Published in the Sept. 3 issue of the medical journal Obesity, the study provides the first concrete evidence linking meal timing and weight gain.
Until recently, most scientists believed that weight loss was a matter of simple mathematics: In order to lose a pound of body weight by dieting alone, you must consume approximately 3,500 fewer calories than your body needs to support itself at your current weight and activity level.
Researchers at Northwestern University challenged this theory. Using laboratory animals, they demonstrated that eating during normal sleeping hours promotes more weight gain than would be predicted by caloric intake alone.
Translated to human terms, this means that eating a slice of leftover pizza as a midnight snack could cause you to gain significantly more weight than if you ate the same slice of pizza at 3:00 in the afternoon.
The scientists' interest in the connection between late-night eating and weight gain arose after noting that nightshift workers, whose schedules force them to eat at times that are in conflict with natural body rhythms, tend to be more overweight than their dayshift counterparts. The researchers suspected that simply changing the timing of food consumption might influence weight gain to a greater degree than previously thought.
To test their hypothesis, the researchers fed two groups of mice a high-fat diet. Mice in both groups were allowed to eat as much as they wanted for a period of 12 hours each day.
While one group of mice was allowed free access to food only during normal rodent feeding times, the mice in the other group were offered unlimited access to food only during their normal sleeping hours. There was no significant difference in the amount of physical activity performed by the mice in the two groups.
Although the mice in both groups ate roughly the same number of calories in each 24-hour period, those that dined during their normal sleeping hours gained more than twice as much weight over the six-week study period as those that ate during the hours they were normally awake.
The mice that were allowed ad lib access to food during their normal sleeping times experienced a 48 percent increase in body weight, while those that were provided ad lib access to food during their typical active hours experienced only a 20 percent increase in body weight.
Based on their findings, the Northwestern University researchers speculate that nighttime eating in humans may disrupt the body's biological clock, known as the circadian rhythm, which governs daily cycles of feeding, activity and sleep. Eating at inappropriate times appears to trigger a chain reaction of hormonal and metabolic events in the body that ultimately leads to excess weight gain.
While a midnight raid on the refrigerator can sabotage your efforts to slim down, eating breakfast is associated with weight-loss success. Although some dieters skip the morning meal in an effort to cut calories, scientific evidence suggests that people who eat breakfast each morning tend to be thinner than those who don't.
In a study published in the American Journal of Clinical Nutrition, researchers at Vanderbilt University found that when dieters ate breakfast regularly, they lost significantly more weight than those who routinely missed the morning meal.
The study included 52 moderately obese women who were placed on a calorie-restricted diet and randomly assigned to one of two groups. The women in both groups consumed an equal number of calories each day, but one group ate breakfast, while the other did not.
Women assigned to the breakfast-eating group lost an average of 19.6 pounds in three months, while those assigned to the breakfast-skipping group lost an average of 13.6 pounds. The women who ate breakfast reported experiencing significantly less hunger throughout the day.
In addition to eating fewer calories at lunch and dinner, the breakfast-eaters were less likely to engage in mindless snacking behaviors, including plundering the pantry before bedtime.
If you're determined to slim down, cutting back on the number of calories you consume is a great place to start. To make the most of your weight-loss efforts, it's important to remember that when you eat may be just as important as how much you eat.
Rallie McAllister, M.D. is a family physician, speaker, and founder of www.MommyMDGuides.com, a Website featuring child-raising tips from trusted doctors who are also moms.