Self-reported onset of puberty and subsequent semen quality and reproductive hormones in healthy young men.

STUDY QUESTION: Is there an association between pubertal onset and subsequent reproductive health in young men?

SUMMARY ANSWER: Self-reported later onset of puberty was associated with reduced semen quality and altered serum levels of reproductive hormones among 1068 healthy, young Danish men.

WHAT IS KNOWN ALREADY: The long-term effects of variations in the onset of male puberty on subsequent reproduction remain largely unstudied.

STUDY DESIGN, SIZE, DURATION: In a cross-sectional study, young healthy Danish men were approached when they attended a compulsory medical examination to determine their fitness for military service from 2008 to 2012.

PARTICIPANTS/MATERIALS, SETTINGS, METHODS: A total of 1068 healthy, young Danish men (mean age 19 years) participated. They were asked to assess whether onset of penile and testicular growth, development of pubic hair and voice break occurred earlier, at the same time as or later than their peers. Their semen quality (semen volume, sperm concentration, total sperm count and percentages of motile and morphologically normal spermatozoa) and serum concentrations of sex hormones (LH, FSH, total testosterone, SHBG, inhibin B) and testicular size were determined.

MAIN RESULTS AND THE ROLE OF CHANCE: The response rate was 29%. Of the 1068 men who then participated, 652 answered the questions about penile growth and pubic hair development and were therefore included in the analysis. Self-reported later onset of puberty was associated with a 25% reduction in sperm concentration (95% CI -41%; -4%), a 40% reduction in total sperm count (-55%; -21%), a 1.6% age point reduction in morphological normal spermatozoa (-2.9; -0.3) and a 1.6 ml reduction in testicular size (-2.4 and -0.8 ml), after adjustment for confounders. Self-reported later onset of puberty was also associated with a 9% (3%; 15%) reduction in free testosterone and a 16% (2%; 31%) increase in FSH, after adjustment for confounders.

LIMITATIONS, REASON FOR CAUTION: Our study was cross-sectional and reverse causality cannot be ruled out. In addition, we cannot rule out the possibility that the men with late puberty onset had not yet fully matured although most were in Tanner stage 5.

WIDER IMPLICATIONS OF THE FINDINGS: Approximately 15% of young Danish men have self-reported later onset of puberty than their peers. We found poorer testicular function in young men with a history of later pubertal development, suggesting that timing of pubertal onset may be a fundamental marker of male reproductive health. However, we cannot exclude the possibility that these men had not fully matured at the time of examination and therefore their semen quality may yet improve, which makes follow-up important.

STUDY FUNDING/COMPETING INTERESTS: This work was supported by the Danish Council for Strategic Research, Program Commission on Health, Food and Welfare (project number 2101-08-0058), Rigshospitalet (grants 961506336 and R42-A1326), European Union, DEER (grant agreement no 212844), the Danish Ministry of Health and the Danish Environmental Protection Agency and Kirsten and Freddy Johansens Foundation (grant 95-103-72087). There are no competing interests.