Could Sunlight Aid Weight Loss?

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Nayak et al. have recently published the findings of their fascinating research into the effects of light on fat metabolism in mice.

Animals have adapted to use light in various ways. The most obvious is our sense of sight – it creates images in the brain through the detection of photons by light sensitive proteins (opsins) in the retina. But there are also non-visual ocular photoreceptors that help regulate our circadian rhythms (body clock), pupillary light reflex and eye development. Interestingly, light sensitive proteins are also found outside the eye. Opsins in our skin can regulate the circadian clock and others can influence blood vessel dilation. In birds, it’s photoreceptors deep within the brain that regulate seasonal breeding behaviour.

There have been suggestions that adipocyte (fat cell) function may be modulated by light. White fat (WAT) acts as a storage site whereas brown fat (BAT) generates heat through a process called non-shivering thermogenesis (NST). During lipolysis, white fat can be broken down into free fatty acids (FFAs) and glycerol. The brown fat can then use the FFAs to generate heat by oxidation. This process plays a crucial role in the regulation of body temperature during cold exposure.

In the current study, Nayak et al. found that lipolysis was brought about by the exposure of light receptors within white fat (encephalopsin, OPN3) to light. OPN3 was particularly sensitive to blue light. The mice lacking OPN3 or light exposure had diminished heat-generating responses when placed in cold environments. The authors conclude: β€œIf the light-OPN3 adipocyte pathways exist in humans, there are potentially broad implications for human health. Our modern lifestyle subjects us to unnatural lighting spectra, exposure to light at night, shift work, and jet lag, all of which result in metabolic disruption. Based on the current findings, it is possible that insufficient stimulation of light-OPN3 adipocyte pathway is part of an explanation for the prevalence of metabolic deregulation in industrialized nations where unnatural lighting has become the norm.”