October 18, 2010

Shining a Night Light on Obesity

(Ivanhoe Newswire) -- If you're wishing upon a star in hopes to stop packing on the pounds, you might already be jinxing yourself.  A recent study finds that continual exposure to light at night may lead to weight gain, even without changing physical activity or eating more food.  Researchers say that instead of wishing upon the stars . . . maybe you should sleep under them.

Researchers found that mice that were exposed to a moderately dim light at night over eight weeks had a body mass that was around 50 percent more than other mice that lived in a standard light-dark cycle. "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," which Laura Fonken, lead author of the study and a doctoral student in neuroscience at Ohio State University, was quoted as saying.

The study revealed that the mice living with light at night eat at times they normally wouldn't, which is a main factor of their weight gain. One study showed that mice exposed to light at night -- but with food 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," which Randy Nelson, co-author of the study and professor of neuroscience and psychology at Ohio State, was quoted as saying. If these results are confirmed in humans, it would verify that late-night snacking is a possible risk factor for obesity.

In another study, mice were housed in one of three condition: 24 hours of constant light, a standard light-dark cycle (16 hours of light at 150 lux, 8 hours of dark), or 16 hours of daylight and 8 hours of dim light (approximately 5 lux of light).

Researchers measured the amount of food the mice ate each day, in addition to how much they moved in their cages each day using an infrared beam crossing system.  Furthermore, body mass was calculated each week. Compared to mice in the standard dark-light cycle, mice that were exposed to dim light at night showed notably higher increases in body mass, beginning in the commencement of the study and continuing to end.

Light-at-night mice had gained 12 grams of body mass by the end of the experiment, compared to 8 grams for those in the standard light-dark cycle.  It was reported that mice in constant bright light additionally gained more than those in the standard light-dark cycle; however the scientists stated that the dim light-at-night mice were better comparisons to the light exposure human generally are exposed to.

What's more, the dim light-at-night mice showed higher levels of epididymal fat as well as impaired glucose tolerance - a marker for pre-diabetes.
Even though the dim light-at-night mice didn't eat more than the others, they did nonetheless change when they ate.  Since these mice are nocturnal, they would in general eat considerably more food at night.  Conversely, the dim light-at-night mice ate 55 percent of their food during the daylight hours, compared to only 36 percent in the mice living in a standard light-dark cycle.

Since the time of eating seemed significant, these 'mighty mouse researchers' did a subsequent study, similar to the initial, with one crucial difference: instead of having food available at all times, food availability was restricted to either times when mice would normally be active or when they would normally be at rest.

In this experiment, mice exposed to the dim light during normal sleeping hours did not have a greater gain in body mass than did the others when their food was restricted to normal eating hours.

"When we restricted their food intake to times when they would normally eat, we didn't see the weight gain," Fonken added.  "This further adds to the evidence that the timing of eating is critical to weight gain." The findings showed that levels of corticosterone, a stress hormone, were not drastically different in dim light-at-night mice compared to those living in a standard light-dark cycle.

Since corticosterone has been linked to changes in metabolism, it's unreservedly important.  Additionally, this shows there doesn't have to be changes in corticosterone levels to have changes in metabolism in the mice. Researchers believe that this may be a result of the light disrupting levels of the hormone melatonin, which is involved in metabolism.  Furthermore, it may disrupt the expression of clock genes, which help control when animals feed and when they are active. Conclusively, these finding could help shine a 'dim night light' on another possible reason for obesity 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," Nelson said. "Societal obesity is correlated with a number of factors including the extent of light exposure at night."

In prior studies, researchers have identified prolonged computer use and television viewing as obesity risk factors, yet focused predominately on their association with 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, disrupting their metabolism," Nelson concluded. "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."

SOURCE: Proceedings of the National Academy of Sciences, October 17, 2010