Short Wave - The truth about intermittent fasting

Episode Date: February 20, 2026

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 and 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 [e-mail] 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.See 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

You're listening to Shortwave from NPR. Hey, Shortwaivers, Regina Barbara here. And Rachel Carlson. 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. 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. Yes, we're talking about running.

also why intermittent fasting might not be a good weight loss solution. And rethinking how to protect people's mental health when they talk with a chatbot. All that on this episode of Shortwave, the science podcast from NPR. Okay, Mary Louise, where should we start? I want to hear all three of these. Okay, I've not had time to get lunch today, so I'm kind of doing intermittent fasting. Oh, no. 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. 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. 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. And what did they find? 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.

Huh. Okay, so it doesn't work. Case closed? Well, not quite. So we asked Matthew Steinhauser 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. But it does suggest that there's not a huge effect on body weight. And certainly nothing approaching what we see with the GLP1 drugs, for example, where patients can lose 10 to 20 percent of their body weight over the course of a few months to a year.

I suppose worth pointing out, people may choose to fast, made choose to change their diet for reasons that go well beyond weight loss, right? 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. 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 chatbots' suicide mental health. 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. OpenAI owns ChatGPT. They told Reto 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. Every three hours, but do those reminders work? 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 Lestadius at the University of Wisconsin, Milwaukee. She says the idea that reminders could be a big solution.

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

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. 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. Mine too. Rachel, if I were doing ultra endurance running, how would it be. changing my body. 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. You said extreme distances. Like,

what are we talking? 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 ultramarathon. 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 ultramarathon group.

The results were published this week in the journal Red Blood Cells and Iron. More damage in the ultramarathon group. So it sounds like the farther they went, the more. more damage or types of damage they found. Do we know why? One of the study authors, Travis Nemcov, 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. 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. But that damage isn't the end of the story. The runner's body starts generating new blood cells. Okay, 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? 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. Here's to keeping healthy and aging well. And Mary Louise, thank you so much for joining us today. It was my total pleasure, as always. Looking forward to the next one. If you or someone you know is considering suicide or in crisis, call or text the suicide and crisis lifeline at 9-8-8.

You can hear more of Mary Louise on Consider This, and PR's afternoon podcast about what the news means for you. And for more science stories just like this one, follow Shortwave on whatever app you're listening to. I produced this episode with Jordan Marie Smith and Hannah Chin. It was edited by Christopher Entaliata, William Troop, and Rebecca Ramirez. Peter, Elena, and Jimmy Keely were the audio engineers. I'm Regina Barber.

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