Even though eating snacks after dinner has long been linked to an increased risk of obesity, less is known about exactly why consuming food too close to bedtime might cause people to gain weight. For the new study, scientists set up a lab experiment to see how changing when people ate — but not what they ate — might impact three factors that can play a role in body weight and the risk of obesity: food cravings and appetite, the ability to burn calories, and the molecular composition of fat tissue. “We wanted to test the mechanisms that may explain why late eating increases obesity risk,” says senior author Frank Scheer, PhD, director of the Medical Chronobiology Program in the Brigham and Women’s Hospital Division of Sleep and Circadian Disorders in Boston. “Previous research by us and others had shown that late eating is associated with increased obesity risk, increased body fat, and impaired weight loss success,” Scheer says. “We wanted to understand why.” For the study, 16 overweight and obese adults participated in two experiments requiring them to live in a lab for several days. During one stint in the lab, they consumed three meals around 9 a.m., 1 p.m., and 4:30 p.m. Then, during the other lab stay, they ate exactly the same meals on a later schedule starting at 1 p.m. and ending at 8:30 p.m. Scientists asked participants to follow strict sleeping schedules and eat identical diets on the same schedule for three weeks before each lab stay. When people were in the lab, they ate the same foods, documented their hunger cravings, provided regular blood samples, and did body-temperature and energy-expenditure assessments. Researchers also collected samples of what’s known as adipose tissue — the dangerous fat that accumulates around our midsection — from several participants to look at how the different eating schedules might impact gene expression in this tissue. The later eating schedule doubled the odds of being hungry and caused people to crave more food overall, according to results published in Cell Metabolism. Eating later also more than doubled cravings for starchy foods and meat in particular, and increased salt cravings by 80 percent, the study found. When people ate later, they also had significantly lower levels of the hormone leptin, which signals satiety. In addition, people on the later eating schedule burned 5 percent fewer calories during the day than they did when they followed the earlier eating schedule. The analysis of adipose tissue samples also found genes expressed that promote fat growth when people ate later in the day. While the study was a controlled experiment designed to help determine whether eating later might directly contribute to weight gain, there were still some limitations to the study. The biggest drawbacks were its small size and the relatively short period of time scientists followed the participants. Another limitation is that people were not given unlimited access to food in the lab — like they have in real life — making it impossible to know how much the timing of meals might directly impact the total amount of calories people might consume. Tightly controlled lab conditions with regulated amounts of sleep, light exposure, calories, and physical activity helped scientists isolate the impact of meal timing on food cravings, but these conditions aren’t the same as what happens when people leave the lab. “In real life, many of these factors may themselves be influenced by meal timing,” Scheer says. “In larger-scale studies, where tight control of all these factors is not feasible, we must at least consider how other behavioral and environmental variables alter these biological pathways underlying obesity risk.”
