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The allure of fasting diets for successful people — and where it falls short

Hilary Brueck,Gabby Landsverk   

The allure of fasting diets for successful people — and where it falls short
  • Intermittent fasting is hugely popular with tech CEOs, longevity-seekers, and movie stars.
  • It's less complicated than dieting. All you have to do is follow a schedule.

Anyone who's anyone has an opinion about fasting.

Chris Hemsworth has done it to boost his longevity and lose a ton of weight for acting roles. Podcaster Andrew Huberman told his more than 5 million YouTube subscribers he uses a "pseudo-intermittent fasting" diet for better focus and energy. Hugh Jackman started fasting for 16 hours a day on the recommendation of Dwayne "The Rock" Johnson, and Terry Crews only eats between 2 and 10 p.m.

It's not just movie stars and health-fluencers. In the C-suite, Open AI CEO Sam Altman has said he rides all morning on espresso and usually ends up fasting for about 15 hours a day. Other tech bosses who love fasting include "father of biohacking" Dave Asprey, Gary Vaynerchuk, Bryan Johnson, and Jack Dorsey.

Fasting speaks to our very 2024 desire for more personalized, or "precision," medicine. Intermittent fasting diet plans, meal kits, fitness trackers, and biological age tests can all offer something to busy people who want to feel in control. More than one in 10 Americans (12%) tried intermittent fasting in 2023, according to one food and beverage industry-backed survey. The business of intermittent fasting phone apps is blossoming and is on track for more growth in the next five to 10 years.

In March, we saw how thirsty we are for clearer answers on what fasting can do for our bodies. A controversial study (with many shortcomings) found that Americans who eat all their calories within an 8-hour window may double their risk of heart attack death. This wasn't even a peer-reviewed paper; it was just a single-page release based on two days of eating surveys, a finding widely panned by scientists and doctors, but it got so much play. News stories, tweets, newsletters, podcasts, forums… For a day or so, the science corner of the internet was absolutely hair on fire.

The drama highlighted the massive gulf between people's passions for fasting and the actual evidence we have. Whatever you think about fasting, don't worry: you can find plenty of studies to back up your point. Fasting is killing us! Wait, scratch that, it's actually a fountain of youth.

So what really happens to your body when you stop eating food for some period of time? And is it good for you, or bad? Here's what we know.

There are many ways to do intermittent fasting — from skipping breakfast to whole days without food

People have been fasting for as long as people have been people. Sometimes fasting happened by necessity (there was no food in sight.) Later, it became ingrained in spiritual practices, religion, and medicine. It is said that Hippocrates recommended fasting to some of his sick patients in ancient Greece.

Today, when people talk about fasting, intermittent fasting, or time-restricted eating, they're usually referring to confining all of their eating to some shorter-than-normal period of the day.

"In its most accessible form, people are really just cutting out evening snacking and then skipping breakfast," longevity researcher Daniel Belsky, an associate professor of epidemiology at Columbia University, told Business Insider. "People feel better, I think, because like many dietary practices, this one — almost as a side effect — substantially reduces some unhealthy, casual eating that people do."

  • The popular 8/16 diet requires people to fit all their meals into an eight-hour window (water and black coffee are still OK in the off hours, but no snacks).

  • There's also 12/12 (fasting for 12 hours a day)

  • OMAD (one meal a day)

  • 5:2 (five days a week of unrestricted eating, two restricted to around 600 calories)

  • alternate-day fasting, and

  • 24-hour fasts, once a week.

Research scientist Satchidananda Panda, who pioneered much of the research that the trendy 8/16 diet is based upon, said people don't have to be super strict with their eating window. In his studies, participants who eat all their meals in a 10-hour pocket of the day see many of the same health benefits that 8/16ers do.

"Eight, nine, 10 are almost equivalent," Panda, a professor of regulatory biology at the Salk Institute for Biological Studies, told BI. "That's why in all of our clinical trials that we do, we don't do eight hours. We ask them to eat within 10. Because 10 has long-term compliance, sustenance. It's easier to do."

The case for fasting to improve sleep, weight loss, and cancer treatment

The clearest immediate effect of time-restricted eating is that you will (probably) eat less. Erasing mindless eating and consuming fewer unhealthy snacks happens by default when you eat on a more rigid schedule — it's basically cutting calories without having to track them.

"I don't think it's magical or special even though I've dedicated my life to studying this," Krista Varady, a professor of nutrition at the University of Illinois, Chicago, told BI. "It's just a diet, like any other."

You can also feel like you have more control over what you eat.

"I think the biggest benefit of time-restricted feeding is the psychological one," Dr. Peter Attia said on an episode of his podcast "The Drive."

But fasting fans say there's more to it. These are the big claims:

  • Autophagy: If you've ever listened to a health influencer like Huberman talk about fasting, you've probably heard the term "autophagy," which basically means "self-eating." Depriving our bodies of nutrients for a little while turns up the dial on this important internal process. It's biology's basic tidying-up service by which our cells recycle old, usable parts and throw out junk. There is some evidence that revving up this process through fasting could improve longevity.

    But much of what we know about autophagy comes from data on yeast and worms because it's difficult to measure it directly in humans. As a result, many claims about how autophagy can boost anything health-related go far beyond what research can prove, according to Varady.

    "It's so over-embellished," she said. "We barely have any data on this. I think a lot of people are trying to sell products, trying to turn on autophagy to make money, not based on science."

  • Fat burn: Some small studies have found fasting helps our bodies burn more fat by trading ketones for energy. One 2016 study of 34 young, muscled men who spent two months on an 8:16 diet (and exercised regularly) showed they lost more fat while keeping their muscles intact during the intermittent fasting study.

    In the long run, though, intermittent fasting often fares no better than calorie counting for human weight loss.

  • Circadian rhythm and sleep: Panda's studies — both mouse and human — have convinced him that fasting has benefits that go beyond any slimming effect. There is a light and dark, night and day pattern that our bodies consistently follow, which means that our gut processes nutrients differently depending on the time of day. Late at night, our bodies become less sensitive to insulin and more sensitive to protein as we shift out of eating-and-doing mode and into rest-and-growth time.

    Fasting for at least a few hours before bed can improve sleep quality and reduce acid reflux. (This makes sense mechanistically: when we secrete more insulin at night by eating, it makes it harder for melatonin to do its thing.) Better sleep then propels a cascade of huge health benefits, including longevity-linked pluses like lower inflammation, improved energy, and better memory, just to name a few.

  • Cancer treatment and prevention: This is where some interesting progress is being made in fasting research. For many years, scientists have had a hunch that fasting may help nix cancer. Now, they're starting to better understand the best methods of attack for pairing fasting with chemotherapy. Some studies also suggest that some amount of fasting (especially at night) may help prevent cancer from popping up in the first place, but much more research is needed on this.

    Valter Longo, a pioneering researcher in fasting for longevity, professor of gerontology at USC, and inventor of the popular "fasting-mimicking diet," said fasting is so promising because it can effectively starve cancer cells while leaving healthy ones alone.

    "What's very exciting is that fasting sends normal cells and cancers in opposite directions," Longo told BI. "With fasting, you protect the normal cells and target the cancer cells."

    But one big hurdle researchers including Longo are increasingly coming up against is that fasting techniques tend to only work sometimes, in some patients. It's not clear why some folks are super responsive to fasting for cancer while other patients just aren't at all.

The case against fasting to lose weight and live longer

You can fast your way to a longer life — if you're a lab rat. Research on worms, flies, fish, mice, rats, and monkeys has shown that caloric restriction through fasting can be a lifespan-extending practice, at least for those animals. The question is, can it lengthen the lives of real people out in the real world? On this, scientists aren't so sure.

"In terms of what do we know about what it does in the long term, the answer is, of course, nothing," Belsky said. "We can look at the examples of populations that have dietary practices that either feature a lot of periodic fasting or that are characterized by more modest caloric intake. And those populations tend to live longer, healthier lives. But we just don't know."

Whether or not fasting might work for you comes down to what you mean by fasting and what your goals are, Longo said.

"It's like saying eating is good for you or bad for you. Fasting is the same way; it depends on what kind of fasting you're talking about," he told BI. "You have to look at the type of fasting and personalization."

Fasting may be a bad idea for people who are trying to build lean muscle mass because you need a lot of energy for that. "People who lose weight lose everything. They lose fat, muscle, and bone," Professor Stephen Kritchevsky, who studies weight loss, fasting, and longevity in older adults at Wake Forest University, told BI.

That's why fasting can be a double-edged sword for longevity: no matter the supposed benefits, losing body mass can make you feel weaker and more depleted.

"If you're restricting calories or protein and losing muscle mass, it's not a good tradeoff," Dr. Mark Cucuzzella, a professor at West Virginia University School of Medicine, told BI. "Anything you do that compromises your muscle mass is not good."

Some intermittent fasting might help people live healthier up until the end of their life if it helps them maintain a stable, healthy weight and great sleep schedule, but there's no evidence it can directly go against the biology of aging and help us all live an extra 10 or 20 years.

Kritchevsky is planning a large-scale clinical trial to better answer whether time-restricted eating is a good idea for older adults. In a pilot study he's running now, 34 older adults are trying out a time-restricted eating plan, where they fast for 16 hours a day.

"The hope would be that the entraining of the eating schedule, and whatever mild caloric restriction that might bring with it, might convey some benefits," he said. "The study to prove longevity in humans would have to be 5,000 to 8,000 people for six, seven, eight years."

One easy way to get the benefits of fasting without depleting your body

Nir Barzilai, one of the preeminent scholars of longevity drugs, started fasting up to 18 hours a day when he hit 65. He swears he has more energy now and can go harder in his workouts. He's moved from 25-pound dumbbells to 35 pounds. He's also lost around five pounds.

Still, he winces at the broad promotion of this as a lifestyle that's good for everybody. Fasting can change how drugs interact with the body, and may be detrimental to longevity for younger people.

"I'm telling you, people are going to experiment with different things, and we'll have to figure out what's true for everyone," Barzilai said. "I am afraid that maybe we're saving a hundred people and killing three. I want to understand which three are going to die before we do it."

For now, one of the safest, most evidence-based strategies is to tap into a kind of fasting you're already doing: sleeping. Waiting an hour or so before eating once you wake up and finishing your last meal of the day two to three hours before bed (no snacking!) is an easy way to take advantage of beneficial processes that happen during the night when your body is at rest and not digesting.

Longo said this roughly 12-hour fasting cycle is one he feels confident in recommending because it closely mimics how people used to eat before access to food became so pervasive. Panda agreed, saying a 12-hour fast is what his grandfather, growing up on a farm without electricity, adhered to automatically.

Our takeaway

Experts interviewed for this story all agreed we still have serious questions about the long-term impacts of fasting on human longevity and health. That's not a problem unique to fasting — nutrition research is notoriously bad at answering with certainty how our eating habits may or may not cause health issues over time.

While intermittent fasting may help you eat less without changing your diet, it's not clear it can make up for the long-term consequences of eating junk food (mice in the lab can eat whatever they want on a fasting diet and still retain health benefits, but it isn't hard to imagine the results might differ for people, who live much longer and more complex lives.)

"People are worrying about whether to skip breakfast or not when they're eating cocoa puffs," Cucuzzella said.

More convincing evidence suggests longevity is a complex mix of the genes we get, the food we eat, plus how our bodies move through the world, exercising and interacting with one another day after day. It may not sound as exciting as "precision medicine" or "fat burning" but eating just enough of the right foods (vegetables, fruits, nuts and seeds, high-quality proteins, and whole grains) is what's strongly linked to a longer life.

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