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Study: Intermittent Fasting Is Not All That Effective for Weight Loss

Don't eat, lose weight. The idea seems simple enough, but a study finds both the type and quantity of weight loss leaves a lot to be desired.

Intermittent fasting is all the rage these days. What better way to lose weight than to fast longer between meals and therefore eat less? This may seem like a logical, if uncomfortable, path to weight loss, but new research finds that you’re probably losing as much muscle as you are fat — and even with that your losses are nothing to brag about. In the end, good old-fashioned calorie restriction is still likely your best bet for getting in fighting shape.

“Many people believe that diets based on fasting are especially effective for weight loss or that these diets have particular metabolic health benefits even if you don’t lose weight,” said James Betts, Director of the Centre for Nutrition, Exercise & Metabolism at the University of Bath, who led the research. “But intermittent fasting is no magic bullet and the findings of our experiment suggest that there is nothing special about fasting when compared with more traditional, standard diets people might follow.”

Celebrities such as Jack Dorsey, Jimmy Kimmel, Jennifer Anniston, and Chris Pratt have credited intermittent fasting for weight loss and maintaining their bods. Betts’ team aimed to put that to the test by comparing a regular calorie restriction diet to two types of intermittent fasting. They found that the normal diet led to more weight loss, even when the number of calories eaten while intermittent fasting was the same, according to the study of 36 people.

The three diets tested were:

  • 1) Intermittent fasting with calorie restriction: Alternate between one day of fasting and one day where you eat 50 percent more calories than usual.
  • 2) Normal calorie restriction diet: Eat 25 percent less calories than normal every day (The total number of calories is the same as in the first diet.)
  • 3) Intermittent fasting without calorie restriction: Alternate between one day of fasting and one day where you eat 100 percent more calories than usual.

Participants who followed the normal diet for three weeks lost an average of 1.9 kg, or 4.2 lbs. Those on the calorie-restriction fasting diet lost 1.6 kg, or 3.5 lbs. The third group did not lose a significant amount of weight.

More striking than the overall difference in weight loss was the type of weight loss. The normal diet group lost their weight almost entirely by shedding fat. But the fasting diet group lost about half of their weight in fat and half in muscle mass.

“Most significantly, if you are following a fasting diet it is worth thinking about whether prolonged fasting periods is actually making it harder to maintain muscle mass and physical activity levels, which are known to be very important factors for long-term health,” Betts said.

This study isn’t the be-all and end-all on the intermittent fasting debate. It was small, and all of the participants were considered lean with a BMI of 20-25. Because the trial was so intensive, it only lasted for three weeks, but the effects of the diets could be different in the long run. Still, the study does suggest that although intermittent fasting can cause weight loss, it’s probably not the best way to drop unwanted pounds.

Not everyone who tries intermittent fasting does it for weight loss, and not all versions of the diet involve calorie restriction. There are other potential health benefits of limiting when you eat, such as mental clarity and reduced inflammation, but the evidence is still thin.

“I think the evidence continues to show that in well done large studies that last a decent amount of time that the amount of improvement in weight, and or metabolic health, is minuscule, relative to other diets,” Ethan Wiess, a researcher, and cardiologist at the University of California, San Francisco, who was not involved with the study, told Inverse.

Weiss used to intermittent fast himself, but after conducting his own 2020 study on the subject that showed no benefits of intermittent fasting over eating three square meals a day, he stopped.