Seminal and molecular evidence that sauna exposure affects human spermatogenesis.

Study Question: What are the effects of continuous sauna exposure on seminal parameters, sperm chromatin, sperm apoptosis and expression of genes involved in heat stress and hypoxia?

Summary Answer: Scrotal hyperthermia by exposure to sauna can induce a significant alteration of spermatogenesis.

What Is Known Already: Several authors have evidenced that high temperature has dramatic effects on spermatogenesis.

Study Design, Size And Duration: A longitudinal time-course study. Data from 10 subjects exposed to Finnish sauna were collected before sauna (T0), after 3 months of sauna sessions (T1) and after 3 (T2) and 6 months (T3) from the end of sauna exposure.

Participants/materials, Setting And Methods: Ten normozoospermic volunteers underwent two sauna sessions per week for 3 months, at 80-90°C, each lasting 15 min. Sex hormones, sperm parameters, sperm chromatin structure, sperm apoptosis and expression of genes involved in heat stress and hypoxia were evaluated at the start, at the end of sauna exposure and after 3 and 6 months from sauna discontinuation. Student's t-test for paired data was used for statistical analysis.

Main Results And The Role Of Chance: At the end of sauna exposure, we found a strong impairment of sperm count and motility (P < 0.001), while no significant change in sex hormones was present. Decreases in the percentage of sperm with normal histone-protamine substitution (78.7 ± 4.5 versus 69.0 ± 4.1), chromatin condensation (70.7 ± 4.7 versus 63.6 ± 3.3) and mitochondrial function (76.8 ± 4.9 versus 54.0 ± 6.1) were also evident at T1, and strong parallel up-regulation of genes involved in response to heat stress and hypoxia was found. All these effects were completely reversed at T3.

Limitations And Reasons For Caution: Absence of subjects with abnormal sperm parameters was the major limitation of this study.

Wider Implications Of The Findings: Our data demonstrated for the first time that in normozoospermic subjects, sauna exposure induces a significant but reversible impairment of spermatogenesis, including alteration of sperm parameters, mitochondrial function and sperm DNA packaging. The large use of Finnish sauna in Nordic countries and its growing use in other parts of the world make it important to consider the impact of this lifestyle choice on men's fertility.

Study Funding/competing Interest(s): No external funding was sought for this study and the authors have no conflict of interest to declare.