Start running.
When British researchers compared 10 men, 55 to 65 years old who ran at least 40 miles a week, with 10 sedentary men of the same age, they found that the runners had higher levels of certain hormones. Testosterone, which fuels sex drive and erectile function, was 25 percent higher. Growth hormone, which helps maintain muscle and bone, was four times higher. You don’t have to run 40 miles; any exercise will help, says study author Steven Hurel, PhD, of London.
Source: Prevention