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].
The findings raise a genuine question for men's health: what can this research tell us about testosterone, aging, and the appropriate use of testosterone replacement therapy?
Table of Contents
The Nature Study
This study was incredibly comprehensive. It pulled together two basic kinds of data.
On one hand, there is data from animals kept in zoos. These animals are often sterilized or placed on some form of hormonal birth control. Zoos maintain careful records for the animals they manage, providing a rich dataset. 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 conducted a comprehensive search for published studies on the effects of sterilization. The 71 studies they found covered 22 species of vertebrates—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.
The data revealed that, among male zoo animals, sterilization significantly extended 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 from the 71 published studies aligned with these zoo findings.
So what is actually going on here? How does castration lead to longer life in animals, and what does it mean for human health?
What stands out first is where gains are not seen. There is no significant benefit in the area of chronic diseases—things like heart disease and diabetes. This is notable both because these conditions are such massive problems in the human population and because they are closely linked to aging. Improvements here would be especially significant. But the data does not show them [1].
Something that does appear to drive improved survival in 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]
That finding is not entirely surprising.
What is more intriguing is the benefit seen in another category: "other causes" of death.
The biggest improvements in survival came from reductions in deaths that could not 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 somewhat mysterious how castration might be contributing to longer survival.
A clue comes from something the researchers observed about the timing of castration.
The greatest increases in lifespan were seen when castration happened early—before puberty. These individuals avoid the profound hormonal changes driven by testosterone. The researchers speculate that castration may contribute to extended survival by altering how the growth hormone pathway develops—a pathway that has been clearly linked 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]
The researchers' hypothesis: when an animal is neutered before puberty, the intervention occurs before testosterone permanently programs growth hormone systems to operate in higher gear. Turning down these systems has broadly been associated with longer lifespans.
For example, using Rapamycin to inhibit the growth enzyme mTOR in mice, worms, and flies has been shown to extend lifespan. The connection to testosterone is that testosterone, acting through the GH–IGF1 axis, keeps mTOR relatively more active—so removing testosterone early may produce some of the same downstream effects as pharmacologically targeting mTOR.
In summary, the researchers suggest two distinct mechanisms are at work:
1. Eliminating testosterone reduces risky and aggressive behavior.
2. It also alters lifespan-linked signaling pathways like GH–IGF1 and mTOR, which are central to aging research.
Human Studies
These animal findings are fascinating, but do they have any implications for human health?
Concerns about low testosterone are common. But does lower testosterone actually translate to extended lifespan?
Replicating a castration experiment in humans is not feasible—randomized controlled trials with this intervention are not going to happen. But historical records provide some genuinely informative data.
The practice of male castration goes back a very long time. The earliest documented records date to approximately 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]
One context where the practice became widespread enough to generate meaningful data 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]
A unique genealogical record survives from the early 1800s: the Yang-Se-Gye-Bo. This document contains lifespan records for 385 eunuchs, of whom 81 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—14.4 to 19.1 years longer than the lifespan of non-castrated men of comparable socioeconomic status [2].
This finding mirrors the zoo animal data: castration appears to extend lifespan, even in humans.
Low Testosterone and Implications
Does this mean low testosterone is a good thing for lifespan extension?
Low testosterone is treated for a reason—and that reason goes well beyond 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]
That last point warrants particular attention. The Korean eunuchs with near-zero testosterone appeared to live longer—which might suggest they were healthier. So why the apparent inconsistency?
First, it is important to acknowledge that the survival benefits seen in that study may not be what they seem.
The researchers had complete lifespan data for only 81 eunuchs out of 385. These 81 may be outliers and not representative of the broader population [2].
That said, the zoo animal study provides reason to think there probably is something real to the Korean figures—they should not be dismissed outright.
Still, living longer does not mean living without health consequences.
A study of a similar eunuch population in China found widespread 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]
Overall, the evidence strongly supports maintaining healthy testosterone levels as we age. The picture emerging from the research is nuanced: near-zero testosterone (as seen in eunuchs) may carry some survival advantages related to reduced aggressive behavior and altered growth signaling, but it also brings significant health costs—most notably to bone and metabolic health. The net picture for most men argues in favor of normal physiological testosterone, not supraphysiological levels and certainly not the near-zero range.
Practically, that means:
- Not chasing supratherapeutic levels with testosterone injections
- Not ignoring the root causes of low testosterone in the first place
Common root causes include:
- Obesity
- Lack of exercise
Research shows that a BMI over 30 was associated with nearly 9 times the risk of low testosterone [8]. This means that, for a substantial proportion of men presenting with low testosterone, body weight and physical inactivity are the primary drivers—not primary testicular failure. Addressing these upstream factors first is both safer and often more effective than proceeding directly to hormone replacement.
"Body mass index of 30 kg/m² or higher was associated with secondary hypogonadism (RRR = 8.74)" [8]
Evidence-Based Clinical Approach
Current clinical guidelines recommend a stepwise approach to managing low testosterone:
1. Address lifestyle factors first — diet, exercise, and sleep.
2. If lifestyle changes are insufficient to reach weight targets, GLP-1 medications may be considered.
3. Once weight targets are met and exercise is consistent:
- For men with true hypogonadism—defined by symptoms plus unequivocally low testosterone levels—testosterone replacement therapy (TRT) with endocrinology oversight may be appropriate [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 TRT has been initiated, careful follow-up is essential to assess patient response and monitor for adverse effects. The goal is restoration of physiological testosterone levels—not maximization [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
