Skip Dinner? Evening Fast May Burn Fat

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Looking to burn more fat? You could give fasting a try, according to results from a preliminary study.

The study found that when participants consumed all of their calories within a 6-hour window, from 8 a.m. to 2 p.m., they burned 6 percent more fat and had more stable hunger levels than participants who consumed the same amount of calories within a 12-hour window, from 8 a.m. to 8 p.m.

"It kind of makes sense," said Courtney Peterson, lead author of the study and an assistant professor in the Department of Nutrition Sciences at the University of Alabama at Birmingham. "Your body's fat-burning ability peaks after you've been fasting for 12 to 14 hours."

For up to 12 hours after the start of a fast, the body is still burning glycogen, a molecule that stores glucose (or sugar). After 12 hours, the body begins burning fat stores, Peterson said. [Dieters, Beware: 9 Myths That Can Make You Fat]

However, Peterson cautioned that burning 6 percent more fat did not meet the researchers' criteria for a meaningful difference between the groups. That means the difference could have been due to chance, and in scientific terms, the finding was not "statistically significant." But this could also be because the study was too small to show a meaningful difference between the groups, Peterson said. So, a larger study would be needed to confirm these preliminary findings, she said.

Dr. Alexandra Johnstone, a senior research fellow at the Rowett Institute of Nutrition and Health at the University of Aberdeen in Scotland, who was not involved with the study, noted that any differences in fat-burning seen in the study are likely because people in the fasting group went all evening and night (18 hours) without eating. But these differences should not be taken to mean that eating food like carbs after a certain time can lead to increased fat production, she said.

In the study, which was presented at the Obesity Society Annual Meeting in late 2016, 11 overweight individuals ages 20 to 45 years took part in two different weeklong phases of the experiment. For one trial, participants would begin, on day 4 of the week, to consume all of their calories between 8 a.m. and 2 p.m. In the control trial, participants would, also on day 4, start consuming all of their calories between 8 a.m. and 8 p.m. Each participant took part in both trials.

While the study found no difference in weight loss between the two trials, Peterson said that there was a nearly 13-hour period, mostly at night, when fat-burning levels were elevated in participants who ate their calories within the 6-hour window.

Besides the increased fat-burning, the study also found that hunger levels were more stable for participants who ate their daily calories within the 6-hour window versus the 12-hour window. Researchers used an arbitrary 100-point scale and asked participants to rate their hunger, with a score of 100 being the highest and 0 the lowest, in each phase of the trial. Results showed that, while average hunger levels were the same for both groups, the group eating all calories within the 6-hour window had hunger levels that varied by 12 fewer points than the group eating all their calories within the 12-hour window.

This may be because those waiting until later in the day to consume their dinner meal still had a third of their calories left to consume, Peterson said. "That might set you up more for binges or unhealthy eating than if you've already eaten all of your food for the day," Peterson said, though she added that more research is needed to confirm this hypothesis.

For people hoping to try fasting, Peterson recommended starting with a 9-hour window of food consumption and gradually working that down to 8, 7 and finally 6 hours.

However, this type of fasting should be avoided for children and women who are pregnant, Peterson said. That's because fasting slows down the rate that cells are dividing, which could harm growing children or fetuses, Peterson noted. She also emphasized that anyone with a major chronic disease would need to talk to a doctor before attempting a fasting regimen. Finally, whether such fasting helps with long-term weight loss is still unknown, the researchers said.

Peterson said she hopes to not only repeat the study with a larger sample size, but also test whether time of day affects fat-burning levels. For instance, she might compare participants who eat their calories in a 6-hour window in the morning against those who do so in a 6-hour window in the evening.

Originally published on Live Science.

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