The truth about intermittent fasting

Summary

Short Wave – “The Truth About Intermittent Fasting”

Date: February 20, 2026
Hosts: Regina Barber & Rachel Carlson
Guest Host: Mary Louise Kelly
Main Theme:
A lively science roundup covering new research on intermittent fasting’s effectiveness for weight loss, potential pitfalls in chatbot interventions for mental health, and the impact of ultra-endurance running on red blood cells.

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Key Topics & Discussion Points

1. Intermittent Fasting: Does It Really Work for Weight Loss?

• What is Intermittent Fasting?

  • Celeste Campos Castillo sums it up: “This is where you restrict the times you eat, right, rather than what you eat.” (02:31)
  • Rachel Carlson explains: “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.” (02:38)

• The Research:

  • Due to lack of robust long-term studies, an international research team conducted a meta-analysis of 22 smaller studies comparing intermittent fasting against other diets or no intervention.
  • Results: “Intermittent fasting did not work for weight loss in overweight or obese adults as compared to either traditional dietary advice or even doing nothing.” – Rachel Carlson (03:14)

• Expert Reaction:

  • Dr. Matthew Steinhauser (not involved with the review):
    • “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.” (03:39)

• Nuance and Caution:

  • Regina Barber: “The small size of all the studies within this larger review makes it hard to know for certain.” (03:26)
  • Rachel Carlson: The 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.” (04:09)
  • Celeste Campos Castillo: “Good evergreen advice there. Always talk to your doctor.” (04:28)

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2. Chatbots, Mental Health, and the Risk of Harmful Interventions

• Backdrop:

  • Recent incidents with AI chatbots allegedly contributing to or not preventing youth suicides; regulatory interventions underway.
  • “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.” – Regina Barber (04:45)

• Policy Solution: Reminders That You’re Talking to AI

  • In states like California and New York, chatbots must ping users every three hours, reminding them they're interacting with AI, not humans. (05:07)

• Does It Work? Concerns from Experts:

  • Linnea Listadius (University of Wisconsin-Milwaukee) & Celeste Campos Castillo (co-author):
    • The idea that reminders could fix the issue “struck us as a bit naive in some ways and potentially dangerously unrealistic in others. And also not engaging with potential downsides of reminders.” – Rachel Carlson quoting Linnea Listadius (05:50)
  • Downsides:
    • “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.” – Rachel Carlson (06:02)
  • Way Forward:
    • “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.” – Rachel Carlson (06:31)

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3. Ultra-Endurance Running: What Happens to Your Blood?

• Study Focus:

  • How running extreme distances affects red blood cells and inflammation markers.

• What Did They Find?

  • “They found that running extreme distances can damage red blood cells. These cells are super important... So damage to those red blood cells can trigger inflammation or lead to anemia.” – Rachel Carlson (07:02)
  • The study compared trail runners completing 24-mile races (just under a marathon) and ultra-marathoners running 106 miles. Both had evidence of blood cell damage, but it was more pronounced in the ultramarathon group. (07:24–07:53)

• Why Does This Happen?

  • Expert Travis Nemkov explains:
    • “When you're doing something like running, your body needs more oxygen, and that means blood is circulating through your [body] 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.” (08:03, 08:15)

• Can Runners Recover?

  • Regina Barber: “But that damage isn't the end of the story. The runner's body starts generating new blood cells.” (08:24)

• Big Picture:

  • The study was small and doesn’t answer long-term health questions.
  • “Travis and the researchers do not know whether this extreme amount of exercise is bad... 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.” – Rachel Carlson (08:45)

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Notable Quotes & Memorable Moments

• On Intermittent Fasting:
  • “Case closed.” – Celeste Campos Castillo, cheerily summing up the negative result (03:24)
  • “Good evergreen advice there. Always talk to your doctor.” – Celeste Campos Castillo (04:28)
• On Chatbots:
  • “A bit naive in some ways and potentially dangerously unrealistic in others.” – Rachel Carlson, quoting Linnea Listadius (05:50)
  • "If someone already feels lonely... a reminder that they're not talking to a real person could destabilize them and make them feel more isolated." (06:02)
• On Running:
  • “My ultra endurance is about four miles these days.” – Celeste Campos Castillo, humorously putting “ultra” in perspective (06:42)
  • “Here's to keeping healthy and aging well.” – Celeste Campos Castillo (09:07)

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Timestamps for Major Segments

• [02:23] Intermittent Fasting—What Is It? & New Study
• [03:14] Study Results: Intermittent Fasting Not Superior for Weight Loss
• [03:39] Expert View: Matthew Steinhauser on Study Shortcomings
• [04:28] Reminder: Always Consult Your Doctor
• [04:45] AI Chatbots & Mental Health Crisis Discussion
• [05:07] Regulation: AI Chatbot Reminders
• [05:50] Potential Downsides of Reminders (Expert Opinion)
• [06:42] Science of Ultra-Endurance Running
• [07:53] Finding: More Blood Cell Damage in Ultramarathoners
• [08:15] Travis Nemkov on Why Red Blood Cells Get Damaged
• [08:45] The Big Picture: Is Extreme Exercise Dangerous?

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Tone & Style

The conversation is accessible and relatable, balancing light humor ("my ultra endurance is about four miles") with thoughtful caution and clear science communication. The hosts foreground nuance, share expert insight, and repeatedly emphasize the limits of available data.

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Takeaways

• Intermittent fasting is no silver bullet for weight loss compared to traditional diets or even making no change at all, according to a meta-analysis of small studies.
• Chatbot reminders about their non-human status may not help—and could even harm—mental health, especially among vulnerable users, suggesting regulation alone isn’t enough without deeper research and policy work.
• Ultra-endurance exercise damages red blood cells more as distances increase, but the body can recover. The health impacts of such extreme exertion remain uncertain, and regular exercise is still highly recommended.

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For more science stories like these, follow Short Wave and keep tuning in for accessible, engaging science you can use.

Transcript

[Ira Glass]

This is Ira Glass on this American Life. We look for stories that are surprising that you won't hear anywhere else. Like, for example, this one astronaut who

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[Celeste Campos Castillo]

No, no, no.

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This is American Life. Unexpected stories. Wherever you get your podcasts, you're listening

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to Short Wave from npr.

[Regina Barber]

Hey, short waivers.

[Rachel Carlson]

Regina Barber here and Rachel Carlson.

[Regina Barber]

And today we have our bi weekly science news roundup featuring the hosts of All Things Considered. And today we have the legend, Mary Louise Kelly.

[Celeste Campos Castillo]

Woo hoo. 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.

[Rachel Carlson]

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

[Regina Barber]

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

[Rachel Carlson]

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

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[Guest Expert (e.g., Matthew Steinhauser or Travis Nemkov)]

Our State of Stigma report helped us understand that believing in mental health is easy, but asking for help is not. Now, with the report on our hands, we can work to make mental health care more accessible.

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[Rachel Carlson]

Louise, where should we start?

[Celeste Campos Castillo]

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.

[Regina Barber]

Oh, no, no.

[Celeste Campos Castillo]

Let's start there. This is just to explain. This is where you restrict the times you eat, right, rather than what you eat.

[Rachel Carlson]

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.

[Regina 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.

[Celeste Campos Castillo]

And what'd they find?

[Rachel Carlson]

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.

[Celeste Campos Castillo]

Huh. Okay, so it doesn't work. Case closed.

[Regina Barber]

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,

[Guest Expert (e.g., Matthew Steinhauser or Travis Nemkov)]

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% of their body weight over the course of a few months to a year.

[Celeste Campos Castillo]

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?

[Rachel Carlson]

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.

[Celeste Campos Castillo]

Good evergreen advice there. Always talk to your doctor. Okay, second story. This is about chatbots and mental health. I wanna mention we are about to discuss suicide. G. There have been a number of high profile cases to do with chatbot suicide. Mental health.

[Regina 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.

[Rachel Carlson]

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.

[Celeste Campos Castillo]

Every three hours. But do those reminders work?

[Regina 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 Listadius at the University of Wisconsin, Milwaukee. She says the idea that reminders could be a big solution struck us as

[Rachel Carlson]

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

[Celeste Campos Castillo]

Potential downsides like what?

[Rachel Carlson]

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.

[Celeste Campos Castillo]

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?

[Rachel Carlson]

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.

[Celeste Campos Castillo]

I'm gonna 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.

[Rachel Carlson]

Mine too.

[Celeste Campos Castillo]

Rachel.

[Regina Barber]

Ultra marathon.

[Celeste Campos Castillo]

Rachel, how? If I were doing ultra endurance running, how would it be changing my body?

[Rachel Carlson]

No, I'm totally with you there. I was really proud of my 10k until I read the 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.

[Celeste Campos Castillo]

You said extreme distances. Like what are we talking?

[Rachel Carlson]

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.

[Regina Barber]

Yeah, Long, long ways. They looked at the blood samples from these runners before and after the RA 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.

[Celeste Campos Castillo]

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?

[Rachel Carlson]

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 quickly, which can lead to the breakdown of red blood cells.

[Guest Expert (e.g., Matthew Steinhauser or Travis Nemkov)]

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.

[Regina Barber]

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

[Celeste Campos Castillo]

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?

[Rachel Carlson]

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.

[Celeste Campos Castillo]

Here's to keeping healthy and aging well.

[Rachel Carlson]

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

[Celeste Campos Castillo]

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

[Regina Barber]

If you or someone you know is considering suicide or in crisis, call or text the Suicide in crisis lifeline at 9. You can hear more of Mary Louise on consider this NPR's afternoon podcast about what the news means for you.

[Rachel Carlson]

And for more science stories just like this one, follow Short Wave on whatever app you're listening to. I produced this episode with Jordan Marie Smith and Hannah Chin. It was edited by Christopher Inteliotta, William Troup and Rebecca Ramirez.

[Regina Barber]

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

[Rachel Carlson]

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

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