Does Intermittent Fasting Really Promote Weight Loss?

One study seems to show that fasting isn't everything we'd hoped.

intermittent fasting
Credit: Photo by Jonathan Pielmayer on Unsplash

Intermittent fasting has been touted high and low as the ideal anti-inflammatory diet; we even did a deep dive into the ins and outs of intermittent fasting (and busted a few intermittent fasting myths). While the practice exists in many forms, the basic principle never wavers: restricting eating to a small window to reap purported benefits including improved health, clearer skin, better sleep, and more. So when a recent study seemed to negate these benefits, the health and wellness world was all aflutter, with the New York Post claiming the “Much-hyped intermittent fasting diet” was “bunk.”

But reporters may have jumped the gun when slamming the practice entirely, according to some experts.

The Facts About Intermittent Fasting

The recently published study was led by researchers at UC San Francisco and was published in JAMA Internal Medicine. It examined one of the most popular forms of intermittent fasting: restricting eating to eight-hour windows separated by 16 hours of fasting.

The study included 116 men and women with a BMI between 27 and 46. It found that those who were randomly assigned to follow the practice lost an average of 2 pounds per day, while those who did not follow the practice lost 1.5 pounds per day. Despite the discrepancy, the study found no significant difference between the two groups in total fat mass, lean mass, fasting insulin, or resting energy expenditure. Some even experienced negative effects of fasting, including lean muscle loss. The study concluded that this practice was not effective on its own as a means of losing weight or improving key metabolic markers.

Corresponding author Ethan J. Weiss, MD, who has been following the practice for years, notes that he was “very surprised” with the results, noting that “we did not see important changes in any circulating biomarkers.” Previous studies, mainly on mice, had promised benefits ranging from weight loss to longevity.

“I am not recommending [intermittent fasting] now,” he says, “but I am going to keep studying it.”

What This Means for Intermittent Fasting

But this study does not fully negate the benefits of the practice. 

Firstly, some of the weight loss benefits of intermittent fasting stem from calorie restriction: fewer opportunities for eating during the day naturally contribute to a lower calorie intake overall. The study did not account for this, seeing both groups consume similar calorie intakes.

Secondly, weight loss is far from the only benefit of intermittent fasting. In fact, according to fasting fan health and fitness expert Drew Manning, author of “Fit2Fat2Fit,” “fasting is not the best tool for weight loss.” Instead, fasting proponents enjoy myriad secondary benefits, including reduced blood sugar, aid in cellular repair, improved digestion, and improved heart health.

“A large body of laboratory and clinical research shows that intermittent fasting, when combined with a healthy diet, can confer biochemical benefits that support our body’s health defenses,” explains William W. Li, MD, physician and author of the New York Times bestseller “Eat to Beat Disease: The New Science of How Your Body Can Heal Itself.” “These benefits have ranged from suppression of cancer development by cutting off the blood supply to tumors to regeneration of immune cells to improving the gut microbiome.”

Dr. Steven Quay, M.D., PhD., Head of Two COVID-19 Clinical Trials (HOPE Study and AT-301 Nasal Spray), agrees.

“Intermittent fasting has so many other benefits even when ideal weight is achieved, and weight loss is no longer happening,” he says. “Glucose homeostasis, insulin usage, immune stem cell activation, epigenetic changes to genes, etc. are all achieved with TRE [Time-Restricted Eating]. In almost any mammalian species studied, TRE leads to increased, disease-free longevity.”

A study published in the American Journal of Clinical Nutrition even showed that fasting can increase metabolic rate, contributing to sustained weight loss over the long term.

How to Fast Properly

Fasting alone is not enough to reap any benefits of the practice – weight loss or otherwise. And one thing that almost all of our experts noted was that the study did not examine what sorts of foods participants were eating in their feeding windows, something that is an essential piece of any intermittent fasting regime.

“Intermittent fasting is a useful tool to help the body regulate its health defenses,” says Li, “but it has to be combined with other factors such as eating healthy foods during feeding period and controlling the volume of food that is eaten.”

Manning recommends associating the practice with a keto diet, which will facilitate fasting, and embracing a whole food approach.

“Also be open to mixing up your routine so that your body is constantly adapting,” he says.

Tara Stiles, yoga expert and author of “Clean Mind, Clean Body: A 28-Day Plan for Physical, Mental, and Spiritual Self-Care,” highlights another limitation of the study: the imposed eating window from 12pm to 8pm – a restriction purportedly imposed as it’s easier for people to skip breakfast than dinner.

“I’d be interested to see if the same study shifted the time window to include breakfast (with no other suggestions) might have a snowball effect of nudging folks to get to bed earlier, go out to big dinners less frequently, and prepare nourishing foods at home more frequently,” she says. “Any real, meaningful change requires a lifestyle shift, and it’s better when it comes from inspiration from a real desire to feel better which leads to stress reduction, than a diet out of fear causing more stress and ultimately failure.”

Ultimately, fasting is like any eating protocol: it works best in tandem with other healthy habits.

“It’s one tool in our tool belt,” says Manning. “It can be used effectively and I’ve had countless people lose weight and fat on this protocol as long as it’s done correctly.”

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Emily Monaco is a food and culture writer based in Paris. Her work has been featured in the Wall... More about Emily Monaco