Research

1. Heat can reduce sperm count and testosterone production.

University of Wisconsin Integrative Medicine

https://www.fammed.wisc.edu/files/webfm-uploads/documents/outreach/im/handout_testosterone.pdf

2. According to this study from Nakamura et al. Human testis perform at their best when they’re around 31-36 °C (87-96 °F). anything hotter than that will negatively impact DNA synthesis, spermatogenesis, and most likely also testosterone production.

http://www.anabolicmen.com/cold-showers-testosterone/

3. Low Testosterone Associated With Obesity and the Metabolic Syndrome Contributes to Sexual Dysfunction and Cardiovascular Disease Risk in Men With Type 2 Diabetes.

American Diabetes Association 

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3120209/

4. A low testosterone [hypogonadism or testosterone deficiency syndrome (TDS)] may be present in 30% of men and present in a number of different ways…There is increasing evidence that TDS is associated with all-cause mortality and in particular cardiovascular death.

European Heart Journal

http://eurheartj.oxfordjournals.org/content/31/12/1436

5. Male Obesity Linked to Low Testosterone Levels,

University of Buffalo

http://www.buffalo.edu/news/releases/2010/05/11311.html

6.“Beyond providing additional evidence that underwear choices may impact fertility, our study provides evidence, for the first time, that a seemingly random lifestyle choice could have profound impacts on hormone production in men at both the level of the testis and the brain,” said Jorge Chavarro, senior author of the study and associate professor of nutrition and epidemiology.

Harvard University

https://www.hsph.harvard.edu/news/press-releases/does-underwear-style-affect-sperm-production/

7. “Any exposure [to heat] that significantly increases temperature is likely to affect spermatogenesis [or sperm production],” says Dr. Jorge Chavarro, associate professor of nutrition and epidemiology at Harvard T.H. Chan School of Public Health, and an author on the study. “That’s the main reason we have scrotums and testes that are external to the abdomen.”

By hanging below the torso, testicles stay cooler — by about 4 to 6 degrees, typically — than the rest of the body. That helps them make happy, functional sperm cells.

https://www.npr.org/sections/health-shots/2018/08/08/636901101/boxers-or-briefs-experts-disagree-over-tight-underwears-effect-on-male-fertility

8. males who spend time in low temperatures prior to mating will produce offspring with more active brown adipose tissue…the offspring of males kept in cool conditions were better protected against excess weight—they gained less weight on a high-fat diet—and against metabolic disorders…

https://medicalxpress.com/news/2018-07-environmental-impact-offspring.html

9.The lower temperature in the scrotum is a prerequisite for normal spermatogenesis.

<Oxford Textbook of Medicine> by David A. Warrell, Timothy M. Cox, John D. Firth.Page 282, (row 32). Published by Oxford University.