Making stew DOES add to available food
Oops, my mistake.
When I made my first stew (a poison one) yesterday, I lost 24 food points. After reading your response, I just experimented and made a non-poisonous one, and (yay!) I gained 22 food points.
Note to self: poison, bad! non-poison, good!
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(If I were more motivated, I would attempt each of the possible recipes several times, and keep track of the food points.)