Men who regularly lift heavy objects at work have higher sperm counts, suggests a new study from researchers at Harvard-affiliated Brigham and Women’s Hospital.

The study, published in Human Reproduction, is part of the Environment and Reproductive Health (EARTH) cohort, a collaboration between the Harvard T.H. Chan School of Public Health and Mass General Brigham to evaluate the effect of environment and lifestyle factors on fertility. EARTH has collected samples and survey data from over 1,500 men and women. The current study focused on a subset of these participants, including 377 male partners in couples seeking treatment at a fertility center.

The researchers found that men who reported often lifting or moving heavy objects at work had 46 percent higher sperm concentration and 44 percent higher total sperm count compared to those with less physical jobs. Men who reported more physical activity at work also had higher levels of the male sex hormone testosterone and, counterintuitively, 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. 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,” said first author Lidia Mínguez-Alarcón, a reproductive epidemiologist in Brigham’s Channing Division of Network Medicine and co-investigator of the EARTH study.

Infertility is a growing problem, and it can be caused by a wide variety of complex factors. However, about 40 percent of infertility cases can be traced to male factors, such as sperm count, semen quality and sexual function. In particular, sperm count and semen quality are thought to be the major drivers of growing infertility rates among males — a previous analysis led by the EARTH study team found that among men seeking fertility treatment, sperm count and quality declined by as much as 42 percent between 2000 and 2017.

While the current study found a relationship between physical activity and fertility in men seeking fertility treatment, it will take further research to confirm if these findings hold true for men from the general population. The researchers also hope that future studies will reveal the underlying biological mechanisms at play.

“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,” said Mínguez-Alarcón. “Uncovering actionable steps people can take to improve their fertility stands to benefit all of us, not just couples trying to conceive.”

Funding was provided by NIH grants R01ES022955, R01ES009718, R01ES033651, and R01ES000002 from the National Institute of Environmental Health Sciences (NIEHS) and Legacy, Inc.