People who regularly do endurance exercise have the same levels of norepinephrine, epinephrine, and insulin in their blood during moderate exercise as people who do not train, and the increased fat breakdown seen in trained individuals is not caused by differences in these hormone levels.

People who regularly do endurance exercise, like running or cycling, show increased fat breakdown in abdominal fat tissue during exercise, even when levels of key hormones in the blood do not change. This suggests the fat tissue itself adapts to exercise rather than relying on hormonal signals.

Men who regularly train for endurance sports show greater fat breakdown in abdominal fat tissue during moderate cycling than untrained men, measured by higher glycerol levels, even when hormone levels like adrenaline and insulin are similar.

In men who are endurance-trained, blocking a specific receptor (α2-adrenergic) does not increase fat breakdown during exercise in abdominal fat, but it does increase fat breakdown in men who are not trained, suggesting that endurance training reduces the effect of this receptor in limiting fat breakdown.

During exercise, a biological signal that normally slows fat breakdown has less effect in people who are endurance-trained compared to those who are not, suggesting that training changes how fat tissue responds to this signal.

When young men do intense leg workouts, their body releases more lactate and cortisol—stress and energy chemicals—but that doesn’t mean they’ll grow bigger muscles or get stronger faster than if those chemicals stayed low.