Professor Alan Pacey (2:58)

So the landmark study which really propelled this debate into the public domain was published in 1992. What that did was look back at sperm quality from the 1940s up until the late 80s early 90s. And that study was heavily criticized. It was heavily criticized because the Populations of men that had been looked at over time were not the same.

Lizzie McNeil (3:21)

Studies from the 1940s looked at men with proven fertility, whereas the studies from the late 80s included samples from men attending fertility clinics. What is also important is that what we considered as a normal sperm count wasn't even defined in the 1940s.

Professor Alan Pacey (3:38)

We didn't actually define what normal was until about 1952. We changed our minds over what was normal several times since the 1950s as well. Counting techniques had changed from the 1940s to the early 1990s. Generally, poor counting technique overestimates the sperm concentration, which means poor counting technique in the 1940s would have automatically elevated the situation in comparison to a modern counting.

Lizzie McNeil (4:08)

RFK was likely quoting a more recent meta analysis published in 2023, and they found that global fertility rates had fallen by as much as 50%. However, the meta analysis relied on data that had the same issues as the 1992 study.

Professor Alan Pacey (4:26)

That data is controversial. All of these populations are subtly different in different ways, and so comparing like with like around the world and over time is actually quite a difficult thing to do.

Lizzie McNeil (4:39)

It's not just differences in counting that can affect results. Sperm counts can fluctuate even within the same individual. For example, abstinence makes sperm count go up as men produce sperm constantly, and different studies require different periods of abstinence. In all, there are so many factors that make it difficult to distinguish true declines from methodological differences.

Professor Alan Pacey (5:03)

There are other meta analyses that show that there has been no change.

Lizzie McNeil (5:07)

The most recent meta analysis published in 2025 showed there had been no discernible decrease in sperm counts in US men across the generations. In fact, if you look at the studies that have been done, 21 show no decline, 8 show a decline, and 6 turned up ambiguous results. Allen says that even the studies which do show a decline do not tie the drop in fertility levels to falling sperm counts.

Professor Alan Pacey (5:34)

If there has been a change in sperm concentration, it's been a change from normal to normal. And therefore, for the vast majority of men, this probably shouldn't have an impact if it is a real effect in the first place.

Lizzie McNeil (5:49)

So no, we do not have evidence that US teenagers have 50% lower sperm counts than 65 year old men. But what about the testosterone claim? 50% of the testosterone is a 65 year old man.

Lizzie McNeil (6:05)

Testosterone does decline with age. Some studies estimate that this drop is around 1% a year from when a man reaches his 30s and 40s. Testosterone replacement therapy, or TRT, is a bit of a craze at the Moment. Fitness influencers and podcasters are touting the benefits of hacking their biology by taking testosterone supplements. Prescriptions for TRT in the US increased from 7.3 million to 11 million between 2019 and 2024. Robert F. Kennedy Jr uses TRT and like him, the majority of users are over the age of 40. But the fastest growing demographic are men in their 20s and 30s. And there's a concern from doctors that younger men are taking TRT when they do not need to.

Edif Aaron (6:49)

Honestly, I'd seen stuff on X and a lot of like influencer type people mentioning that total testosterone had dropp dropped over time.

Lizzie McNeil (6:58)

That's Adif Aaron, a graduate of medicine currently doing his research year at Yale. He decided to see if the rumors were true by looking into the data behind us testosterone rates.

Edif Aaron (7:09)

So I just started saying like, you know what percent of people have a low toll testosterone in the United States every single year and sort of projecting it out to the population.

Lizzie McNeil (7:18)

Traditionally you were considered to have low testosterone if you were under 300 nanograms per deciliter. Edif used data that was collected by the Centers for disease control in 2002, 2004 and then again in 2010 and 2011. According to a basic read of the data, the amount of people with Testosterone levels under 300 skyrocketed.

Edif Aaron (7:41)

Suddenly it spikes about 100% over seven years.

Lizzie McNeil (7:45)

EDIF took a closer look at the data and realized that the methodology had changed. The earlier tests had been conducted using immunoassays.

Edif Aaron (7:53)

The way that the immunoassay works is you stick a compound that binds total testosterone and then you measure fluorescence or radioabsorbance of that.

Lizzie McNeil (8:04)

It's a complicated process to wrap your head around in the time we have, but what you need to know is that this is what the 300 threshold was based off.

Edif Aaron (8:14)

Then over the next 20 years, we sort of switched our methods, you know, multiple times.

Lizzie McNeil (8:18)

Because of the difference in methodology, it's hard to compare testosterone levels from the 70s and 80s to even the 90s and 2000s. And since then we've started using a completely different methodology, mass spectrometry. And this is considered to be much more accurate.

Edif Aaron (8:35)

But that old 300 level really never got changed.

Lizzie McNeil (8:39)

Recently, the endocrine Society recommended that 264 is a much more appropriate threshold for low testosterone. As they were able to measure testosterone levels more accurately, they realized that many men who had no signs or symptoms of a deficiency had levels of around 300.

Edif Aaron (8:57)

So we sort of did a sub analysis where we said okay, we're going to use 300 on these older generation of amino acids and then once the assay gets switched to mass spec in this US study, we're going to switch it to 264.

Lizzie McNeil (9:08)

When they did this, the decline in testosterone levels vanished and you can see.

Edif Aaron (9:13)

That the rates are about constant. And so the overall level of people who are sort of self reported healthy who have this low value don't really change much.

Lizzie McNeil (9:21)

It seems that any huge drop in testosterone levels are likely to do with differences in how data was collected rather than a reflection of what's actually happening. One thing that is concerning is that if people continue to use the higher threshold but with newer technology, then people who do not need to be on TRT might be prescribed it. And there is some irony in that because do you know what is proven to lower sperm count to the point of infertility? Taking testosterone supplements when you don't need them. And that is all we have time for this week. Thank you to our experts and if you have any questions or comments, do write in to more or lessbc.co.uk. until next week. Goodbye.

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Edif Aaron (10:25)

Oatmeal.

Sarah (10:26)

So long, you strange soggy.

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