Researchers have discovered that meal-timing strategies such as intermittent fasting or eating earlier in the daytime appear to help people lose weight by lowering appetite rather than burning more calories, according to a study published in the August issue of the journal Obesity. The study is the first to show how meal timing affects 24-hour energy metabolism when food intake and meal frequency are matched.

“Coordinating meals with circadian rhythms, or your body’s internal clock, may be a powerful strategy for reducing appetite and improving metabolic health,” asserts Eric Ravussin, PhD, one of the study's authors and associate executive director for clinical science at Louisiana State University’s Pennington Biomedical Research Center in Baton Rouge.

“We suspect that a majority of people may find meal-timing strategies helpful for losing weight or maintaining their weight, since these strategies appear to naturally curb appetite, which may help people eat less,” explains Courtney M. Peterson, PhD, lead author of the study and an assistant professor in the Department of Nutrition Sciences at the University of Alabama at Birmingham.

Peterson and her colleagues also report that meal-timing strategies may help people burn more fat on average during a 24-hour period. Early time-restricted feeding (eTRF)—a form of daily intermittent fasting where dinner is eaten in the afternoon—helped to improve people’s ability to switch between burning carbohydrates for energy to burning fat for energy, an aspect of metabolism known as metabolic flexibility. The study’s authors are quick to point out, however, that the results on fat-burning are preliminary. “Whether these strategies help people lose body fat needs to be tested and confirmed in a much longer study,” said Peterson.

The Study

For the study, researchers enrolled 11 adult men and women, ages 20 to 45 years, who had excess weight and were in general good health. Specifically, participants were eligible to participate if they had a body mass index between 25 and 35 kg/m² (inclusive), body weight between 150 to 220 lb (68 and 100 kg), a regular bedtime between 9:30 p.m. and 12 a.m., and for women, a regular menstrual cycle.

Participants tried two different meal-timing strategies in random order: a control schedule where participants ate three meals during a 12-hour period between 8:00 a.m. and 8:00 p.m. and an eTRF schedule where participants ate three meals over a six-hour period between 8:00 a.m. and 2:00 p.m. The same amounts and types of foods were consumed on both schedules. Fasting periods for the control schedule included 12 hours per day, while the eTRF schedule involved fasting for 18 hours per day.

Study participants followed the different schedules for four days in a row. On the fourth day, their metabolism was measured by placing them in a respiratory chamber where researchers measured how many calories, carbohydrates, fat and protein were burned. Researchers also assessed the appetite levels of participants every three hours while they were awake and measured hunger hormones in the morning and evening.

The Results

Although eTRF did not significantly affect how many calories participants burned, the researchers found that eTRF did lower levels of the hunger hormone ghrelin and improved some aspects of appetite. It also increased fat-burning over the 24-hour day.

“By testing eTRF, we were able to kill two birds with one stone,” says Peterson, adding that the researchers were able to gain some insight into daily intermittent fasting (time restricted-feeding), as well as meal-timing strategies that involve eating earlier in the daytime to be in sync with circadian rhythms. The researchers believe that these two broader classes of meal-timing strategies may have similar benefits to eTRF.

Nutrition expert Hollie Raynor, PhD, RD, LDN, who was not associated with the research, finds this research promising, particularly for those who are trying to lose weight. “This study helps provide more information about how patterns of eating—and not just what you eat—may be important for achieving a healthy weight,” she says. Raynor is a professor and interim dean of research in the Department of Nutrition, College of Education, Health, and Human Sciences at The University of Tennessee, Knoxville.

Peterson and colleagues say that prior research was inconsistent on whether meal-timing strategies help with weight loss by helping people burn more calories or by lowering appetite. Studies in rodents suggest such strategies burn more calories, but data from human studies were conflicting—some studies suggested meal-timing strategies increase calories burned, but other reports showed no difference. However, the authors of the current study assert that previous research did not directly measure how many calories people burned or had other methodological problems.

What Does the Research Mean to Health and Exercise Professionals?

While it is outside your scope of practice to recommend specific meal-timing strategies or to urge your clients to try intermittent fasting, you can provide them with the information and sources of research they need to make informed decisions about things that affect their health and well-being. Knowledge is power, particularly when it comes to making choices about what to eat and when. Of course, even the most effective meal-timing strategies won’t work for weight loss if an individual is consistently overconsuming calories. Encourage your clients to thoughtfully consider what they consume and when, and to identify which patterns and habits help them feel most healthy and energetic. Ultimately, these are the strategies that will prevail and produce the greatest benefits over time.