September 28, 2009

Timing of Meal May Affect Weight Gain

photo of a man and his dog looking into a refrigerator late at night

Mice fed a high-fat diet during their normal sleep and rest period gained significantly more weight than mice fed the same diet during their active hours, a new study shows. The finding suggests that when we eat may influence how we pack on pounds.

Many living organisms—from plants to fruit flies to humans—have internal biological clocks, or circadian rhythms, that govern our daily cycles of sleep and activity. These internal clocks are tied to cycles of light and darkness. Several recent studies have found that circadian clocks can affect metabolism in animals, suggesting that the timing of meals might influence whether incoming calories are burned or stored as fat.

To take a closer look, Dr. Fred Turek, Deanna M. Arble and their colleagues at Northwestern University designed a study to search for a direct link between meal times and body weight. Their research is supported by NIH's National Institute on Aging (NIA) and National Heart, Lung and Blood Institute (NHLBI).

For 6 weeks, mice were fed a high-fat diet either during a 12-hour dark phase, when the mice are normally active, or during a 12-hour light phase, when the mice are normally resting. The mice could eat as much as they wanted during their assigned feeding periods, but the food was removed during the remaining 12 hours each day. The study was described in the September 3, 2009, advance online edition of Obesity.

The researchers found that both groups of mice had similar levels of activity and calorie consumption over the 6-week period. However, within the first 2 weeks, the mice who ate during their normal sleeping phase weighed significantly more than the other mice. By the end of the study, the mice who ate when they should be sleeping had a 48% boost to their body weight, on average, compared to a 20% increase in the mice who ate the high-fat diet during their normal waking hours.

The researchers say their study is the first to show that eating at the "wrong" time can influence body weight, although the mechanism is still unclear. The circadian clock is known to affect many other body functions, including body temperature, hormone levels, hunger and sleep, all of which may contribute to changes in weight. Further research will be needed to determine if the findings in mice can shed light on human weight gain.

"How or why a person gains weight is very complicated, but it clearly is not just calories in and calories out. We think some factors are under circadian control," says Turek. "Better timing of meals, which would require a change in behavior, could be a critical element in slowing the ever-increasing incidence of obesity."

—by Vicki Contie

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