Is male testosterone in freefall?

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1:49According to Manosphere Bros, right-wing pro-natalists, and the U.S. Secretary for Health, we're currently experiencing an existential crisis of male fertility. Today, the average teenager in this country has 50 % of the sperm count. 50 % of the testosterone is a 65-year-old man. Usually, we might raise an eyebrow at these sorts of claims, but a new study suggests they might not be wildly off the mark. Over the past 50 years, they found a 54 % decline in total testosterone.

2:27So 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.

2:51Hannah as science correspondent you've reported on this astonishing finding that testosterone levels in men have more than halved between the 1970s and 2020 where's this come from? Yeah so this is work that's being presented at a big fertility conference in London this week and it's from Haggai 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 same question, sort of, you know, how testosterone levels were changing.

3:31And 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. 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? 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.

4:18So typically, you know, 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. 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?

4:57Yeah, so I spoke to an endocrinologist called Chana Giazina, 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, maybe 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.

5:35I have since spoken to another US scientist who specialises in testosterone who was more sceptical, 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, you know, there will be a bit of a range of reaction and how strongly people buy into these findings.

6:16But I 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.

6:34Hannah, if this is a real trend, the key question is why? Why is testosterone falling? Do the authors themselves have any theories as to what's going on? 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 oestrogen and that affects the brain signaling to the body to produce more testosterone.

7:17So I think the question really is whether that explains this entire effect or whether there's something else going on. I spoke to Haggai 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.

8:01And the evidence on things like air pollution and forever chemicals is much more unclear. How plausible do you find this explanation for that trend for those data? And do you think there might be anything else going on? 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.

8:48And 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. Have scientists or have doctors got any sense of what a decline in testosterone like this might mean for male reproductive health? Yeah so I think it's important to say that your testosterone level it doesn't map directly onto your reproductive health.

9:30So 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 that 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 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 who are working in this area

10:25coming up testosterone how low could it go

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11:31Hannah, 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. do results like this suggest that maybe there is a reason for men to be taking extra testosterone yeah it's become a it's become a huge thing it's it's like a real narrative on social media especially you know testosterone naturally declines as as men get older there's been a big move to you know 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?

12:13Do 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 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 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.

13:34The 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, Haggai Levine, he said that we should be applying the precautionary principle here. So in his view, we don't need to be 90 % certain.

14:15We don't even need to be 80 % certain that there is a 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. 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? Yeah, so that was sort of the perspective that Hagel 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 one percent chance that in a hundred 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 hannah is fascinating stuff um it's going to be a great one to watch for the next few years thanks very much yeah it definitely is thanks for having me on thanks again to hannah devlin you can read her piece at theguardian.com

15:43and that's all from us this episode was produced by madeline finley the sound design was by joe cox and the executive producer was ellie burie we'll be back on tuesday see you then

16:02this is the guardian

16:08Thank you.