Background: Sunlight, and ultraviolet radiation (UVR), are essential for life, and have shaped the way energy is acquired. Indeed, humans have evolved under the influence of sunlight. However, our lifestyles have dramatically changed in recent times, where we spend an increasing amount of sedentary time indoors, consuming energy-dense food and drink.
Methods & Results: We observed that regular skin exposure to low (non-burning) doses of UVR reduced weight gain and signs of metabolic dysfunction in mice fed a high fat diet (Geldenhuys et al Diabetes 2014). These findings were independent of circulating 25-hydroxyvitamin D and the effects of low dose UVR were not mimicked by vitamin D supplementation. Instead, we observed that release of nitric oxide bioactivity from irradiated skin was responsible for some of the suppressive effects of UVR. Furthermore, weight gain and hepatic lipid accumulation was reduced when already ‘overweight’ mice (with signs of glucose intolerance) were exposed to low dose UVR, through nitric oxide (Fleury et al J Endocrinology 2017). In more recent studies, we have observed that regular exposure to low dose UVR and physical activity may combine to further reduce weight gain and signs of type-2 diabetes in mice fed a high fat diet. In ongoing mechanistic studies we are characterizing the potential for low dose UVR to modulate the circadian rhythm of heat production (thermogenesis) by brown adipose tissue, which lies immediately beneath UV-exposed back skin. We have established models of maternal obesity and metabolic dysfunction, to examine how the timing of exposure to UVR may affect metabolic function in pregnancy and early life.
Conclusion: Our studies highlight the potential for low dose (and safe) sun exposure (and/or phototherapy) to be used as part of lifestyle interventions, with further research needed to determine whether our observations from experimental animals can be reproduced in people.