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- A new study found that a type of intermittent fasting that limits meals to early in the day could lower appetite and help the body burn fat.
- Participants who ate between 8 a.m. and 2 p.m. had lower levels of the hunger hormone ghrelin and burned more fat than those on a normal meal schedule.
- The study’s small sample size means more research is needed to understand intermittent fasting’s potential benefits for weight loss and metabolic health.
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Intermittent fasting is the latest in a long line of dieting trends, and new research suggests that a version of the eating style that matches your meal times to your body’s natural rhythms may help you burn fat and eat less while feeling fuller.
In the study, published Wednesday in the journal Obesity, researchers tracked 11 overweight men and women, ages 20 to 45, over four days on two meal-timing plans.
One plan limited participants to eating from 8 a.m. to 2 p.m. This window of time was chosen because it combines two types of meal-timing strategies: fasting for at least 14 hours a day, and eating earlier to align with the circadian rhythm, or the body’s internal clock. The six-hour window is also suspected to be the smallest eating window that people can sustain in the long term.
The control group ate according to “typical American meal times,” at approximately 8 a.m., 1 p.m., and 8 p.m. All participants tried both meal plans, in a random order, with about a month in between, and ate the same amount of food on both plans. They were then tested through blood samples, urine tests, and a respiratory chamber that measures energy use so the researchers could look at how each eating schedule affected metabolism.
The researchers found that when participants fasted, they had a decreased appetite – both in self-reported surveys of how hungry they felt and in measurements of the body’s level of ghrelin, a hormone known to stimulate appetite. The limited meal times also prompted their bodies to burn more fat in a 24-hour period.
The fat-burning finding was a surprise to researchers, Courtney Peterson, an author of the study and a professor of nutrition at the University of Alabama, told INSIDER. Her research team intended to better understand why people who try intermittent fasting tend to lose weight.
“What we think this means is that when you lose weight you burn more body fat but keep muscle mass,” she said.
A follow-up study will look at this fat-burning potential, Peterson said, and more research is needed to investigate how fasting might affect muscle mass.
These results suggest that intermittent fasting could work for weight loss by reducing appetite, not by burning more calories
Previous research in animals has suggested that fasting can boost metabolism and burn more daily calories, which was believed to be the reason for its weight-loss benefits.
This study, however, found that one of its primary effects was a lower appetite. In conjunction with the fat-burning effect, this could be the reason for intermittent fasting’s weight-loss potential.
“Maybe people think there’s some miraculous property, that it really revs up your metabolism, but we didn’t find that to be the case,” Peterson said.
More research is still needed to understand intermittent fasting’s potential benefits
Though this study did not directly measure weight loss, its results are part of a growing body of research has touted intermittent fasting as a strategy for helping people shed pounds.
The very small sample size, short time frame, and careful regulation of the study, however, means more research is necessary to better understand how intermittent fasting works in everyday life.
For instance, most people who try intermittent fasting for weight loss won’t have the benefit of a team of researchers planning their meals and ensuring they eat them (and nothing else). For both plans, participants ate specially prepared meals on days three and four of the study, calculated to meet their energy and nutritional needs based on age, gender, and weight.
“In the real world, if someone did time-restricted feeding and ate less, they might be hungrier than we found in our study,” Peterson said.
Fasting is also not for everyone. People with a history of eating disorders or with health complications like diabetes should be wary about trying to fast for 18 hours at a time, and some fasts are more sustainable than others.