A new study from researchers from Brigham and Women’s Hospital suggests that men who regularly lift heavy objects at work have higher sperm counts. The study, published in Human Reproduction, is part of the Environment and Reproductive Health (EARTH) cohort, a clinical study which aims to explore how exposure to environmental chemicals and lifestyle choices affect reproductive health.

“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 first author Lidia Mínguez-Alarcón, a reproductive epidemiologist in Brigham’s Channing Division of Network Medicine and co-investigator of the 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.”

About 40% 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% 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,” said Mínguez-Alarcón.

The researchers found that men who reported often lifting or moving heavy objects at work had 46% higher sperm concentration and 44% 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,” said Mínguez-Alarcón. “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.”