High BMI (obesity) can negatively affect male fertility and sperm quality in several ways:
It can reduce sperm count and concentration - obese men are 42% more likely to have a low sperm count compared to men of normal weight
It increases sperm DNA damage and oxidative stress on sperm, which can impair fertilization and embryo development.
Obesity lowers testosterone levels by increasing aromatization of testosterone to estrogen, which is essential for sperm production.
It raises scrotal temperature, negatively affecting sperm health and motility.
High BMI is associated with lower normal sperm morphology (shape), reducing the ability of sperm to fertilize an egg. However, some studies report conflicting results on the effects of obesity on sperm motility and morphology. The negative impacts seem more pronounced with higher degrees of obesity.
Importantly, weight loss through diet and exercise has been shown to improve semen quality, sperm count, and reproductive hormone levels in obese men, potentially reversing obesity-related male infertility.