Intermittent fasting may be no more effective than traditional forms of dieting and may even reduce lean muscle mass, according to a randomized clinical trial published in JAMA Internal Medicine.
With various forms of alternative diets sweeping nations everywhere, intermittent fasting has quickly risen to the foreground of diets that supposedly affect your metabolism. There are many variations, but all include periods of fasting followed by allocated time to indulge in food of your choice. Some use the diet as a tool to make them more aware of when they are hungry, while others claim it boosts their body's ability to react to calorie intake.
As of 2020, intermittent fasting has become one of the most-followed diet plans in the USA, according to the International Food Information Council (IFIC). However, hard evidence of exactly how successful the diet is and the full mechanisms behind it has been lacking. So researchers from the University of California San Francisco attempted to find out.
The team assembled a clinical trial consisting of 116 adults with a body mass index (BMI) between 27 and 43 and randomly assigned them to either three meals a day or an intermittent fasting routine. The intermittent fasting group could eat to their heart’s content between 12:00pm and 8:00pm, but not outside that time frame. On the flip side, the other group had a structured diet of three meals a day. This group was recommended the time at which to eat, but not what to eat – both groups consumed approximately the same calories daily.
After monitoring the participants for three months, the team measured their weight loss, fat mass, and lean mass among other metrics to identify differences between the groups. Inspecting the results, there was no significant difference between the weight lost by the fasting group compared to those that ate three meals a day. Whilst the fasting group did lose weight, it was a very small amount at just ~0.2 lbs per week.
Furthermore, there was an unexpected result – those participating in intermittent fasting had lost more lean muscle mass compared to the other group. According to Inverse, the results prompted co-author Ethan Weiss to stop intermittent fasting altogether, a diet he has been following for the past 7 years.
"No matter how you look at it, time-restricted eating resulted in very modest weight loss,” Weiss told Inverse. "It did not offer any other metabolic advantage. And then there was the concerning signal over the loss of lean mass."
However, the jury is still out on intermittent fasting, as some previous studies have demonstrated the benefits of the diet, while others align with the results seen here. The study also doesn't take into account other variations of intermittent fasting, such as increased fasting length or the time of day. Three months is also considered short for weight loss research, making it difficult to draw general conclusions. Alongside this, protein and water intake was not accounted for, which could have altered the lean muscle mass between cohorts.
The authors hope that further study with these considerations in mind will clarify whether it is the fasting or a different reason that led to the muscle loss.