Is male testosterone in freefall?

Summary

Podcast Summary: Science Weekly — "Is male testosterone in freefall?"

Host: Ian Sample (Guardian Science Editor)
Guest: Hannah Devlin (Guardian Science Correspondent)
Date: July 9, 2026

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Episode Overview

This episode explores recent scientific findings indicating a dramatic decline in male testosterone levels over the past 50 years—by more than 50%, according to new research. Host Ian Sample and science correspondent Hannah Devlin discuss the evidence, possible causes, implications for male health, and what should be done in response to this trend. The conversation addresses skepticism in the scientific community, the challenges behind the research, and the wider health and societal ramifications of falling testosterone.

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Key Discussion Points & Insights

Introduction: Testosterone in Crisis?

• Context: Concerns about male fertility and hormone health have hit the mainstream, with commentators and officials warning of an “existential crisis” in male testosterone.
• Core Statistic: Recent meta-analysis finds a 54% decline in total testosterone among men since the 1970s.
• [01:18] Ian Sample introduces the controversial debate surrounding male reproductive health and testosterone.

The Study: Scope and Methodology

• Research Basis: The research is a meta-analysis conducted by Hagai Levine and an international team, combining six studies over a 50-year period, examining data from about 120,000 men.
• Significance: This study attempts to overcome previous inconsistencies in testosterone measurement and timeframe, offering the most comprehensive view yet.
• [02:32] Hannah Devlin: “They’ve stitched all that data together to produce a data set with around 120,000 men… they found a 54% decline in total testosterone over that time period.”

Scientific Community Response and Debate

• Mixed Reactions:
  • Some endocrinologists find the evidence convincing, acknowledging measurement challenges but seeing consistent trends across studies.
  • Others caution about unaccounted confounders like age differences and remain skeptical, noting contentious debates over similar claims about global sperm count declines.
• [04:26] Hannah Devlin: “The fact that it’s looking at this long time period and each of the studies kind of consistently find the same pattern, he felt that that was quite convincing.”
• [05:23] “There will be a bit of a range of reaction… but I think what’s clear is this is probably the most comprehensive attempt to date.”

Exploring the Causes: Why Is Testosterone Falling?

• Primary Theories:
  1. Obesity and Diabetes: Rising rates of both are known to decrease testosterone, possibly explaining a quarter to half of the observed decline.
  2. Environmental Factors: Potential roles for endocrine-disrupting chemicals (“forever chemicals”), air pollution, and other exposures. These are controversial and harder to pin down with certainty.
• Complication: Measurement methods, environmental exposures at different life stages, and unstudied factors make attribution challenging.
• [06:13] “Obesity would explain maybe a quarter to a half…but this was very much a sort of educated guess rather than being based on hard data.” — Hannah Devlin
• [07:43] “There’s…exposures during fetal development that set your testosterone and your fertility for life…and there’s also exposures during a man’s lifetime.”

Implications for Male Health and Fertility

• No Simple Relationship: Low testosterone doesn’t directly equate to infertility—normal testosterone levels can coexist with infertility, and vice versa.
• Health Indicator: Low T can both result from and contribute to broader health issues. The decision of when and how to treat low testosterone remains debated even among specialists.
• [08:51] “Your testosterone level, it doesn’t map directly onto your reproductive health…this is something that’s an ongoing debate within even the most specialist scientists here…” — Hannah Devlin

Testosterone Therapy: Solution or Risk?

• Popular Trend: There’s a surge in marketing and social media around “Low T” and testosterone supplementation for men.
• Potential Downside: Supplementing testosterone can actually harm fertility by disrupting the body’s hormone regulation and sperm production.
• [11:21] “If you take testosterone, it can counterintuitively have a negative effect on your fertility…there is a growing issue in fertility clinics of men who are taking testosterone supplements having fertility issues.” — Hannah Devlin

What Can Be Done? Next Steps

• Known Actions: Address obesity and diabetes—modifying lifestyle and health can make a difference.
• Harder Questions: Environmental exposure control is complex; more research is needed, but the precautionary principle might justify tighter regulation even without ironclad proof.
• [12:48] “If we think that we’ve got some evidence that there is [risk], we should be being really careful and doing far more to both investigate, but also banning them.” — Hannah Devlin, attributing Hagai Levine

How Serious Is the Threat?

• Perspective: Even significant declines may not immediately impair fertility, but ignoring potential risk could be irresponsible.
• [14:17] “If there’s a 1% chance that in 100 years it becomes really rare to be able to have a baby…shouldn’t we be taking that really seriously?” — Hannah Devlin referencing Hagai Levine

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Notable Quotes & Memorable Moments

• [02:32] “They’ve done this in what’s called a meta-analysis…with around 120,000 men…they found a 54% decline in total testosterone.” — Hannah Devlin
• [04:26] “This was actually really quite convincing that something has happened, that testosterone levels have declined.” — Channa Jaizena (as reported by Hannah Devlin)
• [06:13] “He thinks obesity would explain maybe a quarter to a half. But this was very much a sort of educated guess…” — Hannah Devlin, quoting Hagai Levine
• [08:51] “You can have low testosterone and have no fertility issues. You can have absolutely normal testosterone and be infertile…” — Hannah Devlin
• [11:21] “If you take testosterone, it can counterintuitively have a negative effect on your fertility.” — Hannah Devlin
• [14:17] “If there’s a 1% chance that in 100 years it becomes really rare to be able to have a baby…shouldn’t we be taking that really seriously?” — Hannah Devlin, referencing Hagai Levine

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Timestamps for Key Segments

• [01:18] – Framing the male fertility/testosterone “crisis”
• [02:32] – Research methods and headline findings
• [04:26] – Scientific reactions and debate over conclusions
• [06:03] – Exploring underlying causes
• [08:41] – Health/reproductive implications of low testosterone
• [11:02] – Commercial and clinical context for testosterone therapy
• [12:34] – What society and science should do next
• [14:04] – Long-term risks and societal implications

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Tone & Style

The conversation is balanced, informed, and pragmatic—skeptical without dismissing the seriousness of the findings. Both Ian Sample and Hannah Devlin convey curiosity, caution, and an awareness of the complexities involved.

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Conclusion

This episode offers a nuanced exploration of alarming trends in male testosterone, presenting robust evidence while highlighting uncertainties and urging both preventative action (where feasible) and continued research. It serves as a valuable overview for anyone curious about the biological, medical, and societal aspects of this emerging health concern.

Transcript

[Hannah Devlin]

This is the Guardian.

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[Ian Sample]

According to Manosphere Bros. Right wing pronatalists and the US Secretary for Health, we're currently experiencing an existential crisis of male fertility.

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Today, the average teenager in this country has 50% of their sperm count. 50% of the testosterone is a 65 year old man.

[Ian Sample]

Usually we might raise an eyebrow at these sorts of claims, but a new study suggests they might not be wildly off the mark.

[Hannah Devlin]

Over the past 50 years, they found a 54% decline in total testosterone.

[Ian Sample]

So if testosterone levels have dropped a whopping 50% in 50 years, why, what does it mean for men's health? And can anything be done to reverse the trend? I'm the Guardian Science editor, Ian Sample, and this is Science Weekly. Hannah, as science correspondent, you've reported on this astonishing finding that testosterone levels in men have more than halved. But between the 1970s and 2020, where's this come from?

[Hannah Devlin]

Yeah, so this is work that's being presented at a big fertility conference in London this week. And it's from Hagai Levine and his team in Israel and an international group of collaborators who've essentially looked at testosterone levels over the past 50 years. And they've done this in what's called a meta analysis. So they've taken six different studies that looked at the the same question, sort of, you know, how testosterone levels were changing, and they've stitched all that data together to produce a data set with around 120,000 men. And looking at their testosterone levels over time. And when they put all that together, they found a 54% decline in total testosterone over that time period.

[Ian Sample]

Let's dig into this. As you said, it's a meta analysis. It brings together a number of previously published papers. Does that mean that endocrinologists have been aware of this for a while?

[Hannah Devlin]

So what they did was they went and looked for previous studies that have tracked testosterone over time. And so I guess by definition there has already been some evidence that testosterone was going down from each of these studies, but they were covering much shorter timeframes, so typically sort of 10 or 20 years, whereas this is looking across a whole 50 year range. And it's been quite difficult to do that previously because the way testosterone is measured has changed quite a lot over that time period. So it's been hard to compare individual readings. So I think what's new here is they've tried really hard to sort of limit it to longitudinal studies which they think will be consistent within themselves, and then kind of patching those studies together.

[Ian Sample]

This is a really big statement to make that testosterone is halving. Have you talked to other scientists about this study and what do they make of it?

[Hannah Devlin]

Yeah, so I spoke to an endocrinologist called Channa Jaizena, who's at Imperial College. You know, he does a lot of work on testosterone and he said, although there are some possible confounders, things like, you know, significant changes in the way things are measured may be having a bit of an effect on the readings that you get. He said that this was actually really quite convincing that something has happened, that testosterone levels have declined. And the fact that it's looking at this long time period and each of the studies kind of consistently find the same pattern, he felt that that was quite convincing. I have since spoken to another US scientist who specialises in testosterone, he was more skeptical, who said they haven't accounted for possible differences in age between the cohorts. And it's worth saying that this work is from the same team as the group who did a very influential paper on declining sperm counts globally in 2017. But within the scientific community, it's been quite contentious and there are definitely people who dispute that claim that there's a declining global sperm count. So I think it's fair to say there will be a bit of a range of reaction and how strongly people buy into these findings. But I think think what's clear is this is probably the most comprehensive attempt to date to look at that question, and so it's going to be seen as A significant finding.

[Ian Sample]

Hannah, if this is a. A real trend, the key question is, is why? Why is testosterone falling? Do the authors themselves have any theories as to what's going on?

[Hannah Devlin]

So I think the kind of simplest and most obvious explanation, which I think will be uncontroversial, is that obesity and diabetes have risen significantly over this time period. And we know that both of those things separately have an effect on testosterone. So excess body fat changes the rate at which testosterone is converted into estrogen in the body, and that can sort of then enter this feedback loop where you've got more estrogen and that affects the brain signaling to the body to produce more testosterone. So I think the question really is whether that explains this entire effect or whether there's something else going on. I spoke to Hagai Levine, the lead author who was presenting the work, and he said that he thinks obesity would explain maybe a quarter to a half. But this was very much a sort of educated guess rather than being based on hard data. And he thinks that there are other environmental factors. So things like endocrine disrupting chemicals, air pollution. That's the bit that is more controversial. Other scientists, I think, would say that's much more of an open question. And we can't rule out that whatever trend is there is just being explained by health factors. And, you know, the evidence on things like air pollution and forever chemicals is much more unclear.

[Ian Sample]

How plausible do you find this explanation for that trend for those data? And do you think there might be anything else going on?

[Hannah Devlin]

I think what this paper does is it kind of sets out that question really clearly. It will perhaps be much easier now to design a prospective study where you're kind of intentionally going out to measure how much do those things really affect total testosterone. But on the environmental side, it's just a really difficult thing to study. It's so complex. So there's changes to environmental exposures during fetal development that set your testosterone and your fertility for life in a way that's irreversible. And then there's also exposures during a man's lifetime. So there was another paper this week at the same conference looking at air pollution and subtle changes to DNA in sperm. You know, there's sort of so much to unravel. It's quite difficult to answer some of the more worrying side of this that the authors are presenting that, you know, they're really strongly saying they think there's an environmental component. And it's quite hard to prove that that's the case.

[Ian Sample]

Have scientists or have doctors got any sense of what a decline in testosterone like this might mean for male reproductive health.

[Hannah Devlin]

Yeah. So I think it's important to say that your testosterone level, it doesn't map directly onto your reproductive health. So you can have low testosterone and have no fertility issues. You can have absolutely normal testosterone and be infertile. The two things are connected, but it's not like a one to one mapping. The lower your testosterone, the lower your fertility. There's a sort of two way relationship with health. So low testosterone can also be an indicator of poor physical health, but it can also have health effects. So, you know, it's something that endocrinologists are sort of grappling with as we speak. Like when are the right times to treat low testosterone? How should it be treated? How does that tie in with kind of broader health and then reproductive health? This is something that's an ongoing debate within even the kind of most specialist scientists here working in this area.

[Ian Sample]

Coming up, testosterone. How low could it go?

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[Ian Sample]

Hannah Recently a lot's been made of male testosterone, not least because companies are selling therapies for low T. And it's something we've talked about on the podcast before. The results like this suggest that maybe there is a reason for men to be taking extra testosterone.

[Hannah Devlin]

Yeah, it's become a huge thing. It's like a real narrative on social media especially, you know, testosterone naturally declines as men get older. There's been a big move to argue maybe that should be treated, maybe men should be taking testosterone supplements to counter that effect. And just, you know, generally there's loads of marketing out there asking men, do you feel a bit tired? Do you have brain fog? Perhaps it's low testosterone. The real issue with that, that's coming up in the clinic is that if you take testosterone, it can counterintuitively have a negative effect on your fertility. So your body sort of has a, almost like a thermostat that detects how much testosterone is circulating. And if it's kind of detecting higher levels, it's going to produce less testosterone internally in your body. And the problem with that is the testosterone, when it's naturally produced, is made in the testes. For sperm production to happen properly, you need that kind of high dose of testosterone in the testes. So there is a growing issue in fertility clinics of men who are taking testosterone supplements having fertility issues because their body's not making sperm in the same way it normally would.

[Ian Sample]

So, Hannah, where do we go from here? I mean, should this be a wake up call for scientists, for doctors, and maybe society even more? How do we begin to figure out exactly what's going on? And if there is a fertility crisis

[Hannah Devlin]

here, I think in a sense it could act as a wake up call. There are some parts of this that we can explain that we know quite well that obesity and diabetes have an impact on testosterone and just health in general. So those are the bits that we can do something about. The other part, the sort of questions around environmental exposures, clearly there does need to be more research and there is really good research going on from lots of teams into those kind of questions. They're just very hard questions to study. And I think the question on kind of when society might need to take action on those factors, that's a real judgment. I mean, how much evidence do you need? How big does the risk have to be before you ban certain substances? And the author who I spoke to, Hagai Levine, he said that we should be applying the precautionary principle here. So in his view, we don't need to be 90% certain, we don't even need to be 80% certain that there is risk from environmental chemicals. His view is that if we think that we've got some evidence that there is, we should be being really careful and doing far more to both investigate, but also banning them.

[Ian Sample]

If we don't tackle this though, Hannah, and if we keep seeing this 1% per year decline in testosterone that these scientists say they've observed, where does that leave us in another 50 years?

[Hannah Devlin]

Yeah, so that was sort of the perspective that Hagai Levine gave me. I mean, he said, let's not kind of overstate things. Men have a lot of sperm to start with. You can kind of have quite significant declines and still be able to have children. But his view was that if there's a 1% chance that in 100 years it becomes really rare to be able to have a baby, to be able to reproduce naturally, shouldn't we be taking that really seriously? I think it is a question that needs to be taken seriously, but I think at this point in time we don't have the full picture of what's going on.

[Ian Sample]

Hannah is fascinating stuff. It's going to be a great one to watch for the next few years. Thanks very much.

[Hannah Devlin]

Yeah, it definitely is. Thanks for having me on.

[Ian Sample]

Thanks again to Hannah Devlin. You can read her [e-mail]. And that's all from us. This episode was produced by Madeline Finlay. The sound design was by Joel Cox and the executive producer was Ellie BL Fury. We'll be back on Tuesday. See you then.

[Guardian Podcast Announcer]

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