Intermittent Fasting Doesn't Promote Weight Loss, Study Shows

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Key Takeaways

  • As a diet trend, intermittent fasting challenges individuals to eat only during specified periods of time per day.
  • A recent study concluded intermittent fasting does not promote weight loss in individuals considered clinically overweight or obese.
  • However, a closer look reveals this study joins the body of largely inconclusive research regarding the practice's potential for weight loss or health benefits.

Fasting has long been included in humans' spheres of ritual for various reasons. In fact, Hippocrates, the father of modern medicine, often prescribed the practice as a means of healing the body.

While restrictive eating patterns have spiritual and medicinal roots, intermittent fasting has now joined the ranks of trending diet practices. The research to back this up, though, isn't quite conclusive.

A recent study set out to determine the effect of time-restricted eating on weight loss, and concluded intermittent fasting yields the same results as eating throughout the day. But a closer look at the study's design reveals these results shouldn't be considered conclusive either.

The Study

The study, published in JAMA Internal Medicine, observed 141 individuals over the course of 12 weeks in a randomized controlled trial.

Participants included men and women aged 18-64 with a body mass index (BMI) of 27-43, a range considered clinically overweight to obese. They were divided into two groups: time-restricted eating (TRE) and consistent-meal timing (CMT). The TRE group was instructed to eat as much as desired from noon to 8:00 p.m., then "completely abstain from caloric intake" from 8:00 p.m. to noon the next day. The CMT group was instructed to eat three structured meals per day.

During the 12 weeks, researchers recorded change in body weight via bluetooth weight scales, and participants self-reported daily adherence through a custom mobile app.

Of the 141 participants, 116 were included in the study. The TRE group experienced a modest decrease in weight that was nearly the same as the CMT group. Researchers concluded that time-restricted eating did not result in weight loss or cardiometabolic benefits.

Flaws and Further Research

The caloric restriction achieved with intermittent fasting has been linked to improvements in various health conditions and increased lifespan in animals. But when it comes to humans, there's a significant lack of data on any long-term benefits. This study echoes that fact.

Natalie Allen, RD

Like any dieting or weight management trend, if you’re cutting out food that you would normally eat, you’re probably going to lose weight.

— Natalie Allen, RD

Exercise physiologist and Director of Science at Precision Nutrition, Helen Kollias, PhD, notes that methodological issues are common in studies of this sort due to lack of control and identifies a few aspects for critical consideration.

First, the 16-hour fasting period implemented in the study is mild. If participants avoid meals between 8 p.m. and noon, they essentially just skip breakfast, which is common practice for many individuals. While some more rigorous forms of fasting call for 18-24 hours periods in which no calories should be consumed, scientists still don't fully understand the impact of this on health overall.

Another important point to consider: Participants were restricted when it comes to the timing of their eating, but not what they ate or how much. Without tracking caloric intake, Kollias says, it's difficult to discern whether the act of fasting affected weight loss.

“Timing is not going to change much if you find yourself bingeing afterward,” Kollias says.

Adding to the confusion surrounding intermittent fasting in general are celebrities and self-proclaimed "life-hackers" that insist adhering to this diet yields life-changing results. Registered dietitian Natalie Allen has a more realistic approach.

“Like any dieting or weight management trend, if you’re cutting out food that you would normally eat, you’re probably going to lose weight,” Allen says.

Helen Kollias, PhD

Timing is not going to change much if you find yourself bingeing afterward.

— Helen Kollias, PhD

Criticism aside, one success of the study is its indicator of reality. Several participants from the TRE group dropped out. Any diet requires commitment over a period of time in order to see results, and intermittent fasting can require considerable dedication. This major lifestyle change isn't an easy or sustainable transition for everyone.

Should You Try It?

Rather than viewing intermittent fasting as a catchall hack, the practice should fit within a constellation of habits that can together provide lasting benefits.

“For some it’s more a mindset, and they become more mindful of what they’re eating," Allen says. "It just controls their calories, there’s nothing magical about it.”

Understanding the cons of intermittent fasting is an important part of the puzzle, as well. There's strong potential for severe hunger and overeating, as well as increased physical and mental fatigue.

“The brain needs glucose to function and think, and it doesn't store this very readily," Allen says. "So if you’re not eating anything until noon, your body’s going to need time to adapt to that."

Determining whether intermittent fasting is right for you comes down to knowing yourself. What does your daily schedule look like? How active are you? How often do you eat out? Are you likely to follow a strict regimen? The answers to questions like these will help you better understand what's realistic for you.

When done correctly, intermittent fasting is largely considered a safe practice. However, it is not recommended for pregnant or breastfeeding women, individuals under the age of 18, those who have a history of disordered eating or people with certain medical conditions like diabetes.

That being said, until more robust research is presented, a healthy lifestyle is the safest and most effective way to lose weight—and these habits are easier to implement.

“Let’s look at things that we know work," Allen says. "They’re basic, but a lot of people don’t do them.”

To promote healthy weight loss, experts recommend diets rich in fiber, antioxidants, and lean protein at every meal; adequate portion sizes and hydration; regular exercise, and prioritizing gut health via pre- and probiotics.

What This Means For You

Research on the benefits of intermittent fasting remains uncertain, and the diet takes considerable commitment. If you're working toward a weight-loss goal, focus your energy on tried and true methods like regular exercise, proper nutrition and ample hydration.

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5 Sources
Verywell Fit uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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