Seasonal light variations impact eating habits and metabolic health, experts flag

17 Oct 2023 --- The findings of a proof of principle study conducted by researchers from the University of Copenhagen demonstrate that light exposure affects how we eat and burn energy. Significant variations in daylight hours across various regions worldwide sparked the researchers’ curiosity to conduct the study.

Their hypothesis was proven in a mouse study and published in Cell Metabolism. The study’s findings can help researchers understand the link between seasonal variation and fluctuations in metabolism. Contrary to popular belief, the study shows that winter eating habits are better for metabolic health than summer.

“I come from Australia, and when I first moved to Denmark, I was not used to the huge difference in light between summer and winter and I was interested in how this might affect both circadian rhythms and metabolism,” says Lewin Small, postdoctoral candidate at Novo Nordisk Foundation Center for Basic Metabolic Research at the University of Copenhagen and lead author of the study.

“Therefore, we exposed laboratory mice to different light hours representing different seasons and measured markers of metabolic health and the circadian rhythms of these animals.”

Seasonal light-hour variations change significantly between summer and winter except at latitudes close to the equator. However, studies about the interplay between energy and metabolism or circadian rhythms use a 12-hour light and dark photoperiod corresponding to the light duration at the equator.

The researchers theorize altering the seasonal photoperiod affects the rhythmicity of peripheral tissue clocks and energy homeostasis.

Influence of light on metabolism
The study breaks ground into investigations on the effect of light hours on metabolism in mice that are not considered seasonal animals because they do not breed in specific seasons, the same as humans. Creatures breeding in distinct seasons gain weight before the breeding season to save energy supplies.

The researchers investigated the effect of different seasonal photoperiods on energy metabolism and diurnal rhythmicity. Both melatonin-deficient and melatonin-proficient mice displayed a modest reduction in body weight and fat mass gain when housed at a short-light photoperiod compared with either an equal-light or long-light photoperiod.

The scientists examined the metabolism and weight of mice exposed to both “winter light” and “summer light.” Small adds: “We found that even in non-seasonal animals, light hours between summer and winter cause differences in energy metabolism. In this case, body weight, fat mass and liver fat content.”The current study’s researchers found that food intake rhythmicity at different seasonal times significantly affects weight gain.

Therefore, they wanted to find out what the seasonal light differences meant for the metabolism. Most people live with at least a two-hour difference in light between summer and winter.

“We found this mostly in mice exposed to winter light hours. These mice had less body weight gain and adiposity. They have more rhythm in the way they eat over a 24-hour period and this then leads to benefits in metabolic health.”

“We study the influence of the time of day on aspects of metabolism such as exercise, obesity and diabetes. However, most studies that investigate this link do so assuming an equal length of day and night all year round,” he says.

Weight gain at that time of the year
Mice in a winter photoperiod had an increase in the amplitude of rhythmic lipid metabolism and a modest reduction in fat mass and liver triglyceride content. When comparing melatonin-proficient and deficient mice, the effect of seasonal light on energy metabolism was primarily driven by differences in the rhythmicity of food intake and not melatonin.

The data indicate that seasonal light impacts energy metabolism by modulating the timing of eating.

Further research is needed to understand how eating patterns are affected by seasonal light. This could help researchers understand why some people gain more weight or if people gain more weight in a specific time of year.

“This is a proof of principle. Do differences in light hours affect energy metabolism? Yes, it does. Further studies in humans may find that altering our exposure to artificial light at night or natural light exposure over the year could be used to improve our metabolic health,” says Juleen Zierath, professor at the Novo Nordisk Center for Basic Metabolism Research and senior author of the study.

“Differences in light between summer and winter could affect our hunger pathways and when we get hungry during the day,” Small concludes.

By Inga de Jong

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