Ultraviolet radiation suppresses obesity and symptoms of metabolic syndrome independently of vitamin D in mice fed a high-fat diet (Geldenhuys et al., 2014)
Sunlight > vitamin D supps. It is known.
Brief review of vitamin D synthesis: TEAMWORK!
In brief: cholesterol -> 7-dehydrocholesterol in skin; sunlight (UV radiation) converts that into D3 which goes to liver on DBP and is made into 25(OH)-D3. That then goes to kidneys and is transformed into the active form, 1,25(OH)2-D3. What's not shown in that graphic is when you have enough D, it's instead converted into an inactive 24,25(OH)2-D3. This prevents toxicity from sunlight D.
Hypervitaminosis D (from supps) is rare but unpleasant.
This study showed sunlight also increases nitric oxide in the skin, which has effects beyond the D... may also be supplemented with NO-inducing skin creams (also speculative), if applied to the right spots...
Big tie-in with our beloved brown adipose tissue.
This was a monster study. Mice divided into low fat and high fat diet groups, then further divided into vitamin D supplemented with no UV, low dose UV, and high dose UV. Twelve groups in all.
1,25(OH)-D3 is tightly regulated, which means it doesn't decline until you're very deficient, so we measure 25(OH)-D3 which more accurately reflects D status. In this study, D supps and UV increased 25(OH)-D3.
The cool part - weight gain was dose-dependently reduced by UV exposure. Food intake wasn't assessed but the authors noted no obvious changes in physical activity.
UV exposure also improved glucose tolerance and insulin sensitivity.
Liver: D supps improved liver histopathology but UV did way more.
Inflammation (as measured by TNF-alpha), on the other hand, was reduced by D supps more so than UV suggesting some independent effects of D and UV.
UV: the authors suggested UV-induced NO may stimulate brown adipose tissue to waste calories which led to suppression of weight gain... what made this more plausible imo is that they exposed the mice to UV right where they have the most BAT (on their backs), and NO is fairly short-lived -- it might not have worked if the NO was induced too far away from any BAT depots.
It wasn't entirely how this would happen in wild mice who are furry (the mice in the study had their backs shaved) and don't go out in the day, but the pathways exist in humans too. UV exposure in humans has been shown to reduce blood pressure, a known effect of NO.
All in all, pretty cool study.
That's all for now!
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