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If you're a man planning a major trek in the Himalayas or some other mountainous region, I have two things to tell you. First, please do not go on and on about it, I promise you no one cares. Second, your balls might shrink.In a letter to the editor published this week in High Altitude Medicine and Biology, researchers out of Italy reported that spending time at high altitudes can have a physical effect on a man's testicles.The researchers used fMRI imaging to measure the testicular volume of seven men between the ages of 24 and 42 before and after a trek up a mountain in Nepal. The men spent 13 days hiking up to a base camp and then another 22 days at an altitude around 3 miles above sea level. Ten days after completion of the trek, the researchers found that the men's testicles shrank around 15 percent on average.The same researchers had previously shown that high altitude treks can also reduce sperm counts, though sperm quality seems to be unaffected. But it's not all bad news: another study looking for long-term effects of such treks found that sperm counts remained reduced up to three months later but returned to normal in two years. So, these testicular effects of high altitudes appear to be reversible. Not a lot of people live at these extreme altitudes and very little research has been done to assess the effects of altitude on fertility in men who live significantly above sea level.Findings from animal studies suggest these effects are due to the reduced oxygen levels at such altitudes. The men in this study did not receive oxygen supplementation during their trek and it's unclear if using oxygen tanks, for example, could prevent these changes. So, if you have any, uh, personal data to share, let us know.