Men’s average testosterone levels have halved over the past 50 years, according to scientists, who say society is facing a male fertility crisis.

Total testosterone levels in men declined by 54% between 1972 and 2019, according to data presented at the annual meeting of the European Society of Human Reproduction and Embryology in London on Tuesday.

Rising levels of obesity and diabetes are expected to play a part, but the team behind the work suggest that environmental factors such as endocrine-disrupting chemicals – which can be found in various household items – and global heating could also be factors in the apparent striking decline.

“I think that we have a major crisis in male reproductive health and it’s currently not given enough attention,” said Prof Hagai Levine, of the Hebrew University-Hadassah Braun School of Public Health and Community Medicine in Israel.

“We saw an over 50% decline in total testosterone over this time period,” he said. “It reflects a more than 1% decline each year, so this is not a fluke, this is not a statistical error. It’s very strong trend.”

The findings will be viewed as a significant contribution in a widespread debate about whether male fertility is in decline and why. Previous work by the same team that concluded sperm counts have steeply declined over the past 40 years have captured public attention, with the US health secretary, Robert F Kennedy Jr, recently describing male fertility decline as an “existential problem”.

However, the claims have been contentious within the scientific community. Prof Channa Jayasena, of Imperial College London and a consultant in reproductive endocrinology, said the latest observations on testosterone should act as “an important reality check”.

“The fact that these studies have been done over several periods in history makes me actually convinced,” he said. “I do think male reproductive health is declining and appears to have been declining over time.”

The relationship between male testosterone and health is two-way and complex. Testosterone regulates sperm production, sex drive, helps build muscle mass, bone density and plays a role in mood, energy levels and metabolism. Health factors such as obesity – where excess body fat increases the conversion of testosterone into oestrogen – can lead to lower testosterone. There is also significant medical debate about the use of testosterone supplements, which can – counterintuitively – suppress sperm production.

“Reproductive health is a very important signal of general health,” said Levine. “We live in an environment that is not ideal for our health in terms of our exposure to chemicals in terms of climate and in terms of health behaviours.”

The meta-analysis combined six previous longitudinal studies tracking testosterone, each of which included at least three time points. Together, they included data from 118,593 individuals from Israel, the US, Brazil, Finland and Denmark, from 1972 to 2019.

Each of the studies, individually, found a decline in testosterone and when the data was combined, the overall drop was estimated at 54%, with the decline appearing to accelerate after 2000.

The individual studies controlled for age, but it is possible that confounding factors such as differences in average ages between the cohorts may have influenced the results. Obesity was also not controlled for, which is known to be strongly correlated with low testosterone.

“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.

Others said that such a conclusion would be premature. “Obesity and diabetes could easily account for all of this,” said Jayasena. “There appears to be a clear reduction in testosterone levels. What we need to do is to clarify, in addition to obesity and diabetes, whether environmental factors are provoking this.”

It is less clear which environmental factors might be involved, with studies on air pollution and endocrine-disrupting chemicals tending to produce inconsistent results. In light of uncertain evidence, Levine said, the precautionary principle should be applied.

“The level of certainty that we need is not 95%,” he said. “We should find better ways to prevent exposure of the general public to hazardous chemicals. And we don’t do it well enough at the moment.”

Prof Allan Pacey, a professor of andrology at the University of Manchester, who was not involved in the work, said a concern about the narrative that men have low testosterone is the increasing promotion of testosterone supplements on social media.

“The solution that’s being promoted is that we give you testosterone,” he said. “But if you give a man testosterone, you switch off his sperm production. I’ve seen that in the clinic.”