And the fact that they ate too much of both, AND the fact that they probably sit on their fat butts watching TV when they're not eating. (And probably when they are, too.)
And let's not forget genetics. Some people have to work harder to stay fit.
Which is why all these anecdotes on this thread are stupid. They prove nothing other than the fact that the human body is extremely adaptable.
No, the human body is SPECIFICALLY made to eat a wide range of foods. To do otherwise, while certain an individual right, is silly.