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.

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 helps keep their hunger hormone from spiking and helps them hold onto muscle, but it doesn’t make them feel less hungry or eat more.

When young men take a high dose of testosterone while eating very few calories, their body starts making more iron available to produce red blood cells, but their metabolism and how they burn food don’t change.

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 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.