Experimental mild increase in testicular temperature has drastic, but reversible, effect on sperm aneuploidy in men: A pilot study.

Plain-language summary

This study asked what a mild, sustained rise in testicular temperature does to the chromosomes inside sperm. Five fertile volunteers wore specially designed underwear that held the testicles up against the body for about 15 hours a day for 120 days in a row, which keeps them warmer than usual while still below core body temperature, and 27 men formed a control group. Semen was collected before, during and after, and 234,038 sperm were examined for aneuploidy (sperm carrying the wrong number of chromosomes, which lowers the chance an embryo develops normally). By day 34 of heating the aneuploidy rate had not moved, and for a stretch after that it could not be measured at all because sperm counts had fallen too far. By 45 days after the heating stopped the rate had doubled against where the men started, and by 180 days it had returned to normal. This is the same five men and the same 120-day experiment as the report on sperm shape by stage of development, which is already in this library, and the two look at different outcomes rather than checking each other.

Key findings

• The share of sperm carrying the wrong number of chromosomes had doubled by 45 days after the heating stopped.
• At day 34 of heating the rate had not yet changed.
• For part of the study the measurement could not be made at all, because the sperm count had fallen too far to score.
• Everything had returned to starting values by 180 days after heating stopped, which is about two full cycles of sperm production.
• This was five men, so the timeline and the direction are the useful parts and the size of the effect is not something to lean on.