Life Wellbeing We know intermittent fasting burns calories – now we know why
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We know intermittent fasting burns calories – now we know why

Intermittent fasting
A new study has found just why intermittent fasting is effective. Photo: Getty
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Researchers, in a small but persuasive study, have discovered the mechanism that leads to weight loss in people who use meal-timing strategies such as intermittent fasting or eating earlier in the daytime.

Prior research has been conflicted. Studies in rodents suggested such strategies burn more calories, but data from human studies was mixed – some studies suggested the strategies increase calories burned, but other reports showed no difference.

A new report finds they primarily work by lowering appetite, which in turn may lead to more fat being burnt, on average, over a 24-hour period.

The study is the first to show how meal-timing affects 24-hour energy metabolism when food intake and meal frequency are matched.

But hang on? Timing your meals won’t burn more calories – but overall burns more fat? Aren’t they the same thing?

Well, no. Fat and calories aren’t the same thing. Calories are a unit of energy, which can come from carbohydrates, protein, or fat.

When you’re working your body hard, calories are mainly burnt from carbohydrates stored in your muscles. When you’re awake and ordinarily active in a fasted state, the calories are burnt from stored fat.

The researchers found that early time-restricted feeding – 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 said, however, the results on fat-burning are preliminary.

“Whether these strategies help people lose body fat need to be tested and confirmed in a much longer study,” Dr Courtney M. Peterson said, lead author of the study and an assistant professor in the Department of Nutrition Sciences at the University of Alabama at Birmingham.

Study participants still ate three meals a day – just within different timeframes. Photo: Getty 

For the study, Dr Peterson enrolled 11 adult men and women who had excess weight. They were adults, in general good health, aged 20 to 45 years old and were eligible to participate if they had a body mass index between 25 and 35, body weight between 68 and 100 kilograms, a regular bedtime between 9.30pm and midnight, 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 with breakfast at 8am and dinner at 8pm, and an early time-restricted feeding schedule where participants ate three meals over a six-hour period with breakfast at 8am and dinner at 2pm.

The same amounts and types of foods were consumed on both schedules.

There was a 12-hour fasting period for the control schedule, while the early time-restricted feeding schedule involved fasting for 18 hours a day.

The different schedules were followed for four days.

Researchers rated the metabolisms of participants by placing them in a respiratory chamber where researchers measured how many calories, carbohydrates, fat and protein were burned.

Researchers also monitored appetite levels of participants every three hours while they were awake, as well as hunger hormones in the morning and evening.

Although early time-restricted feeding did not significantly affect how many calories participants burned, the researchers found that it lowered levels of the hunger hormone ghrelin and improved some aspects of appetite.

Researchers monitored appetite levels and hunger hormones throughout the 24 hours. Photo: Getty

It also increased fat-burning across 24-hours.

Dr Peterson said the researchers had gained some insight into daily intermittent fasting (time-restricted feeding), as well as meal-timing strategies that involve eating earlier in the day to be in sync with circadian rhythms. The researchers believe these two broader classes of meal-timing strategies may have similar benefits to early time-restricted feeding.

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

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