A new study of nearly 900 men found an increase in testosterone levels that correlated with decreasing body weight and waist size.

"Doctors should first encourage overweight men with low testosterone levels to try to lose weight through diet and exercise before resorting to testosterone therapy to raise their hormone levels," said study author Frances Hayes, MD.

"Losing weight not only reduces the risk of prediabetic men progressing to diabetes but also appears to increase their body's product of testosterone."

Source

"Overweight Men Can Boost Low Testosterone Levels by Losing Weight," the Endocrine Society, 6/25/12