In “Super Size Me” Spurlock did include testimony from one man who said he’d subsisted on an all-McDonald’s diet and maintained a normal, healthy weight.
Spurlock himself (experimenting with the McDonald’s diet) was eating far in excess of what any normal person should have eaten.
Spurlock HAD to be taking in more calories than he got from just McDonalds.
I don't think Spurlock was a vegan. He girlfriend was. At one point in Super Size Me, he says "I like meat!".
He just didn't like McDonalds because he is an anti-corporation liberal and wanted to make them look bad.