Check the changelog for relative information in the food category:
viewtopic.php?f=39&t=62967#p7995431. jorb did say at the beginning of the world that most food only only belongs to one food category, and generally only satiate the food to which it belongs. This does not mean all food, though.
2. The food only looks at the highest value for reduction of FEPs, so if meat is at 60, and sausage is at 20, you only get hit for the one with a bigger penalty.
I believe at one point jorb said that if anyone sees any inconsistencies to point them out, however I'm not sure "meat" and "sausage" would be an inconsistency.
Is it a straightforward system? No, and maybe it should be. It does show you what it is affected by if you have your character sheet open to attributes and keep your eye on it. Having such information on the recipe or food tooltip would be much more user friendly.
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