Results of research on mice suggest that late-night eating might be a particular risk factor for obesity.
By JUDY SIEGEL-ITZKOVICH
Persistent exposure to light at night may cause weight gain – even without changing physical activity or eating more food – according to research on animals by scientists at Ohio State University, in which University of Haifa biology Prof. Dr. Abraham Haim took part.The researchers believe the light could disrupt levels of the hormone melatonin, which is involved in metabolism.Exposure to artificial light at night has in numerous other studies been linked to a significantly higher risk in humans of prostate and breast cancer; the explanation is that the production of melatonin – released by the pineal gland in the brain during darkness – inhibits cancer cell development.In addition, melatonin may disrupt the expression of clock genes, which help control when animals feed and when they are active, according to the Ohio study. Overall, the findings show another possible reason for the obesity epidemic in Western countries.“Light at night is an environmental factor that may be contributing to the obesity epidemic in ways that people don’t expect,” Ohio State neuroscientist Prof. Randy Nelson said. “Societal obesity is correlated with a number of factors including the extent of light exposure at night.”Until now, researchers have thought prolonged computer use and TV viewing were risk factors for overweight, because they are associated with a lack of physical activity.“It may be that people who use the computer and watch the TV a lot at night may be eating at the wrong times,” Nelson said.Advertisement“Clearly, maintaining body weight requires keeping caloric intake low and physical activity high, but this environmental factor may explain why some people who maintain good energy balance still gain weight,” Nelson said.The team found that mice exposed to a relatively dim light at night over eight weeks had a body mass gain that was about 50 percent more than other mice that lived in a standard light-dark cycle.The study appears in the online early edition of the Proceedings of the National Academy of Sciences (PNAS) that was released on Monday.Mice were housed in one of three conditions: 24 hours of constant light, a standard light-dark cycle (16 hours of light at 150 lux, eight hours of dark), or 16 hours of daylight and eight hours of dim light (about five lux of light).“Although there were no differences in activity levels or daily consumption of food, the mice that lived with light at night were getting fatter than the others,” Ohio State University doctoral student and lead author Laura Fonken said.“Results suggest that mice living with light at night eat at times they normally wouldn’t. In one experiment, mice exposed to light at night – but that had food availability restricted to normal eating times – gained no more weight than did mice in a normal light-dark cycle.“Something about light at night was making the mice in our study want to eat at the wrong times to properly metabolize their food,” said Nelson.If these results are confirmed in humans, it would suggest that late-night eating might be a particular risk factor for obesity, Nelson said.The researchers measured how much food the mice ate and how much they moved around their cages daily by using an infrared-beam-crossing system. By the end of the experiment, light-at-night mice had gained about 12 grams of body mass, compared to eight grams for those in the standard light-dark cycle.Although the dim light-atnight mice didn’t eat more than others, they did change when they ate, results showed. These mice are nocturnal, so they would normally eat substantially more food at night. However, the dim light-at-night mice ate 55 percent of their food during the daylight hours, compared to only 36% in the mice living in a standard light-dark cycle.Since the timing of eating seemed significant, the researchers performed a second study, similar to the first, with one important difference – instead of having food freely available at all times, food availability was restricted to either the times when mice would normally be active or when they would normally be at rest. In this experiment, mice exposed to the dim light at night did not have a greater gain in body mass than did the others when their food was restricted to times when they normally would be active.“When we restricted their food intake to times when they would normally eat, we didn’t see the weight gain,” Fonken said. “This further adds to the evidence that the timing of eating is critical to weight gain.”
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