One of the first - if not the very first - controlled experiments was actually described in the Bible. In the Book of Daniel, the prophet Daniel tested the effects of eating simply (a diet of pulse or lentils and other seeds) versus a kingly feast of meat and wine:
12Prove thy servants, I beseech thee, ten days; and let them give us pulse to eat, and water to drink.
13Then let our countenances be looked upon before thee, and the countenance of the children that eat of the portion of the king's meat: and as thou seest, deal with thy servants.
14So he consented to them in this matter, and proved them ten days.
15And at the end of ten days their countenances appeared fairer and fatter in flesh than all the children which did eat the portion of the king's meat.
More at Omni Brain: Link - Thanks Steve Higgins!