Across nearly 50 years and five countries, testosterone levels have fallen sharply — and researchers say it may signal a wider crisis in male reproductive health.
Earlier studies pointed to falling sperm counts, while doctors have also tracked links between low testosterone, obesity, diabetes and poor metabolic health.
This new analysis adds another signal: across nearly five decades, testosterone levels appear to have fallen far more steeply than researchers expected.
Testosterone levels can change naturally over a man’s life, and lower levels aren’t automatically a sign of disease.
Most likely, researchers say, it reflects several forces acting at once, and it’s something we’d be wise to keep an eye on.
Image credits: Julien Tromeur.
Something keeps happening to men’s hormones. Across nearly 50 years and five countries, testosterone levels have fallen sharply — and researchers say it may signal a wider crisis in male reproductive health.
In a new analysis presented at the annual meeting of the European Society of Human Reproduction and Embryology in London, researchers reported that average total testosterone levels in men fell by 54 percent between 1972 and 2019.
The work, led by Prof Hagai Levine of the Hebrew University-Hadassah Braun School of Public Health and Community Medicine in Israel, drew together six long-running studies that had repeatedly measured testosterone over time. Together, the studies included 118,593 men from Israel, the United States, Brazil, Finland and Denmark.
“We saw an over 50% decline in total testosterone over this time period,” Levine told The Guardian. “It reflects a more than 1% decline each year, so this is not a fluke, this is not a statistical error.”
A Serious Decline
Scientists have been warning for years that men’s reproductive health may be in decline. Earlier studies pointed to falling sperm counts, while doctors have also tracked links between low testosterone, obesity, diabetes and poor metabolic health. This new analysis adds another signal: across nearly five decades, testosterone levels appear to have fallen far more steeply than researchers expected.
Testosterone is a complex hormone that affects much more than just fertility. It helps regulate sperm production and sexual desire, but it is also tied to muscle mass, bone density, mood, energy levels and metabolism. A sustained fall in testosterone, if confirmed, could therefore be a signal not just about reproduction but about men’s general health.
Testosterone levels can change naturally over a man’s life, and lower levels aren’t automatically a sign of disease. But a sharp population-wide drop across generations is different from ordinary aging. It may suggest that more men are being exposed to conditions that suppress testosterone, from obesity and diabetes to other pressures in the modern environment.
The new analysis focused on longitudinal studies, meaning studies that tracked testosterone measurements across multiple points in time rather than capturing a single snapshot. That type of comprehensive study design gives the research more weight than a simple comparison of unrelated groups.
What Could Be Causing It?
The biggest suspects are obesity, diabetes and environmental factors.
Obesity is strongly linked to lower testosterone. As body fat increases, more testosterone can be converted into oestrogen, lowering the amount of testosterone circulating in the body. Obesity is also closely tied to inflammation and metabolic dysfunction, which can interfere with normal hormone regulation.
Diabetes may play a role as well. Insulin resistance and chronically high blood sugar can disrupt the hormone signals that run from the brain to the testes. When those signals weaken, the body may produce less testosterone.
The new analysis did not control for obesity, which is an important limitation. That means the study can show a striking decline in testosterone over time, but it cannot say exactly how much of that decline is due to rising obesity or diabetes rates.
“If I had to guess — and it’s an educated guess — I would say that maybe one quarter to one half of the decline would be explained by obesity and metabolic syndrome,” Levine said.
Other researchers think the metabolic explanation could be even larger. “Obesity and diabetes could easily account for all of this,” said Prof Channa Jayasena, a reproductive endocrinologist at Imperial College London who was not involved in the work, also for The Guardian.
But Levine and his colleagues also point to possible environmental pressures. Endocrine-disrupting chemicals, which can interfere with hormone systems, are found in some plastics, personal-care products, pesticides and household materials. Air pollution and heat have also been studied as possible threats to male reproductive health.
The evidence for these environmental factors remains less settled. Studies have not always produced consistent results, and it is difficult to separate chemical exposure from diet, body weight, age, stress, sleep and other parts of modern life.
Supplements Aren’t the Solution
The finding lands in the middle of an already heated debate over male fertility. Levine’s group has previously reported steep declines in sperm counts, research that drew public attention. But it also drew criticism from scientists who questioned how much could be concluded from the available data. The new testosterone analysis will likely intensify that debate.
It is still not clear just how big of a problem this is for male fertility, and the answer will not be the same for every man. Testosterone is only one part of a much larger reproductive system, and lower levels do not always mean infertility. But the trend may feed a growing market for testosterone supplements — a response that fertility specialists say can backfire.
Testosterone treatments have become more visible in advertising and on social media, often marketed to men worried about fatigue, aging, low mood or declining masculinity. The “manosphere” frequently promotes testosterone replacement therapy (TRT) and supplements as a cure-all for a manufactured “masculinity crisis.”
Yet medically, the picture is more complicated. Taking testosterone can cause the brain to reduce the signals that tell the testes to make sperm.
Administering testosterone therapies creates a biological feedback loop that actually reduces sperm production and can reduce male fertility even more.
For now, the new analysis does not identify one cause or one solution. Instead, it points to a broad change unfolding across countries and decades.
The decline may turn out to be partly a story of obesity and diabetes. It may also involve chemicals, heat, lifestyle and other environmental pressures. Most likely, researchers say, it reflects several forces acting at once, and it’s something we’d be wise to keep an eye on.
The study was presented at the annual meeting of the European Society of Human Reproduction and Embryology in London and has not been published in a peer-reviewed study.
