The leaves (and snowflakes) are falling, the wind has a chill, and the days are getting shorter. Autumn is here at last and winter doesn’t seem so far away for many across the nation – and although you might be looking forward to Christmas and bonfires, you might have noticed your motivation waning as the day wears on.
You’re certainly not alone. There is something about the shorter days that messes with your motivation – and most of it has to do with variations in your biological clock. So much of how our body functions relies on environmental cues driven by the sun.  All life revolves around it – figuratively and quite literally. In the summertime, we take it for granted, but with winter right around the corner, odds are you might be feeling a little blue – but it turns out your gut might be under the weather, too.
Flu season? Cold season? Sound familiar?
When you were young, you might have been plagued with parents reminding you not to forget a jacket, or to keep dry during wetter seasons so you don’t catch a cold. But getting wet doesn’t necessarily lower your immune system. Although fighting cold temperatures can put physiological stress on you and make you more susceptible to getting sick, the weather changes might be affecting other things beneath your hood as well – and it all has to do with Vitamin D and your gut. [2,3]
Sunlight and Vitamin D
In spring and summer, when we walk outside and catch some rays, that sunlight is causing all sorts of beneficial biochemical reactions in your skin. You might be familiar with UV rays – those things that sunblock is supposed to protect you from – but did you know that they also play an important role in keeping you healthy?
One of the few vitamins humans can create themselves is Vitamin D, essential for growth and immune function.  We make this vitamin when certain UV rays hit our skin, converting cholesterol - used in our skin cell membranes to keep them fluid and mobile – into Vitamin D.  Although you can ingest sources of Vitamin D in food or supplements (like fortified milk, yogurt, and fatty fish), the kind our skin produces is unique and is easier for us to process. 
Vitamin D is then used to promote a healthy immune system. When your Vitamin D status is low, you can find yourself more at risk for infection and more susceptible to disease.
The Immune System Organ
You might be familiar by now that upwards of 70% of your immune system is housed in your gut, making Vitamin D very interesting to some studies looking at gut health. Recently, a study published in the scientific journal Frontiers noted that female participants in a small clinical study exposed to UVB light showed significant improvements in gut microbiome diversity. The women showed improvements in symptoms associated with IBD and MS, both diseases associated with chronic inflammation. This study showed a new perspective on how sunlight may impact our gut microbiome – something we never expected to see the light of day!
The exact way this impacts gut health, however, is proving to be a little more elusive - though the researchers in this study speculated it probably has to do with an improved immune system. Their next step is to study how by examining the mechanisms that might be improving gut microbiome balance.
If you’re feeling SAD – or seasonal affective disorder – you’re probably feeling tired, lethargic, and – well – a little sad. It always seems like some people feel it worse than others, but no one ever seems immune.
Many scientists speculate lack of Vitamin D may be to blame since it contributes to our serotonin levels or our “happy” neurotransmitter.  Serotonin works to regulate mood, sleep, appetite, digestion, and social behavior, making it a strong link in depression studies. If you’re looking for another gut link – you don’t have to look hard: scientists estimate that 90% of the serotonin in our body is produced right in your gut.
In fact – most of that serotonin is made right from your gut microbes. 
If your gut is experiencing an imbalance, it could have a larger impact on your mood – something evidenced in studies exploring the gut-brain axis(8). It’s all connected!
Could it be that sunlight has a larger role in our gut health than previously thought? Researchers seem to think so.
The next time you’re feeling those seasonal blues, it’s not all in your head – in fact, it might even be in your gut!
What Can You Do?
The first step is remembering how much your body changes and adapts to the seasonal tides. We can’t bring back the sun, but you can certainly check in with your gut and see what’s happening inside- and Viome can help. If you’ve noticed a little more bloating or weight gain this season, it might not just be those second portions of pumpkin pie. Your gut might be experiencing changes based on the time of year and need a little extra attention – just like you do when it’s 5pm and dark outside, and the only thing you want is a heated blanket and your favorite Netflix show. We all need a little extra self-care this season!
*The information on the Viome website is provided for informational purposes only and with the understanding that Viome is not engaged in rendering medical advice or recommendations. Viome is providing this educational information to share the exciting developments being reported in the scientific literature about the human microbiome and your health. Viome products are not intended to diagnose, treat, or prevent any disease.
Honma K, Honma S, Kohsaka M, Fukuda N. Seasonal variation in the human circadian rhythm: dissociation between sleep and temperature rhythm. Am J Physiol. 1992;262:R885-891.
van der Lans AA, Boon MR, Haks MC, et al. Cold acclimation affects immune composition in skeletal muscle of healthy lean subjects. Physiol Rep. 2015;3.
Aranow C. Vitamin D and the immune system. J Investig Med. 2011;59:881-886.
Wacker M, Holick MF. Sunlight and Vitamin D: A global perspective for health. Dermatoendocrinol. 2013;5:51-108.
Jungert A, Spinneker A, Nagel A, Neuhauser-Berthold M. Dietary intake and main food sources of vitamin D as a function of age, sex, vitamin D status, body composition, and income in an elderly German cohort. Food Nutr Res. 2014;58.
Penckofer S, Kouba J, Byrn M, Estwing Ferrans C. Vitamin D and depression: where is all the sunshine? Issues Ment Health Nurs. 2010;31:385-393.
Yano JM, Yu K, Donaldson GP, et al. Indigenous bacteria from the gut microbiota regulate host serotonin biosynthesis. Cell. 2015;161:264-276.
Clapp M, Aurora N, Herrera L, Bhatia M, Wilen E, Wakefield S. Gut microbiota's effect on mental health: The gut-brain axis. Clin Pract. 2017;7:987.