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Mikhail Kogan, MD, regularly prescribes a form of intermittent fasting known as time-restricted eating (TRE), among other interventions, to his patients, about 30% of whom are obese.

“I prescribe it to most people,” he told Medscape Medical News, “…because it’s one of the core impacts or core effects on improving metabolism over time.”

Kogan, the chief medical officer for the GW Center for Integrative Medicine in Washington, DC, doesn’t just prescribe TRE to patients; he is a practitioner of it as well. He eats generally between the hours of 11 AM and 9 PM 5-7 days a week, and once every other week he also does a 24-hour, water-only fast.

“I do it for longevity, he said. “My weight is perfect.”

“TRE does optimize metabolism a lot, and also it teaches [the] body to be comfortable with switching from burning sugar to burning fat,” Kogan said in an email.

The recommendation Kogan generally gives patients — to eat for around 8-10 h/d and fast the rest of the time — may make him somewhat of an anomaly among his peers.

Despite some research that shows a benefit of intermittent fasting for weight loss in obese and overweight individuals, not all doctors embrace it. Medscape Medical News reached out to doctors around the world to get their assessment. But first, we will delve into what intermittent fasting entails and some of the supporting research.

What Is Intermittent Fasting?

Intermittent fasting is a broad term encompassing various meal timing approaches that alternate periods of eating with extended fasting. The most widely studied regimens are alternate-day fasting, 5:2 intermittent fasting (restricting calories 2 d/wk, which generally means consuming about 500-700 calories on those days), and TRE, according to a 2019 review article published in The New England Journal of Medicine.

One form of TRE — eating within up to a 10-hour daily window and fasting the rest of the time (14 or more hours) — is the most popular form of intermittent fasting, Olivia Altonji, PhD, and Courtney Peterson, PhD, wrote in a recent editorial in Nature Medicine. That form of TRE is also easier for people to incorporate into their routines, added Mark Mattson, PhD, who worked as chief of the laboratory of neurosciences at the National Institute on Aging from 2000 to 2019.

How Does Intermittent Fasting Work?

“The key defining feature of intermittent fasting eating patterns is that they include periods of time sufficient to cause that metabolic switch from glucose to ketones,” Mattson said. It takes about 12 hours for the switch to occur, he said. (Mattson also wrote the book, The Intermittent Fasting Revolution: The Science of Optimizing Health and Enhancing Performance.)

There are signals with intermittent fasting “that over a period of a couple of weeks to a month, they cause a switch in your brain’s responses to your energy state so that you’re no longer hungry during time periods you’re not eating,” Mattson said.

On the other hand, if you’re eating three meals a day and you have reduced calories, you’re going to feel hungry all the time long term, he explained.

There are a decent number of studies on intermittent fasting, said Peterson, an associate professor in the Department of Nutrition Sciences at The University of Alabama at Birmingham.

“The sort of general problem in the field is most of these studies are pretty small,” she said, adding that, “it hasn’t been until about 3 years ago that we really got to see consistent evidence that intermittent fasting helps you lose weight.”

A recent randomized-controlled trial of 197 participants published in Nature Medicine in February found that TRE was better for losing weight in overweight and obese individuals than diet change alone, although it offered no additional benefit over education about a Mediterranean diet in reducing visceral adipose tissue.

“Studies are not fully consistent in their results, but meta-analyses underline a weight loss effect and benefits on blood pressure and diabetes type 2,” Andreas Michalsen, MD, wrote in an email. He regularly recommends intermittent fasting for patients with obesity and metabolic diseases in his work as an internist and professor at Charité Medical University Berlin in Berlin, Germany.

What Do Doctors Think of Intermittent Fasting?

With the widespread use of glucagon-like peptide 1 receptor agonist (GLP-1 RA) medications such as Wegovy (semaglutide) and Zepbound (tirzepatide), which can act quickly to help obese individuals lose weight, we wondered what doctors think of intermittent fasting — a process that could be far slower to yield any results if it does at all. Here’s what we found out.

‘It’s Not Something I Prescribe to People’

Scott Kahan, MD, director of the National Center for Weight and Wellness in Washington, DC, said he does not generally prescribe intermittent fasting or timed-restricted eating to his patients who are obese or overweight for the purpose of weight management, but he also wouldn’t tell a patient necessarily not to do it either.

Intermittent fasting is a broad term that can mean many things, he said.

“Some studies suggest that it’s not necessarily any worse than just more traditional healthy eating and tracking one’s portions and calories and so forth, but also not necessarily any better.”

“It’s not something that I prescribe to people, and I try to help them appreciate that most of what they’re reading — perhaps all of what they’re reading about it — is probably either inaccurate or at least poorly contextualized,” he said.

Taking an ‘Intermittent Fasting-ish’ Approach

Mihir Patel, MD, on the other hand, said he brings up intermittent fasting with his patients sometimes, but it’s not usually his first suggestion. That’s because some of the earlier data on it showed it didn’t differ much from portion and calorie control, he said. Intermittent fasting works if you also change the quality of your diet and how much you’re eating.

“I think as an adjunct, I think that’s where the benefit lies,” he said.

Patel said he believes in the physiology of intermittent fasting “so I kind of interweave intermittent fasting principles.” When your insulin levels decrease, “that allows you to break down adipose tissue,” said Patel, an obesity medicine specialist at the University of Maryland School of Medicine, Baltimore.

“I try to get people to eat two or three meals a day. I limit the snacking. So, it’s kind of intermittent fasting-ish,” he said.

‘Highly Effective’ Approach

Whereas Patel generally offers GLP-1 RAs to his patients, most of whom have class 2 and 3 obesity (with a body mass index of at least 35), on their first visit, Michalsen, the internist in Germany, offers a stepwise approach. He regularly recommends intermittent fasting for patients with obesity and metabolic disease such as hypertension and type 2 diabetes.

He starts his patients with a fasting mimicking diet (FMD) — an intervention that mimics the effects of fasting but provides key nutrients — followed by a 14:10 TRE plan. This refers to a 14-hour fast followed by 10 hours of eating. He encourages patients who cannot have an early dinner at 6 PM or 7 PM to have a late breakfast — and don’t skip it — around 9 AM or 10 AM.

Generally, he sees from this regimen a “mostly moderate weight loss” — around 4%-5% of body weight — within 6 months, some reductions in blood pressure and A1c levels, and improved sleep.

Many patients stick with the plan, he said in an email, “as they experience better vitality and quality of life.” He reinforces that with “dietary treatment” and repeated cycles of FMD. If sufficient weight loss does not occur after some months, then he starts patients on a GLP-1 RA medication, he wrote.

Kogan is also a proponent of intermittent fasting. Many of his patients at the GW Center for Integrative Medicine have conditions that are not well managed by standard-of-care approaches, and those who are obese often have metabolic disturbances.

“So for metabolic optimization, particular of energy production, intermittent fasting is one of the dozen or so approaches that’s highly effective. It’s not the only approach. It’s not going to fix everything. But it definitely has benefit,” he said.

“Intermittent fasting is good for, I would say, the majority of people with mitochondrial dysfunction if they are relatively young and strong,” he said. In conjunction with intermittent fasting, he recommends that the first meal of the day is high in protein. He also often prescribes strength training.

On a personal note, Kogan started practicing TRE about 15 years ago. “My entire life I was very sugar sensitive; if I go without one meal, I get hangry (hungry and angry),” he said in an email. “Doing TRE completely stopped all that; now I can do for 24 hours without eating, working normally, exercising, etc.”

Intermittent Fasting for Type 2 Diabetes

Canadian nephrologist Jason Fung, MD, prescribes some type of intermittent fasting regularly to his patients with type 2 diabetes alongside other weight loss tools.

He does so even though there aren’t “giant randomized controlled trials” showing a benefit. “When you’re treating a patient, what’s important is that they get better, right?” he said. He has personally witnessed people with type 2 diabetes reverse it through intermittent fasting, he says. There’s no risk and only a potential benefit, Fung asserted.

He doesn’t have just one protocol but individualizes the approach.

“I’m going to be more aggressive with younger people who are on less medication. I’m going to be less aggressive with older people,” he told Medscape Medical News. It’s all a discussion. “I try to get people to whatever they can do.”

He sometimes uses forms of intermittent fasting along with GLP-1 RA medications.

Fung, who works at Scarborough Health Network,Toronto, Ontario, Canada, is co-owner of The Fasting Method, an online coaching site for intermittent fasting that started in his clinic, he said.

He doesn’t suggest intermittent fasting or TRE in children or in people who are undernourished, underweight, pregnant, or breastfeeding. Mattson said it shouldn’t be used in those with type 1 diabetes. It’s also not recommended in those with eating disorders or gallstones, added Michalsen.

You could try it or “wait 10 years for somebody to do a New England Journal type of study, if it ever gets done,” he said, but by that time, your patient with type 2 diabetes might be on dialysis.

“So it’s like clinically, why wouldn’t you do it?”