Research in human volunteers has found that keeping mealtimes in tune with the body’s natural circadian rhythms could help weight loss, not by increasing energy expenditure, but by reducing appetite. The results, reported in Obesity, showed that sticking to an early time-restricted feeding (eTRF) regimen—a meal timing strategy that involves intermittent daily fasting—was linked with reduced levels of the appetite hormone, ghrelin, and increased levels of the satiety hormone, PYY. Study participants also reported more “even-keeled” hunger, increased feelings of fullness, and less inclination to eat.

The researchers say the study is the first to show how meal timing affects 24-hour energy metabolism under conditions of matched food intake and meal frequency. “We suspect that a majority of people may find meal timing strategies helpful for losing weight or to maintain their weight since these strategies naturally appear to curb appetite, which may help people eat less,” suggested research lead Courtney M. Peterson, PhD, an assistant professor in the department of nutrition sciences at the University of Alabama at Birmingham. “Coordinating meals with circadian rhythms, or your body’s internal clock, may be a powerful strategy for reducing appetite and improving metabolic health,” added Eric Ravussin, PhD, associate executive director for clinical science at Louisiana State University’s Pennington Biomedical Research Center, and first author of the researchers’ published paper, which is titled, “Early Time-Restricted Feeding Reduces Appetite and Increases Fat Oxidation But Does Not Affect Energy Expenditure in Humans.”

Studies in humans and rodents indicate that eating at times that are out of sync with the body’s circadian rhythms—essentially “during the biological night”—promotes weight gain and metabolic dysfunction, while eating earlier in the day can increase weight loss, the authors wrote. One pivotal human trial designed to investigate any impact on weight loss of “eating breakfast like a king, lunch like a prince, and dinner like a pauper” found that women did lose more weight if they ate a larger proportion of their daily calories as breakfast, compared with those who ate a smaller breakfast, but the same number of overall daily calories.

What isn’t known, however, is whether the effects of meal timing on weight loss are driven by mechanisms that decrease appetite, or that increase calorie burning. Some human studies have suggested that the primary mechanism is decreased appetite, while rodent studies indicate that the primary weight loss mechanism is elevated energy expenditure. To try and clarify this Peterson and colleagues designed what they described as “the first randomized controlled feeding trial to determine whether meal timing affects 24-hour energy expenditure under conditions of matched food intake and meal frequency.”

The trial enrolled 11 participants who satisfied the criteria that they were in good health, aged 20–45 years old, had a body mass index of between 25 kg/m2 and 35 kg/m2, and a bodyweight of 68–100 kg. The volunteers followed one of two mealtime schedules for four days. One group ate their three daily meals at set times over the course of 12 hours, with breakfast at 8 a.m. and dinner 8 p.m. This schedule matched typical American mealtimes. The second group followed an eTRF schedule, which spread the three meals over a 6 hour period, starting with breakfast at 8 a.m. but ending with dinner at 2 pm, so the participants effectively then had to fast until the next morning.

Courtney M. Peterson, PhD, department of nutrition sciences [University of Alabama at Birmingham]
The volunteers followed their mealtime schedules for four days, during which they all went to bed at the same time at night, and got up at the same time in the morning. On day four the participants went into a respiratory chamber for 24 hours, to measure the total amount of calories, carbohydrate, fat, and protein burned. During their time in the chamber they were given three meals, comprising a strawberry yogurt smoothie with whey protein and skim milk, and a peanut butter and jelly sandwich.

Blood levels of hormones including ghrelin, PYY, leptin, and insulin were measured in the evening of day three and on leaving the respiratory chamber in the morning of day five, and the volunteers also answered regular questionnaires about their appetite and hunger levels. After completing either the eTFR or 12-hour mealtime schedule and respiratory chamber testing, they then had a 3.5–5 week break, before the trial was repeated, but this time swapping to the alternate meal schedule.

The results showed that although eTRF didn’t have any major impact on the total number of calories burned over the course of a day, it did reduce levels of ghrelin and was linked with less spiking in hunger. “We found that eTRF lowered mean values of the hunger hormone ghrelin (mostly in the morning) as well as increased levels of the satiety hormone PYY in the middle of the evening,” the investigators stated. “eTRF also tended to decrease the mean desire to eat and increase fullness across the waking day.”

The team stressed that, contrary to expectations, they found no evidence that daily intermittent fasting increased mean appetite levels in eTRF participants, when compared with control volunteers who were eating the same number of calories per day. “Surprisingly and paradoxically, eTRF reduced swings in hunger (i.e., reduced the diurnal amplitude of hunger), making hunger levels more even-keeled throughout the day, which may reduce overeating or binge eating.”

Credit: University of Alabama at Birmingham

The results also indicated that eTRF may increase fat burning over 24 hours, by improving the body’s ability to switch between burning carbohydrates and burning fat, a feature known as metabolic flexibility. “The subset of meal-timing interventions that involve intermittent fasting, such as time-restricted feeding, may confer additional metabolic advantages by improving metabolic flexibility and increasing 24-hour fat oxidation,” the authors wrote, “… which may independently reduce the risk of obesity and type 2 diabetes.” These findings were preliminary, however, and will need validating, as the team acknowledged. “Further research is needed to determine the effects of meal timing on energy and fat metabolism.” Peterson further commented, “Whether these strategies help people to lose body fat need to be tested and confirmed in a much longer study.”