Weight Loss Could Be Boosted by Eating at These Times of the Day, Scientists Discover

Eating earlier in the day appears to decrease appetite and could, therefore, help with weight loss, according to the authors of a small study.

Past studies suggested that our circadian system, or body clock, plays a role in how we burn energy and our appetite. And now it appears eating earlier in the day lines up with these processes, and can aid weight loss. The authors of the new paper published in the journal Obesity wanted to understand whether people lose weight because they are using more energy, eating less, or whether something else entirely is at play.

The study comes as fasting diets, such as 5:2 and 16:8, have gained popularity, although the evidence to prove they are beneficial is relatively thin.

The researchers recruited 10 adults to try a 12-hour and 18-hour fasting schedule, featuring the same sorts of food and amounts, in a random order. The participants were overweight but healthy; had a BMI of between 25 and 35kg/m2; weighed between 68 to 100g; and were aged between 20 and 45. They also fell asleep between 9:30 p.m. to 12 a.m., normally. The meals were 50 percent carbohydrate, 35 percent fat, and 15 percent protein. Each diet lasted four days and was separated by a 3.5- to 5-week washout period.

The first diet was a form of intermittent fasting called early time-restricted feed (eTerf), where the participants ate three meals between 8 a.m. to 2 p.m. Breakfast was at 8 a.m., and dinner at 2 p.m. The team thought a 6-hour eating period was different enough to the control, while being sustainable in the long-term.

The other diet acted as the control, where participants consumed the three meals between 8 a.m. to 8 p.m., or typical American mealtimes.

On day four of each regime, the researchers invited the participants to spend 24 hours in a respiratory chamber, which enabled them to measure how much energy they burned. They also collected blood and urine samples to measure hormone levels, and periodically asked the participants to rate factors including their hunger, desire and capacity to eat, and fullness.

pasta, spaghetti, food, diet, stock, getty,
A stock image of a woman eating pasta. Scientists have investigated the potential benefits of intermittent fasting-style diets. Getty

ETerf appeared to lower levels of ghrelin, a hormone related to hunger, and seemed to help with fat burning. It also appeared to reduce their desire to eat, feel fuller on average.

The authors acknowledged their study was limited in ways including being able to draw blood two times, meaning it was harder to examine metabolic hormones; the diets were only followed for four days which might not be long enough for the metabolism or circadian rhythm to adapt; and the sample size was relatively small and skewed towards men.

Co-author Eric Ravussin, associate executive director for clinical science at Louisiana State University's Pennington Biomedical Research Center in Baton Rouge, commented in a statement: "Coordinating meals with circadian rhythms, or your body's internal clock, may be a powerful strategy for reducing appetite and improving metabolic health."

Courtney M. Peterson, lead author of the study and an assistant professor in the Department of Nutrition Sciences at the University of Alabama at Birmingham, told Newsweek: "Our research suggests that time-restricted feeding and/or eating early in the daytime helps lower appetite and can therefore help people lose and maintain their weight."

"I was surprised that early time-restricted feeding lowered the hunger hormone ghrelin so much! It's a large effect for only four days. I was also surprised that subjective hunger levels were more even-keeled throughout the day, despite the extended daily fasting. I would have expected that people have large swings between being very hungry and very full when they practiced the early time-restricted feeding, but we found the opposite to be true."

Commenting on the consensus among the scientific community on fasting diets, Peterson said: "While studies in rodents mostly report that intermittent fasting is beneficial for health, the studies in people have been mixed. From what we can tell so far, some of the more extreme forms of intermittent fasting—such as alternate-day fasting and alternate-day modified fasting—don't appear to be as sustainable long-term and/or are not any more effective than regular dieting.

"By contrast, and quite encouragingly, most of the studies on time-restricted feeding in people have reported positive results when people practiced time-restricted feeding by eating early in the day (i.e., finishing dinner by mid-afternoon) or the middle of the day (i.e., 10 am – 6 pm; 11 am – 7 pm)," she explained.

Peterson warned studies where participants eat later in the day, skipping breakfast and lunch and eating only dinner, had no benefits or worsened their health. She also stressed such practices aren't safe for pregnant women or children.

Hollie Raynor, professor and interim dean of research in the Department of Nutrition, College of Education, Health, and Human Sciences at The University of Tennessee, Knoxville, who did not work on the paper comment in a statement: "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."

This article has been updated with comment from Courtney M. Peterson.