You are what you eat
What you eat determines how healthy of a person you are. So to be fit you must eat healthy. It was first used in the 1960's (during the hippy era). It originally referred to eating wholefood.

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the idiom "you are what you eat" or "Der Mensch ist was er isst" originated
with Ludwig A. Feuerbach, a student of the popular philosopher Hegel. Feuerbach was known for being a materialist, believing that only matter bound by space and time is reality. Accordingly, the material we consume becomes us.

(来源：英语麦当劳－英语杂志 http://www.EnglishCN.com)