Hitler's vegetarianism is a myth, a part of the public image carefully crafted by Goebbels of Hitler as an asket only living and working for the sake of his country. As alanabit says, he may have been dieting at times but nothing more. (According to biographies, Hitler's public image as not drinking, not womanising is equally wrong. The nonsmoking bit, however, seems to be correct)

One source of that myth in the English speaking world is an article from the New York Times on May, 30th, 1937:

(my retranslation): It is well known that Hitler is vegetarian, doesn't drink and smoke. His meals therefore mainly consist of soups, eggs, vegetables, and mineral water, occasionally completed by (white) sausages (a Munich specialty which is everything but vegetarian), caviar, and a slice of ham.

A kind of liberal use of the word vegetarian, I'd say.

Wolfgang