If young men eat much less and exercise a lot for four weeks while taking a high dose of testosterone, they gain a few extra kilograms of muscle compared to those taking a fake pill—even though their strength and leg function don’t get better.

When young men eat way less food than usual, giving them a high dose of testosterone makes some muscle-building signals in their cells stronger, but it doesn’t stop their muscles from breaking down — just like in guys who get a fake pill.

Taking a lot more testosterone than your body normally makes—without working out—can make your muscles bigger and add about 7 pounds of muscle mass, showing that testosterone alone can build muscle.

If a guy who already lifts weights takes a high dose of testosterone and keeps training hard for 10 weeks, he’ll gain a lot more muscle and strength than if he just took the hormone or just trained alone.

If a guy who already lifts weights takes a high dose of testosterone for 10 weeks without working out, he’ll get stronger on the bench press and squat—meaning testosterone alone can make muscles stronger, even without training.

When overweight men eat a very low calorie diet for 12 weeks, their testosterone levels go up a lot, which can help improve their health.