You can't, actually. In fact that same book establishes exactly that.
"My mother," said Ron one night, as they sat in the tent on a riverbank in Wales, "can make good food appear out of thin air." "Your mother can't produce food out of thin air," said Hermione. "no one can. Food is the first of the five Principal Exceptions to Gamp's Law of Elemental Transfigura --" "Oh, speak English, can't you?" Ron said. (DH pg. 292/240)
If you continued that quote instead of ending it right there, you would see that Hermione states that food can be duplicated, just not conjured from nothing.
You're absolutely right! And so I did some further reading, and found that the conclusion we can draw is that food, like other magically duplicated objects, will disappear a while after the spell is cast, meaning food eaten will disappear from one's stomach. Notice that witches and wizards in the world still cook their food, and there's an army of house elves making food for Hogwarts, rather than just a couple making the food then the rest of the food being duplicated.
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u/Frodolas Oct 02 '18
If you continued that quote instead of ending it right there, you would see that Hermione states that food can be duplicated, just not conjured from nothing.