
Berries are meant to be a filler food, but they are not really worth it to use at the moment because of the hunger levels that they give
0.1 perc FEPs but 1.5% hunger
This means that eating one berry will give you basically nothing other than a variety bonus, but the other food is still comparable in terms of hunger level they add
Example:

This sausage is giving 3.27 hunger for nearly 9 FEP
2 berries give 0.2 FEP while using up 3 hunger
The hunger levels for berries need to be lowered significantly or they are not effective filler foods