In your body, "break down sugar" doesn't mean grind up the crystals
into powder, and if it did, you couldn't do it with oxygen, you'd need
a hammer or a grinder.
"Break down sugar" means take a molecule of sucrose or lactose or fructose,
use oxygen to rip it apart, and then put the pieces back together as a molecule
of glucose.
It should be pretty obvious that this is a chemical change.