I think what she's saying is that because of the composition of McDonald's food, the body doesn't metabolize it, and the food material remains in the body instead of being expelled, leading to obesity. As she says: "Once it gets in our thighs, there's no breakdown. It doesn't go anywhere."
In fairness, I wonder if that's actually a plausible description of what happens in the body? I have the impression (perhaps mistaken) that obesity is a bit more complicated than that.
Rain soddens what is kept wrapped up,
But never soddens what is open;
Uncover, then, what is concealed,
Lest it be soddened by the rain.