The truth about intermittent fasting

From TikTok and Instagram influencers to celebrities like Hugh Jackman and Kourtney Kardashian, intermittent fasting has gotten a lot of hype. The diet restricts what time you eat rather than what or how much you eat. The idea is that short periods of fasting cause your body to burn through stored fat reserves. But is that conventional wisdom true? And can it really contribute to weight loss? Regina G. Barber https://www.npr.org/people/1082526815/regina-g-barber and Rachel Carlson https://www.npr.org/people/1112796909/rachel-carlson tackle those questions — plus why some researchers are rethinking how to protect people's mental health when talking to chatbots and how ultra-endurance running changes the human body. Have a scientific question you want us to answer? Email us your question at [email protected] mailto:[email protected]. Listen to every episode of Short Wave sponsor-free and support our work at NPR by signing up for Short Wave+ at plus.npr.org/shortwave http://plus.npr.org/shortwave. See pcm.adswizz.com https://pcm.adswizz.com for information about our collection and use of personal data for sponsorship and to manage your podcast sponsorship preferences. NPR Privacy Policy https://www.npr.org/about-npr/179878450/privacy-policy

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• 0:12Regina Barber?

  You're listening to Short Wave from NPR. Hey, Short Wavers. Regina Barber here.

• 0:19Speaker 02

  And Rachel Carlson.

• 0:21Regina Barber?

  And today, we have our biweekly Science News roundup, featuring the hosts of All Things Considered. And today, we have the legend, Mary Louise Kelly.

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  Woohoo! I'm so excited to be here and excited because I hear one of the stories you brought is about running, which is like the only sport I can actually do. So I'm here for you.

• 0:39Speaker 02

  Yes, we're talking about running. Also, why intermittent fasting might not be a good weight loss solution.

• 0:44Regina Barber?

  And rethinking how to protect people's mental health when they talk with a chatbot.

• 0:50Speaker 02

  All that on this episode of Short Wave, the science podcast from NPR.

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• 2:11Speaker 02

  Okay, Mary Louise, where should we start?

• 2:13Speaker 03

  I want to hear all three of these. Okay, I have not had time to get lunch today, so I'm kind of doing intermittent fasting.

• 2:18Speaker 02

  Oh, no.

• 2:19Speaker 03

  So let's start there. This is, just to explain, this is where you restrict the times you eat, right? Rather than what you eat.

• 2:26Speaker 02

  Yeah, so some people might fast every other day. Others might eat between the hours of 10 to 6, but do a 16-hour fast outside of that. The idea is that these short periods of fasting will cause your body to start burning through stored fat reserves.

• 2:41Regina Barber?

  The issue is, we don't have a big, long-range study on how it compares with other types of dieting. So, an international team of scientists did the next best thing. They looked at 22 smaller studies that compared intermittent fasting to other dietary interventions, like eating less or eating specific types of foods. They also compared intermittent fasting to doing nothing.

• 3:02Speaker 03

  And what'd they find?

• 3:03Speaker 02

  They concluded that intermittent fasting did not work for weight loss in overweight or obese adults, as compared to either traditional dietary advice or even doing nothing.

• 3:12Speaker 03

  Huh, okay, so it doesn't work. Case closed?

• 3:14Regina Barber?

  Well, not quite. So, we asked Matthew Steinhouser about it. He does metabolic research at the University of Pittsburgh. He wasn't involved in this research, and he said that the small size of all the studies within this larger review makes it hard to know for certain.

• 3:27Speaker 05

  But it does suggest that there's not a huge effect on body weight. And certainly, nothing approaching what we see with the GLP-1 drugs, for example, where patients can lose 10 to 20% of their body weight over the course of a few months to a year.

• 3:49Speaker 03

  I suppose worth pointing out, people may choose to fast, may choose to change their diet for reasons that go well beyond weight loss, right?

• 3:57Speaker 02

  Yeah, that's exactly right. The results of this literature review really focused in on weight loss as the standard. But, like you said, that's not the only reason people choose to try intermittent fasting. And no matter what, Matthew told us that, in medicine, very few things are risk-free. So definitely talk to your doctor before making any big changes to your diet.

• 4:16Speaker 03

  Good evergreen advice there. Always talk to your doctor. Okay. Second story, this is about chatbots and mental health. I want to mention we are about to discuss suicide. Gina, there have been a number of high-profile cases to do with chatbot suicide mental health.

• 4:34Regina Barber?

  Yeah, it's really scary for a parent like me. Last year, a number of parents testified to Congress about the dangers of AI chatbots. A couple of those families believe that AI chatbots pushed their teenage sons to kill themselves. Our colleague, Ritu Chatterjee, reported that one family testified that one of the chatbots, ChatGPT, even offered to help write their son's suicide note.

• 4:56Speaker 02

  OpenAI owns ChatGPT. They told Ritu at the time that it's redesigning its platform to be safer for minors. One intervention some parents and others have advocated for is chatbots regularly reminding users that they're talking to an artificial intelligence agent and not a human. Laws in California and New York require AI chatbots to send these regular reminders every three hours.

• 5:18Speaker 03

  Every three hours. But do those reminders work?

• 5:20Regina Barber?

  So some social scientists argue that at best, these reminders probably don't work, and at worst, they might be harmful. One of the authors of a recent opinion piece is social scientist Linnea Lestadias at the University of Wisconsin-Milwaukee. She says the idea that reminders could be a big solution.

• 5:37Speaker 06

  Struck us as a bit naive in some ways and potentially dangerously unrealistic in others, and also not engaging with potential downsides of reminders.

• 5:48Speaker 03

  Potential downsides? Like what?

• 5:50Speaker 02

  So Celeste Campos-Castillo co-wrote that opinion piece in Trends in Cognitive Sciences with Linnea. She warns that if someone already feels lonely, or if they're struggling with their mental health, a reminder that they're not talking to a real person could destabilize them and make them feel more isolated.

• 6:07Speaker 03

  Huh. So if these reminders pinging you, you're actually talking to AI, not a human. If those don't work, if they may even be, make things worse, what's the way forward?

• 6:19Speaker 02

  Yeah, it's a good question. Celeste and Linnea are urging these companies to share their data more openly so they can study these interventions better and hopefully help shape more effective policy.

• 6:30Speaker 03

  I'm going to move us to our last topic, which is how ultra endurance running changes the body. I'm totally intrigued. I'm a runner, although my ultra endurance is about four miles these days.

• 6:42Speaker 02

  Mine too.

• 6:43Regina Barber?

  Not ultra marathon.

• 6:45Speaker 03

  How, if I were doing ultra endurance running, how would it be changing my body?

• 6:50Speaker 02

  No, I'm totally with you there. I was really proud of my 10K until I read this study. And they found that running extreme distances can damage red blood cells. These cells are super important. They carry oxygen throughout people's bodies and transport waste. So damage to those red blood cells can trigger inflammation or lead to anemia.

• 7:09Speaker 03

  You said extreme distances. Like, what are we talking?

• 7:12Speaker 02

  They're not talking about us. That meant trail runners who raced either 24 miles, so a little under a marathon, or who raced 106 miles in ultra marathon.

• 7:22Regina Barber?

  Yeah, long, long ways. They looked at the blood samples from these runners before and after their races, and runners in both groups had damaged red blood cells. But distance did matter. There were more inflammation markers and types of damage in the ultra marathon group. The results were published this week in the journal Red Blood Cells and Iron.

• 7:41Speaker 03

  More damage in the ultra marathon group. So it sounds like the farther they went, the more damage or types of damage they found. Do we know why?

• 7:51Speaker 02

  One of the study authors, Travis Nemkov, put it like this. When you're doing something like running, your body needs more oxygen. And that means blood is circulating through your body more quickly, which can lead to the breakdown of red blood cells.

• 8:03Speaker 07

  The damage that is obtained by these red blood cells causes some of those red blood cells to be removed from circulation more quickly than they would have otherwise been.

• 8:13Regina Barber?

  But that damage isn't the end of the story. The runner's body starts generating new blood cells.

• 8:19Speaker 03

  OK, so it sounds like the body starts to recover. But you did just tell me that damage to red blood cells can cause inflammation, can cause possibly anemia. Where does this leave us? Do we think extreme exercise is dangerous?

• 8:33Speaker 02

  Well, Travis and the researchers do not know whether this extreme amount of exercise is bad. The study was very small. It only looked at 23 runners. And it doesn't tell us anything about the long-term impact of these kinds of races. And then more broadly, Travis says, definitely do not stop doing regular exercise. It's the best tool we have to keep our bodies healthy and age well.

• 8:56Speaker 03

  Here's to keeping healthy and aging well.

• 8:59Speaker 02

  And Mary Louise, thank you so much for joining us today.

• 9:03Speaker 03

  It was my total pleasure, as always. Looking forward to the next one.

• 9:11Regina Barber?

  If you or someone you know is considering suicide or in crisis, call or text the Suicide and Crisis Lifeline at 988.

• 9:24Regina Barber?

  You can hear more of Mary Louise on Consider This, NPR's afternoon podcast about what the news means for you.

• 9:30Speaker 02

  And for more science stories just like this one, follow Short Wave on whatever app you're listening to.

• 9:37Speaker 02

  I produced this episode with Jordan Marie Smith and Hanachin. It was edited by Christopher Intagliata, William Troop, and Rebecca Ramirez.

• 9:45Regina Barber?

  Peter, Elena, and Jimmy Keeley were the audio engineers. I'm Regina Barber.

• 9:49Speaker 02

  And I'm Rachel Carlson. Thanks for listening to Short Wave, the science podcast from NPR.

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