Sperm DNA fragmentation (DFI) is a lab measure of how much of the DNA in a semen sample is damaged. In studies, heat, smoking, cannabis and a recent fever are linked with poorer semen measures, and in one small sauna study heat also affected how sperm DNA was packaged. These changes eased after the exposure stopped.

What hasn't been shown: that a lifestyle program lowers an individual man's DFI, or that changing these habits improves embryo quality or pregnancy rates. Antioxidant supplements are a good example: in the MOXI randomized trial (174 men), they didn't lower DNA fragmentation or improve live birth compared with placebo.

Staying cool, not smoking, sleeping well and eating a Mediterranean-style diet are low-risk and good for his health either way. Whether DFI testing makes sense for him is a question for his clinician.