That's because weight training increases testosterone levels immediately after a session, rising 25 percent after 45 minutes, The Sun reported. About 30 minutes later, testosterone levels drop as the hormone is converted to dihydrotestosterone or DHT, which shrinks hair follicles and roots.

"We have seen an increase in men in their 20s and 30s who regularly circuit train," Rogers Medical Group, a practice specializing in hair transplants, said in a statement. "There's no doubt it is often a factor in hair

loss."

While inheriting genes for baldness is still the biggest single cause of hair loss, scientists say training with weights is No. 2. Women who lift weights are also likely to end up with thinner hair.

Heavy-set men who eat high-fat diets are at the most risk of losing hair because of weight-lifting.