Research shows that low testosterone causes men to lose muscle mass while keeping or gaining fat, and significantly weakens bones, especially in the pelvis. A 2026 study in male monkeys found that removing testosterone led to substantial muscle loss and decreased bone mineral density that persisted for 15 months, demonstrating a direct link between low testosterone and the body composition problems seen in aging men.
Researchers studied what happens to the body when testosterone levels drop dramatically. Using male monkeys on a Western diet, scientists removed their testicles to create very low testosterone conditions. Over 15 months, the animals lost muscle mass but gained body fat percentage, and their bones became weaker, especially in the pelvis. This research helps scientists understand how low testosterone in aging men can lead to muscle loss, weight gain, and osteoporosis, and it could help develop treatments to prevent these problems.
Key Statistics
A 2026 research article published in JBMR Plus found that surgical removal of testosterone in diet-induced obese male monkeys resulted in significant lean muscle loss while fat mass remained unchanged, causing body fat percentage to increase over the 15-month study period.
According to the 2026 study, orchiectomy (surgical testosterone removal) caused long-lasting bone loss lasting at least 15 months, with the most severe decreases in bone mineral content and density occurring in the pelvic region of male primates.
Research reviewed by Gram found that low testosterone induced transient elevation of bone turnover markers in the studied monkey colony, indicating accelerated bone breakdown that contributed to osteoporosis-like changes over the follow-up period.
The Quick Take
- What they studied: How removing testosterone affects body weight, muscle, fat, and bone strength in male monkeys eating a Western-style diet high in processed foods
- Who participated: A colony of male cynomolgus macaques (a type of monkey) that were already overweight from eating a Western diet. The exact number of animals wasn’t specified in the abstract, but they were a well-studied group
- Key finding: When testosterone was removed, the monkeys lost significant muscle mass but kept or gained fat, and their bones became weaker over 15 months, with the biggest bone loss in the pelvis
- What it means for you: This research suggests that low testosterone in men may cause muscle loss and bone weakness even if weight stays the same. Men with low testosterone should talk to their doctor about bone and muscle health, though this study was in monkeys and may not apply exactly to humans
The Research Details
Scientists studied a group of male monkeys that were already overweight from eating a Western-style diet. They surgically removed the testicles of these monkeys to create a state of very low testosterone, similar to what happens in some aging men or men with hormonal problems. Before surgery and at regular intervals for 15 months after surgery, the researchers measured the monkeys’ body composition using a special imaging technique called DXA (dual-energy X-ray absorptiometry), which shows how much muscle, fat, and bone a body has. They also measured hormone levels in the blood to confirm testosterone was low and to track other hormones that respond to low testosterone.
This approach allowed researchers to carefully track what happens to the body over time when testosterone drops. The monkeys were kept in controlled conditions and fed the same diet throughout the study, so scientists could isolate the effects of low testosterone from other factors. Because monkeys are biologically similar to humans in many ways, this model helps researchers understand what might happen in men with low testosterone.
This research design is important because it shows cause and effect. By removing testosterone and watching what happens, scientists can be more confident that low testosterone actually causes the body changes they observed, rather than just being associated with them. The 15-month follow-up period is long enough to see lasting effects, not just temporary changes. This type of animal research helps scientists understand human biology before testing treatments in people
This study was published in a peer-reviewed scientific journal, meaning other experts reviewed the research before publication. The researchers used objective measurement tools (DXA imaging and blood tests) rather than relying on what animals reported. The study followed animals for a long time (15 months), which strengthens confidence in the findings. However, the exact number of animals studied wasn’t provided in the abstract, which makes it harder to assess the study’s statistical power. Animal studies, while valuable, don’t always translate perfectly to humans
What the Results Show
After testosterone was removed, the monkeys experienced significant changes in body composition. Most notably, they lost muscle mass but did not lose fat mass, meaning their percentage of body fat actually increased even though they lost overall weight. This is an important finding because it shows that low testosterone doesn’t simply cause weight loss. Instead, it causes the body to lose the good stuff (muscle) while keeping the bad stuff (fat).
The effects on bone were even more concerning. The monkeys’ bones became significantly weaker, with decreased bone mineral content (the amount of minerals in bones) and bone mineral density (how tightly packed those minerals are). The pelvic bones were hit the hardest, showing the most dramatic losses. These changes persisted throughout the entire 15-month study period, suggesting that low testosterone causes long-lasting bone damage.
Blood tests confirmed that testosterone was very low after surgery, while other hormones that respond to low testosterone (follicle-stimulating hormone and luteinizing hormone) increased as expected. The researchers also saw temporary increases in bone turnover markers, which are signs that bones are being broken down faster than they’re being rebuilt.
The study found that inhibin B and estrogen levels also dropped significantly when testosterone was removed. This is important because estrogen, even in males, plays a role in bone health. The temporary elevation of bone turnover markers suggests that the body initially responds to low testosterone by breaking down bone faster, though this response may change over time. The fact that these changes lasted for 15 months shows that the effects of low testosterone on bone are not temporary but rather persistent and potentially progressive
According to Gram Research analysis, this study supports existing knowledge that low testosterone and obesity are connected in a two-way relationship. Previous research has shown that obesity can cause low testosterone, and this study demonstrates that low testosterone can worsen body composition by reducing muscle and increasing fat percentage. The bone loss findings align with what researchers know about osteoporosis in aging men with low testosterone. However, this study provides new detailed information about how quickly these changes happen and how severe they can be, particularly in the context of an already unhealthy diet
The study was conducted in monkeys, not humans, so the results may not apply exactly the same way to people. The abstract doesn’t specify how many animals were studied, making it difficult to assess whether the findings are statistically reliable. The monkeys were all on a Western-style diet, so the results may not apply to men eating healthier diets. The study doesn’t test any treatments to prevent these problems, so it doesn’t show whether interventions could help. Finally, while this model is relevant to human biology, individual humans vary greatly in how their bodies respond to low testosterone
The Bottom Line
Men concerned about low testosterone should discuss bone and muscle health with their doctor. If testosterone is low, doctors may recommend hormone replacement therapy, exercise programs focused on strength training, and adequate calcium and vitamin D intake. The evidence from this study is strong that low testosterone causes bone and muscle problems, but treatment decisions should be made with a healthcare provider who knows your individual situation. Confidence level: High for the connection between low testosterone and bone/muscle loss; moderate for specific treatment recommendations based on this animal study alone
This research is most relevant to aging men, men with diagnosed low testosterone, and men who are overweight and experiencing unexplained muscle loss. Men taking medications that lower testosterone should be aware of these potential side effects. Healthcare providers treating osteoporosis or sarcopenic obesity (having both weak muscles and excess fat) should consider testosterone levels as a factor. This research is less directly applicable to women, though some findings about the relationship between hormones and bone health may be relevant
Based on this study, significant changes in muscle and bone can occur within 15 months of low testosterone. However, the timeline for seeing benefits from treatment would depend on the specific intervention. Bone typically takes 6 to 12 months to show improvement with treatment, while muscle can respond more quickly to exercise, sometimes showing changes within 4 to 8 weeks
Frequently Asked Questions
Does low testosterone cause weight gain or muscle loss?
Low testosterone causes muscle loss while fat is preserved or increases, so body fat percentage rises even if overall weight drops. A 2026 primate study found this pattern persisted for 15 months after testosterone removal, showing the effect is long-lasting and significant.
Can low testosterone weaken bones?
Yes, low testosterone significantly weakens bones by decreasing bone mineral density and content. The 2026 research found the most severe bone loss occurred in the pelvis and lasted throughout the entire 15-month study period, indicating persistent damage.
What happens to hormones when testosterone is very low?
When testosterone drops, follicle-stimulating hormone and luteinizing hormone increase as the body tries to compensate. Estrogen and inhibin B also decrease, which matters because estrogen helps maintain bone strength even in males.
Is this monkey study relevant to human health?
Yes, cynomolgus macaques are biologically similar to humans in hormone systems and bone metabolism, making this model relevant to understanding age-related body composition changes and hypogonadism in men, though individual human responses may vary.
How quickly do bones weaken when testosterone is low?
The study showed significant bone loss within the 15-month follow-up period, with bone turnover markers initially elevated, suggesting bone breakdown accelerates quickly after testosterone removal and persists long-term.
Want to Apply This Research?
- Track body composition metrics monthly if possible (weight, muscle mass percentage, and fat percentage), along with energy levels and muscle strength during workouts. Note any symptoms of low testosterone such as fatigue, mood changes, or decreased muscle tone
- Users should log resistance training sessions (strength exercises) at least 3 times per week, track protein intake to support muscle maintenance, and record any discussions with doctors about testosterone levels. Set reminders for calcium and vitamin D intake, which support bone health
- Create a quarterly review of body composition trends, bone health markers if available from medical tests, and muscle strength improvements from workouts. Compare these metrics over 6-month and 12-month periods to identify whether interventions are working
This research was conducted in monkeys and while relevant to human biology, results may not apply identically to humans. Low testosterone is a medical condition that requires professional diagnosis and treatment. Do not use this information to self-diagnose or self-treat. If you suspect you have low testosterone or are experiencing unexplained muscle loss, bone weakness, or other symptoms, consult with a qualified healthcare provider. Any treatment decisions should be made in consultation with your doctor who understands your individual health situation, medical history, and risk factors.
This research translation is published by Gram Research, the science division of Gram, an AI-powered nutrition tracking app.