Could be a lot of things. Lots of long distance runners consume a lot of sugary gels to keep going. Not sure what the typical composition is, but likely lots of glucose and no fibre.

The marathon runners I know also seem to eat tons of junk food, they can get away with it from a weight perspective because a long run will burn it off, but it could have other consequences.

Point being: there's a lot about long distance runners that's quite different from other people.