Such is the finding of new research from the University of Buffalo, which carried out a small test across 25 obese and 25 lean males aged between 14 and 20.
Published online in Clinical Endocrinology, the study was controlled for age and level of sexual maturity.
Morning fasting blood samples were taken from the young men to test for concentrations of total and free testosterone, as well as the oestrogen hormone estradiol.
"We were surprised to observe a 50 per cent reduction in testosterone in this paediatric study because these obese males were young and not diabetic," explained Professor Paresh Dandona, first author on the study.
"The implications of our findings are, frankly, horrendous because these boys are potentially impotent and infertile," he added.
In 2004, the same researchers at the University's School of Medicine and Biomedical Sciences also reported the presence of low testosterone levels in obese and type-2 diabetic males.
This was confirmed in 2010 in more than 2,000 obese men who were both diabetic and non-diabetic, reinforcing evidence that excessive fat can harm testosterone levels.
As well as having reproductive consequences, the researchers pointed to low levels of testosterone as contributing to a tendency towards abdominal fat and reduced muscle, which in turn leads to insulin resistance.
Professor Dandona said that these hormone levels return to normal in men that have undergone gastric bypass surgery.
He continued: "It's possible that levels will also return to normal through weight loss as a result of lifestyle change, although this needs to be confirmed by larger studies."
As well as being important for fertility, testosterone plays a major role in hair growth patterns, muscle growth and bone development.