After intense cycling, young men burn through a lot of muscle fuel (glycogen), but that doesn’t change how water is split between cells and surrounding areas in their arms, trunk, or legs—measured by...

When young guys burn off a lot of energy during intense cycling, their muscles lose stored fuel (glycogen), but this doesn’t seem to change how water is distributed in their arms, legs, or body — at least not enough for common body scanners to detect.

Even when athletes burn off more than half their muscle sugar, a body scan tool called BIS doesn’t see fake changes in muscle water — so it’s a reliable way to track body changes during training.

Even when young guys burn off a lot of muscle fuel (glycogen) in their legs over a day, a special body scan that measures water doesn’t pick up any change in leg water levels — meaning it might not notice the water shifts that happen when fuel stores drop.

After a really tough workout, young guys lose about half a liter of fluid from outside their cells within a day — and it happens whether their energy stores are low or full, so fluid changes aren't just about fuel levels.

After a really tough workout, young guys lose a little bit of fluid from outside their cells — and it happens whether their energy stores are low or back to normal, so something other than fuel levels is affecting fluid changes.