Male infertility issues likely are directly linked to the hormonal abnormalities which are common with obesity. 

Specifically, excess estrogens (from increased conversion of androgens by aromatase enzyme from increased number of fat cells) may have negative effects on sperm production. Different versions of the aromatase gene in men may account for different effects on estradiol and sperm count between obese men.

Additionally, estradiol affects the brain production of “fuels” (called luteinizing hormone, LH, and follicle stimulating hormone, FSH) which normally increase testosterone production from the testes. However, in obese men, these “fuels” are decreased and there is less testosterone production both within the testis and in the blood resulting in reduced testicular function. The decrease in testosterone is proportional to obesity degree. Low testosterone associated with obesity also contributes to a cycle of excess fat accumulation and muscle mass reduction.

Additionally, the increased inflammatory state and increased risk of diabetes may also affect the hormonal profiles and male infertility of obese men.

Finally, leptin (produced by fat cells affects appetite and energy, and may be related to genetic abnormalities in obese men) also affects the part of the brain (pituitary) which affects testosterone production.

Decreased physical activity and increased upper abdominal and upper thigh fat can lead to hotter testicular temperature which can further harm sperm production and male infertility. Hotter testicular temperatures have been noted specifically in men with sedentary occupations such as taxi/truck drivers, cyclists, and office workers.

Environmental toxins (such as organochlorines which are found in insecticides) can preferentially accumulate in fat cells and have been shown to correlate with decreased sperm production and motility.

Increased inflammation caused by fat cells, diabetes, and elevated cholesterol can lead to an increase in agents (called reactive oxygen species-ROS) that damage sperm and decrease sperm counts.

Finally sleep apnea, a disorder with interrupted breathing during sleep, which occurs commonly with obesity, leads to interruption of hormonal fuel patterns from the brain which may also affect male infertility.

Source: Fertility Authority