Unexplainable – “The Metabolism Myth”

Podcast: Unexplainable
Host: Noam Hassenfeld, Vox
Episode Date: September 17, 2025

Overview

This episode of Unexplainable, “The Metabolism Myth,” investigates the widespread belief that metabolism is the central factor behind body weight and weight loss. Through personal stories, historical science, shocking experiments, and modern studies (including “The Biggest Loser”), host Noam Hassenfeld and guest Julia Belous (health reporter and author) unravel the complex reality behind metabolism—and explain why the truth is much stranger, and simpler, than the myth.

Main Themes & Purpose

• Challenging the narrative that metabolism determines body weight.
• Exploring the science and history of metabolism.
• Examining real-world studies that expose surprising truths about metabolism and weight loss.
• Discussing why society clings to the metabolism myth and what we should focus on instead.

Key Discussion Points & Insights

1. Julia Belous’s Personal Journey with Metabolism

[00:57–02:29]

• Julia shares her childhood experience internalizing messages about weight.
  • “I was this chubby kid… I kind of internalized that. And I thought… maybe I have a slow metabolism.” – Julia Belous ([01:16])
• She tried extreme diets and mysterious supplements as a teenager, hoping for a “fix.”
  • “God only knows what was in them. But I had this hope that these supplements might be something that helped me finally lose weight.” – Julia ([01:43])
• As a science reporter, Julia remains obsessed with understanding metabolism’s real role.

2. The “Metabolic Chamber” Experiment

[03:05–05:49]

• Julia spends 23 hours in a metabolism chamber at NIH—measuring her breath to analyze calorie burn.
  • “It’s 5:30 now, and I’ve just laid down to relax once again… to see how much energy my body burns.” – Julia ([04:06])
• Results: Julia’s metabolism is “perfectly normal,” upending her lifelong belief.
  • “Your metabolism is perfectly normal.” – NIH researcher ([05:44])
  • “My mind went to, well, then, what the hell? … what is the reason and what else is going on?” – Julia ([05:58])
• Raises the episode’s central questions: What is metabolism? How does it really relate to weight?

3. The Science (and Misunderstandings) of Metabolism

[06:14–10:48]

• Historical perspective: Lavoisier’s 18th-century experiments link breath, heat, and energy.
  • Lavoisier discovers that bodies, like burning charcoal, use oxygen and produce CO₂—“both powered by combustion. Ours is just a lot less explosive.” – Noam ([09:22])
• Modern understanding: Metabolism is the sum of countless chemical reactions in all our cells. Most daily energy is used to simply exist (not exercise).
  • “Two-thirds or more [of calories burned] are from our body just existing, sitting around, doing nothing.” – Noam ([09:52])
  • “[Metabolism] converts everything we eat into everything we are and everything we do.” – Julia ([10:26])

4. The Connection Between Metabolism and Weight—A Flawed History

[10:55–15:15]

• In the 20th century, DNP (an industrial chemical) is used off-label for weight loss—dangerously speeding up metabolism and leading to deaths.
  • “If you can calibrate or speed up the metabolic rate in people, perhaps that is one way to help people lose weight. But it also suggested that… you don’t want to mess with this process.” – Julia ([11:49])
• Minnesota Starvation Experiment (WWII): Semi-starved men display physical and psychological effects from a slowed metabolism.
  • “Everything slows down, particularly the metabolism rate.” – Participant ([14:05])
  • “Their body responds… by going into power saving mode.” – Julia ([14:09])
• These studies birth the idea: Faster metabolism = easier weight loss, slower = harder.

5. Breakthrough: “The Biggest Loser” and a Metabolism Shock

[19:15–24:18]

• The TV show “The Biggest Loser” becomes an accidental scientific study on extreme weight loss.
  • “Not exactly the place you’d expect to find breakthrough science…” – Noam ([20:30])
• Researchers (including Julia’s co-author Kevin Hall) track contestants’ metabolism and weight:
  • After losing massive weight, contestants’ metabolisms slowed dramatically—but those with the greatest slowdowns also lost and kept off more weight.
  • “The people who had lost the most weight at the end of the contest had the greatest metabolic slowing.” – Julia ([22:33])
  • Even after 6 years, those with “slowest” metabolisms had kept off the most weight.
    • “So if I can understand that, the people with the slowest metabolisms lost the most weight?” – Noam ([23:01])
    • “Yes.” – Julia ([23:01])
• This upends the assumption that a “slow” metabolism always makes lasting weight loss impossible.

6. Why Did Contestants Regain Weight?

[24:18–26:47]

• Despite initial weight loss, many regain weight—but not because of metabolism.
• Former contestant Danny Cahill describes extreme restriction:
  • “I ended up after… five or six weeks… at 800 calories… burning near 8,000 a day. I lost 160 pounds in 90 days.” – Danny Cahill ([24:28])
• Return to regular life and food environments makes maintenance difficult.
  • “I was motivational speaker… took a toll on me… it was really hard to exercise for three hours A and to subsist on very little food when the demands of real life crept up…” – Danny ([26:08])
  • “What’s changed is the food environment.” – Julia ([26:31])
• For most, weight struggles are about the environment, not a defective metabolism.

7. The Data Problem—Why the Myth Persists

[27:14–28:29]

• Early nutrition science relied on unreliable self-reports—leading to the false “slow metabolism” narrative.
  • “We drastically under report what we eat…Do you remember the butter you put on your toast?…It’s extremely hard to be precise.” – Julia ([27:32])
  • Controlled studies show the metabolism-weight association mostly disappears when diet is properly measured.

8. Metabolism: It’s About Life, Not Weight

[29:18–30:18]

• Metabolism is foundational for life: movement, healing, speech, and every cellular process.
  • “It’s about life. …the reason we can blink… walk… heal wounds… the reason I can talk to you now. …I think it’s been a big distraction [to focus on weight].” – Julia ([29:23])
• The “weight loss = metabolism” fixation is a misdirection. The real focus should be on our modern food environment.

Notable Quotes & Memorable Moments

• “My metabolism is perfectly normal.” – NIH Researcher ([05:44])
• “Two-thirds or more [of daily calories burned] are from our body just existing, sitting around, doing nothing. Exercise burns less than a quarter.” – Noam ([09:52])
• “If you can calibrate or speed up the metabolic rate in people, perhaps that is one way to help people lose weight. But… you don’t want to mess with this process.” – Julia ([11:49])
• “Everything slows down, particularly the metabolism rate.” – Minnesota Starvation Participant ([14:05])
• “The people who had lost the most weight… had the greatest metabolic slowing… and six years later… the same finding again.” – Julia ([22:33–22:46])
• “When it comes to weight, it’s not metabolism. We need to stop obsessing about metabolism and pretending that’s the thing…when we’re thinking about our own weight struggles or the weight struggles of populations.” – Julia ([23:26])
• “We have this line in the book: ‘it’s not about metabolism, stupid.’ …we need to start looking at where the problem really lays for most people, and that is the food environment.” – Julia ([28:29])
• “It’s about life. …I think it’s been a big distraction [to focus on weight].” – Julia ([29:23])

Timestamps for Key Segments

• Julia’s personal struggle & myth of the ‘slow metabolism’: [00:57–02:29]
• Metabolic chamber insights: [03:05–05:49]
• Lavoisier and metabolism science history: [07:31–10:48]
• DNP and Minnesota starvation experiments: [10:55–15:15]
• Biggest Loser study and findings: [19:15–24:18]
• Why weight regain happens (Danny Cahill): [24:18–26:47]
• The myth’s persistence & unreliable data: [27:14–28:29]
• Reframing metabolism: it's about life: [29:18–30:18]

Final Takeaways

• The long-standing belief that a “slow metabolism” causes most people’s weight struggles is not supported by modern science.
• Dramatic weight loss can slow metabolism, but those with the slowest metabolisms (post-weight loss) were most likely to keep weight off.
• Most people’s metabolisms fall within a normal range; weight gain is driven largely by the modern food environment.
• Metabolism is fundamental to life—it’s been mistaken for an explanation of weight when it’s so much more.
• Focusing on “fixing your metabolism” as a path to weight loss is a distraction from the bigger factors at play.

Want more?
Julia Belous’s book, Food Intelligence: The Science of How Food Both Nourishes and Harms Us, co-authored with Kevin Hall, offers deeper insights into these issues.