Food science

New study reveals an unintended side effect of intermittent fasting

Researchers challenge the assumption that this popular diet regimen promotes healthy weight loss.

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In the ever-evolving pursuit of living longer and healthier, we know diet plays a pivotal role. In recent years, scientists have also focused on the potential of intermittent fasting — a diet regimen that hinges on when you eat, not what you eat.

New research, however, suggests it doesn't deliver on what some intermittent fasters are looking for when they adopt it as an eating pattern.

Growing evidence, primarily in animals and relatively small groups of people, suggests intermittent fasting (sometimes referred to as time-restricted eating) can improve metabolic function, stave off certain diseases, and may even extend lifespan. Based on these findings intermittent fasting — alternating periods of eating normally and fasting — has soared in popularity. In 2019, it was the most Googled diet on the planet.

Intermittent fasting is also billed as a simple strategy to lose weight: No counting calories, complicated recipes, or cutting out certain food groups involved.

However, in a recent randomized clinical trial, one of the most rigorous studies on the eating pattern to date, researchers tracked 116 obese and overweight individuals over three months to see if scientific evidence lives up to the hype.

The team, in turn, discovered that time-restricted eating or intermittent fasting did not help people lose weight significantly more than simply eating three meals a day. Instead, it had an unintended side effect: the loss of lean muscle.

Can intermittent fasting actually help you lose weight?

Ethan Weiss is one of the study's co-authors and a cardiologist at the University of California, San Francisco. His team published their findings published Monday in the journal JAMA Internal Medicine.

Prior to this work, Weiss practiced time-restricted eating for the past seven years. After analyzing these results, he stopped.

"No matter how you look at it, time-restricted eating resulted in very modest weight loss [about .2 pounds per week over 12 weeks on average]," Weiss tells Inverse. "It did not offer any other metabolic advantage. And then there was the concerning signal over the loss of lean mass."

Victoria Catenacci, an endocrinologist and metabolism expert at the University of Colorado, Anschutz Health and Wellness Center, says the study was "nicely executed," but does say that factors like the timing of the eating window, lack of behavioral support, and decreases in physical activity in the fasting group may have impacted the results. Catenacci was not involved in the study.

"This well-designed study demonstrates that late time-restricted eating alone without behavioral support does not result in clinically meaningful weight loss over 12 weeks," Catenacci tells Inverse. She notes that this study is another reminder of the critical need to "rigorously evaluate weight-loss interventions, such as time-restricted eating, before recommending them to the general public."

Catenacci also acknowledges there are still many open questions when it comes to this way of eating, arguing the study doesn't "close the door on time-restricted eating as a weight loss intervention," or as an intervention that can prompt other positive effects, including longevity.

What the study examined — To determine how intermittent fasting, or time-restricted eating, influences weight and metabolism, Weiss and his team recruited a group of 116 individuals who were classified as overweight or obese.

The researchers split the group into two dietary patterns. Half practiced daily time-restricted eating — eating all their meals between noon and 8:00 p.m. then fasting for 16 hours. Only noncaloric beverages were permitted outside of the eating window.

The other half ate three meals a day, including snacks. Neither group was given any instruction about what or how much to eat throughout the day. They also weren't instructed to workout or abstain from exercise either.

Over three months, researchers tracked both group's weight and metabolic outcomes using a mobile app, surveys, and a Bluetooth scale. Each participant weighed themselves daily in the morning before eating or drinking.

The results and the explanation — The study's researchers predicted the fasting group would lose more weight than the three meals a day group. But over 12 weeks, there was no greater weight loss with time-restricted eating compared with the consistent meal group.

There were also no significant observed differences in fat mass, fasting insulin, blood sugar, HbA1C, or blood lipids between the two groups.

"... there is nothing magical about a late time-restricted eating window itself for short-term weight loss."

"The minimal weight loss suggests they did not achieve a significant energy deficit," Catenacci says. "I think this suggests that there is nothing magical about a late time-restricted eating window itself for short-term weight loss."

Over the course of the study, participants in the fasting group lost about three and a half pounds. But the bulk of pounds lost wasn't fat.

Crucially, it was lean mass, including muscle. Typically, about 20 to 30 percent of total weight loss is lean mass. In this study, the proportion of lean mass loss was 65 percent.

"You don’t want to lose lean mass. You want to lose fat mass," Weiss says. "Here we found that two-thirds of weight loss came from lean mass."

There are a number of potential factors driving fasting participants to shed lean mass, one being that it's possible the participants weren't eating enough protein or getting enough physical activity. On average, fasting participants walked about 2,500 fewer steps per day, a shift in activity that could contribute to their unchanged weights and dwindling lean mass.

It's also possible that fasting later in the day didn't curb energy intake that would result in weight loss.

"Preclinical data suggests an early time restricted eating window may have greater metabolic benefits, and practically speaking an early [eating] window may be more likely to result in greater caloric restriction as most people currently consume the majority of their calories later in the day and in the evening," Catenacci explains.

She recommends that, in the future, rigorous trials need to evaluate other intermittent fasting nuances, like eating earlier in the day or combining it with a reduced-calorie diet.

What's next for intermittent fasting — After publishing, Weiss and his team's study prompted immediate discussion, with experts evaluating the results on social media. Some argue the study participants didn't fast enough or early enough in the day to see a real benefit. Others say that to see weight loss results, the eating pattern should be combined with other diets.

Ultimately, the trial shows what many nutrition and exercise studies do: That there's no singular solution or magic strategy to lose weight and live a long healthy life.

"This is just one regimen but it highlights how easy it is to think something works when it doesn’t," Weiss says. "Or maybe more accurately, that anything works."

The findings certainly do not support that there will be some kind of major effect of fasting, Weiss adds.

"Longevity? Who the heck knows," Weiss says. "Weight and metabolic endpoints are hard enough!"

In the meantime, if people do want to incorporate fasting into their routines, experts say a fast isn't an excuse to throw out healthy eating completely.

"As I tell my patients, the point is not to eat whatever you can during the [eating] window, but to use the window as a strategy to reduce total daily calorie intake," Catenacci says. "However, future studies are needed before we can confidently state that time-restricted eating, even with this mindset, is an effective weight loss strategy."

IMPORTANCE: The efficacy and safety of time-restricted eating have not been explored in large randomized clinical trials.
OBJECTIVE: To determine the effect of 16:8-hour time-restricted eating on weight loss and metabolic risk markers.
INTERVENTIONS: Participants were randomized such that the consistent meal timing (CMT) group was instructed to eat 3 structured meals per day, and the time-restricted eating (TRE) group was instructed to eat ad libitum from 12:00 PM until 8:00 PM and completely abstain from caloric intake from 8:00 PM until 12:00 PM the following day.
DESIGN, SETTING, AND PARTICIPANTS: This 12-week randomized clinical trial including men and women aged 18 to 64 years with a body mass index (BMI, calculated as weight in kilograms divided by height in meters squared) of 27 to 43 was conducted on a custom mobile study application. Participants received a Bluetooth scale. Participants lived anywhere in the United States, with a subset of 50 participants living near San Francisco, California, who underwent in-person testing.
MAIN OUTCOMES AND MEASURES: The primary outcome was weight loss. Secondary outcomes from the in-person cohort included changes in weight, fat mass, lean mass, fasting insulin, fasting glucose, hemoglobin A1c levels, estimated energy intake, total energy expenditure, and resting energy expenditure.
RESULTS: Overall, 116 participants (mean [SD] age, 46.5 [10.5] years; 70 [60.3%] men) were included in the study. There was a significant decrease in weight in the TRE (−0.94 kg; 95% CI, −1.68 to −0.20; P = .01), but no significant change in the CMT group (−0.68 kg; 95% CI, -1.41 to 0.05, P = .07) or between groups (−0.26 kg; 95% CI, −1.30 to 0.78; P = .63). In the in-person cohort (n = 25 TRE, n = 25 CMT), there was a significant within-group decrease in weight in the TRE group (−1.70 kg; 95% CI, −2.56 to −0.83; P < .001). There was also a significant difference in appendicular lean mass index between groups (−0.16 kg/m2; 95% CI, −0.27 to −0.05; P = .005). There were no significant changes in any of the other secondary outcomes within or between groups. There were no differences in estimated energy intake between groups.
CONCLUSIONS AND RELEVANCE Time-restricted eating, in the absence of other interventions, is not more effective in weight loss than eating throughout the day.

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