patterns in the severity of disease.  Specifically, the relapsing and remitting nature of IBD and MS are strongly associated with vitamin D levels.”[iii] You do have to wonder just how much this vitamin D/microbiome alterations piece plays a role in the ever increasing rates of inflammatory disease.  There are other major differences that play a role:  diet, lifestyle, helminth exposure, and so forth.  Remember this blog post from this past June, which showed two contrasting world maps, one illustrating the rates of autoimmune disease, the other, in total contrast, the levels of helminth exposure? Well, it might as well apply to vitamin D too:  those of us living in North American, Europe, Asia also have far less sun exposure than our southern hemisphere neighbors. This is yet another example of just how incredibly complex this question is:  what factors are a significant part of the inflammatory epidemic and to what degree?!
  2. Seasonal affective disorder also immediately came to mind.  We know it’s associated with vitamin D levels.  We know depression is associated with microbiome alterations.  This all cannot be a coincidence!

To sum up: while this study certainly shows that sun exposure, via vitamin D production, has an effect on the microbiome – and probably a significant one too – it would be a mistake to blame this factor entirely for the inflammatory disease epidemic we currently face.  Still, considering that “Winter is Coming” for us in the north 🙂 , in yet another “things you can do now,” maybe a little vitamin D supplementation for the next few months?




[iii] Bosman, ES, Albert, AY, Lui, H, Dutz, JP, Vallance, BA.  Skin exposure to narrow band ultraviolet (UVB) light modulates human intestinal microbiome. Frontiers in Microbiology. 2019;10(2410).  doi: 10.3389/fmicb.2019.02410

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