"We wanted to see if we could predict which group would lose more weight," says Frank Scheer, director of the Medical Chronobiology Program at Harvard Medical School and an author of the study. "When we compared other aspects between early and late eaters that may influence weight-loss success - total intake, activity levels, and sleep - we found that there was no difference between the two groups. Our unpublished data indicate that meals influence energy expenditure differentially depending on the time of food intake, offering a possible mechanism for differences in weight regulation. Together with other recent studies, this finding supports the idea that meal timing is an important factor beyond calories."
"It's becoming clear," Scheer adds, "that when you consume food, it's not processed in the same way in the body. It depends on when the food is consumed."
Other papers bolster this finding. A study by Tel Aviv University researchers found patients who consumed a larger breakfast early in the day that included dessert items had a larger weight loss than those who ate a smaller meal.
In the study, 93 obese women were randomly assigned to one of two controlled-calorie groups eating either a big breakfast or a big dinner. Participants in the large-breakfast group lost an average of 17.8 pounds each, compared with 7.3 pounds for participants in the big-dinner group.
There is also evidence that upending the circadian patterns of eating in nocturnal mice can lead to obesity. In a paper in Nature Medicine last year, Garret FitzGerald, professor of medicine and pharmacology at Penn's Perelman School of Medicine, found mice with broken clock genes - those that drive and suppress appetite in the hypothalamus - shifted about 20 percent of their caloric consumption into their rest period. Eating off their circadian schedules caused the mice to bulk up in weight.
Calling the rodent findings "intriguing," FitzGerald says it's "too early to instruct people about when they should eat; we need to do the clinical studies, which are still few in number and essentially fragmentary." But "timing seems an important factor and the mouse data raises plenty of questions that we should address in clinical trials."
In a second paper published this year in Obesity, Scheer and colleagues showed the specific influence of our body clock drives hunger to be stronger in the later hours of the day.
In the study, 12 participants lived on a 20-hour wake/sleep cycle that systematically scheduled their fasting and eating periods across their internal circadian cycles. The result showed a peak in hunger and appetite corresponding to about 8 p.m.
"The way the circadian rhythms can be seen conceptually is that we may eat our largest meal at the end of the waking day because that would help permit and sustain an overnight fast while we sleep," says Scheer.
He speculates that this cycle may have been beneficial in times when evolutionary pressures were taxing, and people weren't living, as now, in a state of nutritional excess.
"This fits into the concept of a healthy physiological system. But the flip side of the coin may result in an increased caloric intake in the evening, which may lead to increased body mass."
Underlying all of this research is a paper published in 2009 by scientists at the Northwestern University Department of Medicine showing that nocturnal mice fed a high-fat diet during their normal 12-hour resting period gained significantly more weight than those fed during their 12-hour waking phase.
Deanna Arble, a postdoctoral fellow at the Metabolic Diseases Institute at the University of Cincinnati and the paper's lead author, stresses that, "considering the timing of food intake is an important factor in metabolic homeostasis - this ranges from body weight and fat mass to healthy glucose management."
"Abnormally timed meals (ones that occur at a time that conflicts with the circadian control drive to eat) result in an increased risk for weight gain and glucose intolerance." She suggests it's possible eating at the wrong time of day "can be a direct consequence of lowering energy expenditure."
Even more intriguing, Arble notes that timing could be a critical component that determines the ultimate fate of a calorie.
"Consumed at one time, this calorie may be sequestered to fat; consumed at another, it may be quickly utilized for daily function."
"This doesn't mean that you can lose weight by eating 15 cheeseburgers a day but eating them at the 'right time,' " Arble notes. "But pairing this advice with smart food choices and exercise may aid weight loss."