A massive new study published in Nature found that castration increases lifespan across vertebrate species, including zoo mammals, rodents, and wild animals [1].
This aligns with historical human data: Korean eunuchs lived 14–19 years longer than their peers [2].
Now unless you’re Bryan Johnson, I’m sure you wouldn’t consider chopping off your balls in pursuit of longevity. But here’s the question: are there ways we can apply this knowledge to our own health and in relation to testosterone replacement therapy?
Table of Contents
The Nature Study
This study was incredibly comprehensive. It pulled together two basic kinds of data.
On the one hand, there’s data coming from animals kept in zoos. These are often sterilized or put on some form of hormonal birth control. Zoos tend to keep careful records for the animals they manage, so there’s a treasure trove of data here. The researchers drew on records for 117 different types of zoo animals [1].
“In zoos, many animals with well-documented birth and death dates experience ongoing hormonal contraception or permanent surgical sterilization to inhibit the production of surplus animals, to maintain genetic diversity, to manage animal behaviour or for health reasons.” [1]
On the other hand, the authors also did a comprehensive search for published studies on the effects of sterilization. The 71 studies they found covered 22 species of vertebrates—that’s animals with a backbone—including:
- 4 primate species (including humans)
- 3 carnivores (including domestic cats and dogs)
- 3 ungulates
- 5 rodents
- 1 lagomorph
- 2 marsupials
- 1 reptile
- 3 fish species [1]
These studies provided 159 different treatment vs. control comparisons.
Here’s what the data revealed: when it came to male zoo animals, sterilization significantly extended the average lifespan. The impact was around 10% [1].
“We found that the life-prolonging effect of these interventions in zoo-housed animals was similar in males and females... female: β = 0.093, 95% confidence interval [−0.005, 0.190]; male: β = 0.098, 95% confidence interval [0.005, 0.191], R² < 0.01%” [1]
The numbers they got from the 71 existing studies were similar.
So what’s actually going on here? How does castration lead to longer life in animals? And what does it mean for us humans?
What jumps out at first is where we don’t see gains. And that’s in the area of chronic diseases—things like heart disease and diabetes. This is important both because these are such massive problems in the human population and because they are linked to aging. So improvements here would be especially interesting. But again, we don’t see them in this study [1].
Something that does seem to be driving improvements in lifespan for castrated males is less risky and aggressive behavior [1].
“We observed that in male mammals housed in zoos, surgical sterilization, particularly castration, leads to improved survival partly through reduced death occurring as a consequence of behavioural interactions, suggesting that a lowering of androgen actions on male behaviour (such as aggressive behaviours and risk seeking) may contribute to improved survival.” [1]
But that’s hardly surprising.
What’s more intriguing is the benefit we see in another area: "other causes" of death.
The biggest improvements in longevity came from avoiding deaths that couldn’t be classified into known categories [1].
“The remaining deaths (around 65%) in animals undergoing necropsy were undetermined and could not be linked to a specific cause of death or were the result of a disease or mortality source that occurs in such small proportions so as not to be recorded as a category within the database.” [1]
By itself, this leaves it a bit mysterious how castration might be helping.
But we may get a clue from something the researchers noticed about the timing of castration.
Interestingly, the greatest increases in lifespan were seen when castration happened early—before puberty. These individuals avoid a series of profound changes brought about in the body by testosterone. The researchers speculate that castration may contribute to lifespan by altering how the growth hormone pathway develops. This pathway has been clearly connected to the aging process [1].
“Of note, the greatest increase in survival occurred when males underwent surgical sterilization early in life, prior to puberty, when gonadal hormone exposure can have organizing effects on body size and programs the adult functionality of the brain and other hormonal systems, such as the growth-hormone insulin-like growth factor 1 (GH–IGF1) signalling axis. The GH–IGF1 axis and related cellular signalling targets (for example, the mTOR signalling complex) directly influence lifespan.” [1]
Their basic idea is this: when we neuter an animal before puberty, we’re intervening before testosterone permanently programs growth hormone systems to run in higher gear. Turning down these systems has, broadly speaking, been associated with longer lifespans.
For example, using Rapamycin to turn down the growth enzyme mTOR in mice, worms, and flies extends lifespan.
In short, the researchers suggest we’re looking at two distinct mechanisms:
1. Shutting down testosterone reduces risky behavior.
2. It also alters longevity-linked signaling pathways like GH–IGF1 and mTOR, which are central in aging research.
Human Studies
So all this is fascinating. But does this research have any implications for us?
We hear worries all the time about having too little testosterone. Is that actually a good thing for lifespan extension though?
It would be great if we could somehow replicate the experiment in humans. But there’s an obvious hurdle here: we’re not going to be seeing randomized controlled trials with castration as an intervention any time soon. And even if Bryan Johnson tries it out, that’s just one data point.
But it turns out we have access to some intriguing data from historical records.
The castration of males, for various reasons, goes back a very long time. The earliest records of the practice are from around 4,000 years ago in Sumeria [3].
“The earliest records for intentional human castration to produce eunuchs are from the Sumerian city of Lagash in the 21st century BC.” [3]
In some contexts, the practice became widespread enough that we can gather some significant data. One such context is the imperial court of the Chosun Dynasty in Korea (1392–1910), where eunuchs—men who had been castrated—often held important government roles [2].
“Eunuchs of the Chosun Dynasty lived with privileges: Korean eunuchs were conferred with official ranks…” [2]
And it just so happens that a unique record survives from the early 1800s. It records the birth and death dates of generations of eunuchs. This document, called the Yang-Se-Gye-Bo, contains the records of 385 eunuchs, but only 81 of them had identifiable birth and death dates [2].
“We examined the lifespan of Korean eunuchs by analyzing the Yang-Se-Gye-Bo—a genealogy record of Korean eunuchs…” [2]
The average lifespan of these 81 eunuchs was 70 years. That may not sound extraordinary by today’s standards, but it was 14.4 to 19.1 years longer than the lifespan of non-castrated men of similar socioeconomic status [2].
So this study fits the same pattern we saw in the study of zoo animals. Castration seems to boost longevity—even in humans.
Low Testosterone and Implications
So again, is low testosterone actually a good thing for lifespan extension?
The problem with this line of thinking is that we treat low testosterone for a reason. And it isn’t just about mood or vitality.
Low Testosterone is Linked to:
- Type 2 diabetes
“Hypogonadism, in particular testosterone deficiency, is associated with metabolic disorders, and predicts an increased risk of developing incident MetS and T2-DM.” [4]
- Osteoporosis in older men
“Testosterone signaling stimulates osteoblasts to form trabecular bone and helps osteocytes prevent trabecular bone loss…” [5]
- Elevated risk of all-cause mortality
“Lower testosterone levels are associated with higher risk of death in men.” [6]
And that last one should particularly catch our attention. Those Korean eunuchs with practically zero testosterone appeared to live longer. Which gives the impression they were healthier.
So why the apparent inconsistency?
First, it’s important to acknowledge that the longevity benefits seen in that study may not be what they seem.
The researchers had full lifespan data for only 81 eunuchs out of 385. These 81 may be outliers and not representative of the whole population [2].
But equally, we don’t want to dismiss it either. The zoo study gives us reason to think there probably is something to the Korean figures.
Still, just because they lived longer doesn’t mean they didn’t suffer health consequences.
A study of a similar eunuch population in China found frequent evidence of osteoporosis. In a sample of 31 eunuchs aged 45–57:
- 21 developed kyphosis
- They had sparse body hair and no beard growth [7]
“…21 of the 31 had developed kyphosis as a clear sign of osteoporosis.” [7]
All things considered, it’s highly likely that maintaining healthy testosterone levels as we age is desirable. But that doesn’t mean we should:
- Chase supratherapeutic levels with testosterone injections
- Or ignore the root causes of low testosterone in the first place
Common root causes include:
- Obesity
- Lack of exercise
One study found that a BMI over 30 was associated with nearly 9 times the risk of low testosterone [8].
“Body mass index of 30 kg/m² or higher was associated with secondary hypogonadism (RRR = 8.74)” [8]
Clinical Approach
The approach I take in the clinic is:
1. Address lifestyle factors first — diet, exercise, and sleep.
2. If those aren’t enough to reach weight targets, we’ll consider GLP-1 medications.
3. Once weight targets are met and exercise is consistent:
- If the patient has true hypogonadism, defined by symptoms plus unequivocally low testosterone levels, we’ll consider testosterone replacement therapy (TRT) with endocrinology oversight [9].
“We recommend diagnosing hypogonadism in men with symptoms and signs of testosterone deficiency and unequivocally and consistently low serum total testosterone and/or free testosterone concentrations.” [9]
Once treatment has been initiated, careful follow-up is important to assess how the patient is responding and to watch for adverse effects [9].
“In hypogonadal men who have started testosterone therapy, we recommend evaluating the patient after treatment initiation to assess whether the patient has responded to treatment, is suffering any adverse effects, and is complying with the treatment regimen.” [9]
References
1. https://www.nature.com/articles/s41586-025-09836-9
2. https://www.cell.com/current-biology/fulltext/S0960-9822(12)00712-9
4. https://www.frontiersin.org/journals/endocrinology/articles/10.3389/fendo.2019.00345/full
5. https://pmc.ncbi.nlm.nih.gov/articles/PMC5376477/
6. https://pmc.ncbi.nlm.nih.gov/articles/PMC9938530/
7. https://pmc.ncbi.nlm.nih.gov/articles/PMC3955324/
8. https://academic.oup.com/jcem/article-abstract/95/4/1810/2597149
