It’s not a revelation that late-night snacking on foods high in salt, fat and sugar can pack on the pounds. Obesity affects about 42 percent of the U.S. population, and evening snacks is a favorite pastime. Late-night eaters tend to favor high-calorie processed foods rather than vegetables, but is it just what you eat or the time it’s eaten that affects weight gain? Researchers recently sought out some answers.

A new study from the Brigham and Women’s Hospital Medical Chronobiology Program has discovered how and why evening eating helps us gain weight in a rigorously controlled, randomized crossover trial comparing early and late eating. In this type of trial, all the people in the study receive the same treatment or intervention, but they receive it at different times.

Body Mass Indexes and Sleep Cycles

The researchers investigated the effects of late-night eating on three key factors for gaining weight: the regulation and intake of calories, number of calories burned and changes in fat cells that favor increased storage of fat. The study not only confirms that when you eat impacts all these factors, it also reveals some of the reasons why.

The results of the study are published in the journal Cell Metabolism. The study included 16 patients with a body mass index (BMI) putting them in the range of overweight to obese. During the first three weeks of the study, all the patients maintained regular sleep and wake cycle hours. Other factors, such as light exposure and the amount of physical activity, were also kept consistent or nonexistent. In the last three days before the diet phase of the study, all the patients were on the same diets and were eating at the same time.

After this final lead-in period, patients were randomly assigned to an early or late meal schedule for the day. The only difference between the late meals and the early meals was a four-hour delay.

Why Is Late Eating Bad for Us?

During the laboratory diet, all the patients reported how hungry they were, provided frequent blood samples, and had their body temperature and energy expenditure documented. To measure the effect of the diet on fat cells, the research team took small fat tissue biopsies. After the diet ended, there was a wash-out period of several weeks before the patients repeated the study protocol again, but with the opposite-timed diet (so that each patient did both the early and late meal schedules).

These were the key results…

  • Late-eating patients reported twice as much hunger as the early eaters.
  • Blood tests found appetite hormones were increased in the late eaters, indicating increased hunger.
  • Late eaters burned fewer calories and used less energy.
  • Late eaters had lower body temperatures, indicating less energy expenditure.
  • Genetic study of fat cells that were sampled suggests that fat cells of the late eaters broke down less fat and stored more fat.

Taken together, these findings support early eating for prevention of obesity. The findings also may explain why late eaters have an increased risk for obesity. The research team plans to continue their studies with larger numbers of patients and to study what effect that time between last meal and sleep has on obesity. Stopping food intake earlier rather than later in the day may be one way to improve both prevention and treatment of obesity.

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