Men with physically demanding jobs found to have higher sperm concentration and total sperm count

Published 14 March 2023

A study based on self-reported answers to a questionnaire cross-referred to semen samples and hormone measurements has found that physically demanding jobs and rotating or evening shift working may be associated with higher testicular function in men as seen by higher sperm concentrations and counts as well as higher serum testosterone and estradiol levels.

Male factor infertility has become a 21st century global problem, not only because of its link with reproductive health but also with a general risk of chronic disease such as diabetes and cancer. Concerns over declining sperm count and quality have been fuelled largely by the meta-analysis of Levine et al which, in an update last year, presented salutary new evidence of an acceleration in pace of this downswing.(1,2)

Various factors have been analysed to establish why sperm counts and concentration appear to have more than halved in 40 years. Numerous studies have focused on lifestyle, BMI, environment including endocrine-disrupting chemicals, and diet in the search for what might improve or impair sperm health. A recent RCT has even suggested that eating nuts might reduce sperm DNA fragmentation, a cause of reduced fertilisation rates and embryo quality.(3)

Now a new study suggests that men’s occupations might be added to this long list of factors contributing to a decline in male reproductive health.(4) Observational data based on more than 300 participants show that sperm concentrations and serum testosterone levels among men who do physically demanding work are nearly double those of employees who never lift or move heavy objects. Certain work shift patterns also result in these positive associations. The findings are from the Environment and Reproductive Health (EARTH) study at Brigham and Women’s Hospital and Harvard Medical School.

All participants in this latest EARTH study were also male partners of couples seeking fertility treatment, with a minority diagnosed with conditions such as cryptorchidism (5%) and varicocele (6%). The majority (87%) were of white ethnic background, with a median BMI of 27, and age ranging from 33 to 39 years. Enrolment took place between 2005 to 2019 with an aim of exploring the relationship between self-reported occupational factors and several markers of testicular function.

The men completed questionnaires about work shift patterns, how often they lifted or moved heavy objects, and physical level of exertion at work (eg, sitting, working in office etc); and were asked about lifestyle (eg, drug use, exposure to chemicals) and other factors such as diet. Semen samples were analysed following WHO guidelines, and multiple samples collected from most men. Of the 377 participants who completed the survey, 12% reported often lifting or moving heavy objects at work, 6% reported heavy physical exertion at work, and 9% reported evening or rotating shifts.

Overall, results showed most semen samples exceeded the WHO reference values. Men who reported often lifting/moving heavy objects at work had 46% higher sperm concentrations (P = 0.01) and 44% higher total counts (P = 0.01) than those who reported never lifting heavy objects. Results were similar for men on rotating shifts (defined as an alternating cycle of days/nights) compared to those on day shifts, and among men involved in heavy physical exertion compared to those engaged in light levels.

A subset of 145 participants had hormone measurements taken from blood samples. Of these workers, those involved in heavy/moderate levels of physical exertion in the workplace had higher circulating testosterone concentrations than those with lighter exertion, and men who often moved/lifted heavy objects had higher estradiol concentrations than those who never did. In addition, higher testosterone concentrations were observed among male workers on evening/rotating shifts: their levels were 24% greater than those of men on day shifts and estradiol concentrations 45% higher. No associations were observed for ejaculated volume, total motility, morphologically normal sperm, or serum FSH and LH concentrations.

The authors note that these results appear to conflict with earlier studies, which generally found work-related heavy exertion was linked to lower sperm concentration/total sperm count, with shift work not associated with semen quality. However, referring to a similar study reported in 2015, the authors note that participants here had generally higher median sperm concentrations/total sperm counts, and were younger but heavier than the men from the EARTH study.(5)

However, semen parameters among the EARTH study participants were comparable to those among young healthy men from the general US population, but the authors acknowledge a limiting factor in only sampling the male partners of couples having fertility treatment. ‘It may not be possible to generalize our findings to men from the general population,’ they concede. However, they still propose that physically demanding jobs and rotating or evening shift occupations may be associated with higher testicular function in men as indicated by higher sperm concentrations and counts as well as higher serum testosterone and estradiol levels. But confirmation of these findings would only be derived from other non-fertility clinic study populations, and from subjects of different ethnic backgrounds.

1. Levine H, Jørgensen N, Anderson M-A, et al. Temporal trends in sperm count: a systematic review and meta-regression analysis. Hum Reprod Update 2017; 23: 646-659.
2. Levine H, Jørgensen N, Anderson M-A, et al. Temporal trends in sperm count: a systematic review and meta-regression analysis of samples collected globally in the 20th and 21st centuries. Hum Reprod Update 2022;
3. Salas-Huetos A, Moraleda R, Giardina S, et al. Effect of nut consumption on semen quality and functionality in healthy men consuming a Western-style diet: a randomized controlled trial. Am J Clin Nutr 2018; 108; 953-962.
doi.org/10.1093/ajcn/nqy181. PMID: 30475967
4. Mínguez-Alarcón L, Williams PL, Souter I, el al. Occupational factors and markers of testicular function among men attending a fertility center; Hum Reprod 2023; https://doi.org/10.1093/humrep/dead027
5. Eisenberg ML, Chen Z, Ye A, et al. Relationship between physical occupational exposures and health on semen quality: data from the Longitudinal Investigation of Fertility and the Environment (LIFE) Study. Fertil Steril 2015; 103; 1271-1277; doi.org/10.1016/j.fertnstert.2015.02.010

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