system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: These Olympic marathon runners look like they have no body fat. Where are they getting the energy to run 27 miles?

I mean, they probably gave at least a little fat, but is that the only place our bodies can store energy for something like this?
gpt
Generally speaking - 

Energy for the body is stored in the body as fat or as glycogen, which is a complex sugar, and as glucose in the blood. The fat is stored under the skin and around the organs, and the glycogen is stored in the muscle cells and in the liver.

During exercise the body uses the glucose, and when the level starts to drop both fat and glycogen are used - the amount of each is determined by loads of factors like how hard they're working, the temperature, their own metabolism etc.

For Olympic endurance athletes they are mostly using glycogen in the muscles and glycogen in the liver is being turned into glucose to keep blood sugar up too - giving them energy to work. Additionally energy from food or drink consumed during or before a race is digested and goes straight into the blood ready for use. These things might include energy drink, or foods, though there is some debate about the efficacy of such supplements.