Men who have physically demanding jobs and regularly lift heavy objects at work also have higher sperm counts and testosterone levels.
A new study that is part of a broader effort to explore how exposure to environmental chemicals and lifestyle choices affect reproductive health found the link.
"We already know that exercise is associated with multiple health benefits in humans, including those observed on reproductive health, but few studies have looked at how occupational factors can contribute to these benefits," said study author Lidia Mínguez-Alarcón. She is a reproductive epidemiologist at Brigham and Women's Hospital in Boston and co-investigator of the broader Environment and Reproductive Health (EARTH) study.
"What these new findings suggest is that physical activity during work may also be associated with significant improvement in men's reproductive potential," she said in a hospital news release.
The EARTH study is a collaboration between the Harvard T.H. Chan School of Public Health and Mass General Brigham healthcare system.
For the EARTH study, the researchers have collected samples and survey data from more than 1,500 men and women.
The latest study, published recently in the journal "Human Reproduction," focused on a subset of this overall group, including 377 male partners in couples seeking fertility treatment.
The investigators found that men who reported often lifting or moving heavy objects on the job had 46% higher sperm concentration. Their sperm counts were 44% higher than those of men with less physical jobs. These men also had higher levels of the male sex hormone testosterone and, surprisingly, of the female hormone estrogen.
"Contrary to what some people remember from biology class, 'male' and 'female' hormones are found in both sexes, but in different amounts," Mínguez-Alarcón explained. "In this case, we hypothesize that excess testosterone is being converted into estrogen, which is a known way for the body to keep normal levels of both hormones."
About 40% of infertility cases can be traced to male factors. These include sperm count, semen quality and sexual function. Sperm count and semen quality are thought to be the major drivers of growing infertility rates among men.
Earlier research from the EARTH group found that among men seeking fertility treatment, sperm count and quality declined by as much as 42% between 2000 and 2017.
"Further, there is increasing evidence that male infertility is associated with common chronic diseases such as cardiovascular disease and autoimmune disease, highlighting the broader importance of male reproductive health," Mínguez-Alarcón said.
More research is needed to confirm these findings for men from the general population, the study authors said. They hope that future studies will reveal the underlying reasons.
"Reproductive health is important in its own right, but more and more evidence suggests that male infertility can give us insight into broader public health issues, including the most common chronic diseases," Mínguez-Alarcón said. "Uncovering actionable steps people can take to improve their fertility stands to benefit all of us, not just couples trying to conceive."
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