A new scientific study has warned of a significant decline in average testosterone levels in men over the past five decades, a development that researchers say may be an additional indicator of a growing crisis affecting men's health and fertility worldwide, amid increasing concerns about the impact of modern lifestyles and environmental factors, according to The Independent.

Testosterone is the cornerstone of reproductive health in men, as its role is not limited to sperm production and regulating libido, but extends to maintaining muscle mass and bone density, regulating metabolism, supporting energy levels, and psychological stability and mood.

The study relied on a systematic review of six long-term studies that examined hormone levels in men, and included data from approximately 118,600 men, collected between 1972 and 2019 in five countries: the United States, Israel, Brazil, Finland, and Denmark.

The results showed that the average testosterone levels in men in 2019 had dropped by more than 50 percent compared to 1972. The researchers also observed similar indicators of hormone decline in all studies included in the analysis, which reinforces the hypothesis that the decline represents a continuous trend, rather than a transient change or a result of different measurement methods.

The researchers, led by Hagai Levine from the Braun School of Public Health at the Hebrew University-Hadassah in Israel, said the average decline since 1979 has been about 54 percent, noting that the rate of decline accelerated markedly since the beginning of the new millennium.

According to the study, testosterone levels are declining at a rate of nearly 1 percent per year, a rate that researchers describe as alarming, given the potential long-term repercussions on public health and reproductive capacity.

Researchers believe that obesity and diabetes may be among the most prominent factors associated with this decline. As fat accumulates in the body, part of the testosterone is converted into estrogen, leading to a decrease in its levels. Additionally, insulin resistance and chronically high blood sugar levels may affect the functions of the hypothalamus and pituitary gland in the brain, which are responsible for regulating hormone production.

The study did not rule out the role of environmental factors, as researchers suspect that endocrine-disrupting chemicals found in some household products, including what are known as 'forever chemicals,' may contribute to this decline, along with other factors still under investigation.

These findings support a previous study conducted by the same research team, which concluded a sharp decline in sperm counts in men over the past four decades, reinforcing concerns about declining male reproductive health indicators worldwide.

In contrast, experts warned against resorting to testosterone supplements as a general solution, explaining that using them without medical need may inhibit the natural production of the hormone, because the brain responds to high levels by stopping the signals that stimulate its production.

The study, titled 'Temporal trends in total and free testosterone levels (1972-2019): A systematic review and meta-analysis of change trends,' was presented at the annual conference of the European Society of Human Reproduction and Embryology, hosted by London, but it has not yet undergone peer review, meaning its results are still preliminary and require further scientific scrutiny before being finalized.