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 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.

Women with more ghrelin — the hunger hormone — actually eat less each day, which seems surprising but was observed in this group.

When overweight men eat fewer calories, it raises a protein in their blood that changes how their body uses testosterone, which can impact things like energy and health.

Older people don’t feel hungry after eating because their body doesn’t release the right hunger hormone the way younger people do.

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.