Discover how reduced light exposure may raise metabolic syndrome risk and how light therapy can support your internal biological clock and metabolic health.
Yes, fat cells deep under your skin can sense light. When the body receives too little of the right kind of light, these cells behave differently.
The finding, published 21 Jan 2020 in Cell Reports, emerged while Cincinnati Children’s scientists were studying temperature control in mice. The results reach well beyond how mice keep warm.
The study shows that light exposure helps coordinate two types of fat cells that supply the raw materials all other cells use for energy. Disrupting this basic metabolic step may mirror an unhealthy feature of modern life—spending most of the day indoors.
“Our bodies evolved under sunlight, including light-sensing genes called opsins. Now we live under artificial light that lacks the sun’s full spectrum.”
Richard Lang, PhD, developmental biologist and senior author.
Lang directs the Visual Systems Group at Cincinnati Children’s and has published more than 120 papers, many on eye development and non-visual effects of light.
“This paper changes how we think about light’s influence on the body,” Lang says.
Most people know that some wavelengths can harm—gamma radiation from a nuclear source or excess ultraviolet that burns skin. This study outlines a different, beneficial role for light.
Despite mouse fur or human clothing, photons penetrate the skin, scatter, and reach deeper layers, where they can alter cell behavior.
Lang’s earlier work, a 2013 Nature paper, showed that light exposure guides eye development in fetal mice. In 2019 his team published two related studies: one in Nature Cell Biology on possible light-therapy benefits for preterm infant eye development, and one in Current Biology on how skin light receptors help mice set their internal clocks.
The new Cell Reports study also includes contributions from Russell Van Gelder, MD, PhD, and Ethan Buhr, PhD, University of Washington, and Randy Seeley, PhD, University of Michigan.
“The idea that light reaches deep tissue is still new, even to many scientists. Yet we and others keep finding opsins in diverse tissues. We’re only at the beginning.”
In the latest experiments, mice were chilled to about 40 °F. Like humans, mice warm themselves by shivering and by burning fat. The team found that this internal heating falters when the gene OPN3 is missing and when 480-nanometer blue light—plentiful in sunlight but scarce in most indoor lighting—is absent.
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When light exposure occurs, OPN3 prompts white fat cells to release fatty acids into the bloodstream. Various cell types can then use these fatty acids as fuel. Brown fat, however, burns the fatty acids through oxidation, producing heat that warms cold mice.
Mice bred without the OPN3 gene could not warm up when placed in chilly conditions. Surprisingly, even genetically normal mice failed to warm up under light that lacked the blue wavelength.
These findings led the team to conclude that sunlight is required for normal energy metabolism—at least in mice. Although the scientists suspect a similar light-dependent pathway exists in humans, further experiments are needed to confirm it.
“If the light-OPN3 adipocyte pathway exists in humans, there are potentially broad implications for human health,” the study notes. “Our modern lifestyle exposes us to unnatural lighting spectra, light at night, shift work, and jet lag, all of which can disrupt metabolism. Insufficient stimulation of this pathway might partly explain the high rate of metabolic deregulation in industrialized nations where artificial lighting is ubiquitous.”
Several more years of research will likely be needed to build on this discovery. In theory, “light therapy” might one day help prevent metabolic syndrome from progressing to diabetes, and full-spectrum indoor lighting could support public health, Lang says.
Yet questions remain: how much sunlight is needed for healthy metabolism, whether people with obesity might carry an impaired OPN3 gene, and when light therapy would be most effective—during pregnancy, infancy, childhood, or adulthood.
Cincinnati Children’s Hospital Medical Center
Nayak, G., et al. Opsin 3-Dependent Adipocyte Light Sensing Enhances Adaptive Thermogenesis in Mice. Cell Reports (2020). doi.org/10.1016/j.celrep.2019.12.043
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