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Intermittent Fasting Less Effective at Burning Body Fat Than Daily Dieting

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A new study suggests that the popular dietary technique intermittent fasting (IF) is less effective that traditional daily dieting at reducing body fat.

What is intermittent fasting?

IF has become a well-known dietary plan in the Western world. The calorie restriction technique has been boosted by celebrity practitioners ranging from the Tonight Show host Jimmy Kimmel to Twitter CEO Jack Dorsey. IF is an umbrella term for various planned eating schedules: some routines dictate not eating between certain hours of the day, while the 5:2 diet, popularized over the last decade, mandates a heavily restricted diet for two days of the week.

Despite IF’s popularity, its health benefits remain unclear. Advocates suggest that IF should stimulate evolutionarily conserved mechanisms that could help burn fat. Studies conducted in mice have shown clear metabolic benefits from restricted food availability in certain periods. Small studies in humans have repeatedly shown that improvements to insulin sensitivity, a metabolic marker that can protect against diabetes, can be realized with just a few weeks of IF. Nevertheless, a review article from the National Institute on Aging points out that large scale studies of fasting remain scarce and that calorie restriction diets should be clinically monitored.

In a new study, researchers led by the University of Bath’s Professor James Betts examined the effects of a type of IF called alternate day fasting, where calorie intake is reduced every other day. They wanted to discover whether alternate day IF would have health benefits in humans, and, importantly, whether those benefits would go above and beyond any improvements seen through normal daily calorie restriction. To do so, they designed a first-of-its-kind trial.

The authors recruited study participants who were all lean and of healthy weight – most IF studies look at people who are overweight. For three weeks, 36 volunteers were separated into three equal groups:

  • One group undertook IF eating, with 24-hour fasts followed by days in which they were asked to eat 150% of their usual calorie intake
  • A second group followed a traditional daily dieting regime, eating 75% of their normal calorie intake every day (matching the first group for overall energy consumption)
  • A final group fasted for 24 hours and then ate 200% of their normal calories on the second day, resulting in no overall calorie reduction

The three groups were monitored for their body mass, fat mass and metabolomic performance. The daily energy restriction group lost weight, on average 1.91 kg. This was almost entirely due to loss of fat, which decreased by 1.75 kg (which translated to a 1.8% body fat reduction). While the body mass of the second group, who reduced their overall calories in an alternate fasting regimen, also dropped, just 0.74 kg of their average 1.6 kg weight loss was through fat loss (a 0.6% reduction in body fat). The group who practiced alternate fasting, but did not reduce their overall calorie intake, unsurprisingly lost neither body weight nor fat. The three groups’ cardiometabolic measures, which included assessments of gut hormones, showed no differences between groups.

So why might IF be a less efficient way of burning fat? On average, the group that fasted and reduced their total energy intake also reduced their low-to-moderate physical activity levels over the three-week period, meaning they somewhat offset their reduced energy intake with lower calorie burning. They suggest that maintaining energy expenditure should be a critical part of IF dieting.

One mystery of this study’s findings is the lack of additional change among cardiovascular and metabolic health measures in the fasting participants compared to regularly dieting individuals. Supposedly, fasting-induced changes have been noted repeatedly in other studies. The authors think the most likely explanation is that their participants were too healthy at the start of the study to record any significant improvement to these measures, whereas previous studies have used overweight participants.

Finally, they speculate that alternate-day fasting might not entrain the body’s circadian rhythms to produce fasting responses in the same way that an everyday time restriction fast may do. There still could be a place, the authors suggest, for this kind of diet for people who find daily dieting to be unsustainable. “Given that human physiology operates to defend against the desired energy deficit that is the objective of dieting, there may still be some value in deliberately misaligning and, thus, impairing that natural protective response via a more erratic fasting schedule, perhaps in overweight or obese individuals for whom there is more excess body fat to lose or preexisting health conditions to rectify,” they write. 

Meet the Author
Ruairi J Mackenzie
Ruairi J Mackenzie
Senior Science Writer