Transient scrotal hyperthermia affects human sperm DNA integrity, sperm apoptosis, and sperm protein expression.

Plain-language summary

This study set out to see what repeated heat does to sperm. Twenty men with normal semen results were randomly split into two groups, and both had their testicles warmed in a 43 degrees Celsius (109 degrees Fahrenheit) water bath for 30 minutes at a time, ten times over. One group did it on ten days in a row, the other once every three days. Sperm were checked before the heating and every two weeks afterwards for several months. Both schedules left more sperm with loosely packed DNA and more sperm dying off, and both damaged the sperm's energy machinery. The daily schedule did the more serious damage, and the spaced-out schedule was the slower of the two to recover. All of it reversed.

Key findings

• Both heating schedules raised the share of sperm with high DNA stainability (loosely packed DNA, a sign the sperm did not finish winding its genetic material tightly).
• Both schedules raised sperm cell death and disrupted the sperm's mitochondrial membrane potential (a measure of whether the sperm's energy-producing parts are still working).
• Heating on ten days in a row did more damage than heating once every three days.
• The spaced-out schedule was the slower of the two to recover, not the faster.
• In the daily group, which is the only group where proteins were measured, proteins tied to heat stress and to mitochondria went up while proteins that build and run the sperm tail went down.
• The damage was reversible in both groups.