Continuously eating very little/ or eating very much/ will have visual influence on the character, a specific form of diet might also affect this.
Visual impact:
Gaining or losing weight will obviously make the player look either starved, slim, fat, ...
Surrounding impact:
For the beginning this could just have an impact on mounts and how much the player-weight affects riding.
Being starved will have the mount run faster and the opposite would slow it down instead
This might allow to even mount animals which aren't supposed the mounted because of how tiny or "weak" they are, but requiring you to be very light (starved).
Balancement:
Everything that has advantages should also have disadvantages and the other way around.
*Starved *= Better mounting but weaker
*"Big-boned" * = Worse mounting and slower, but stronger in melee and a larger hunger bar/slower hunger loss indicating the big fat reserves
configuarable