A new Israeli study has found that average testosterone levels in men have fallen by 54% in less than five decades.
The findings, presented at the annual meeting of the European Society of Human Reproduction and Embryology in London, raise concerns that the well-documented decline in sperm counts is accompanied by a profound and ongoing deterioration in the male hormonal system.
The study was conducted as part of Alon Levy's master's thesis in public health at the Hebrew University of Jerusalem.
Researchers performed a meta-analysis, a statistical method that combines data from multiple previous studies, incorporating six longitudinal studies conducted in Israel, the United States, Brazil, Finland, and Denmark. Each study included testosterone measurements taken at at least three different time points. In total, data from 118,593 men examined between 1972 and 2019 were analyzed.
All six studies found declining testosterone levels over time.
Testosterone levels drop 54% worldwide
After combining the data, the researchers estimated that total testosterone levels had fallen by 54%. The trend reflects an average decline of more than 1% per year, with the rate of decline appearing to accelerate after 2000.
The scale of the decline is particularly striking because the studies were conducted in different countries, populations, and time periods.
Researchers adjusted for participants' ages, since testosterone naturally declines with age, but the observed decrease was substantially greater than could be explained by age alone.
According to Prof. Haggai Levin, chairman of the Israel Association of Public Health Physicians at the Hebrew University and Hadassah Medical Center, who led the research team, the trend is strong and cannot be explained as a random fluctuation or statistical error.
The findings add to previous studies by his group that identified a significant decline in sperm counts among men worldwide in recent decades.
What is the cause of the testosterone decline?
"More research is needed," Prof. Levin told Maariv. "We know that obesity and metabolic syndrome contribute to the decline.
Other studies have shown that environmental exposures, such as endocrine-disrupting chemicals, impair testosterone production both during fetal development and later in adult life.
Lifestyle habits that harm health, including smoking, poor nutrition, and a sedentary lifestyle, are also associated with lower testosterone levels and reduced male fertility. This study did not directly examine male fertility, but the two go hand in hand. There are likely common underlying factors."
Testosterone is generally associated with male libido and sexual function, but its role in the body extends far beyond that. The hormone helps regulate sperm production, build muscle mass, maintain bone density, and support metabolism. It is also linked to energy levels, mood, body hair growth, and the development of male sexual characteristics.
Low testosterone levels may manifest as fatigue, reduced sex drive, erectile dysfunction, loss of muscle mass, increased body fat, and reduced bone density. Some men may also experience mood disorders, difficulty concentrating, and decreased sperm production.
However, a single testosterone measurement is not sufficient for diagnosis, and physicians must also consider symptoms, the time of day the test was performed, and other medical findings.
The relationship between testosterone and overall health works in both directions. Chronic diseases, obesity, diabetes, and sleep disorders can reduce hormone production.
Conversely, low testosterone has been linked to metabolic disorders, declining muscle strength, and reduced quality of life. It remains unclear in many cases whether low testosterone causes disease, results from it, or is part of a cycle in which both conditions worsen each other.
One of the leading explanations for the decline is the global rise in obesity.
Fat tissue contains an enzyme that converts some testosterone into estrogen, and the accumulation of abdominal fat may disrupt hormonal signaling between the brain and the testes. Obesity is also associated with insulin resistance, chronic inflammation, type 2 diabetes, and metabolic syndrome, all of which may affect testosterone production.
Prof. Levin estimated that obesity and metabolic syndrome could explain between one-quarter and one-half of the decline observed. However, because the studies included in the analysis did not fully adjust for body weight, it is not yet possible to determine what proportion of the decline is attributable to rising obesity rates.
Among the other factors under investigation are endocrine-disrupting chemicals. These substances may mimic the activity of natural hormones, block them, or alter their production in the body. They may be found in some plastics, food packaging, pesticides, personal care products, cleaning agents, and other household products.
Researchers are concerned that exposure to these chemicals during fetal development could affect the formation of the reproductive system, while exposure during adulthood may impair testicular function and hormone production. However, studies examining the relationship between environmental chemicals and testosterone levels have so far produced inconsistent findings, and no specific substance has been identified as responsible for the trend.
Air pollution and global warming are also being investigated as possible contributors. The testes are located outside the body because normal sperm production requires temperatures slightly lower than core body temperature. Prolonged exposure to heat, extreme heat events, and climate change may affect testicular function, although evidence regarding their long-term impact on testosterone levels remains limited.
Modern lifestyles may also play a role. Reduced physical activity, prolonged sitting, diets high in processed foods, smoking, alcohol consumption, insufficient sleep, and chronic stress have all been linked in various studies to poor metabolic health and impaired reproductive function. While the impact of each factor individually may be limited, their combined effects over many years could be significant.