i suggest that food eaten should result in a percentage return of it's weigh as fresh dung delivered instantly to your inventory be it your meal of the day or when you heal up. c'mon you know this would be amusing and might even create a waste disposal system to get rid of it, not everybody wants to keep it and some rulers might get upset when a potatoe eating metropolis drops thiers on the ground
maybe something between 10% and 25% would be a proper range
this would also make good on the dirtyness meter, if you wallow in dung you get dirty right? so we should self produce dung so when dirtyness becomes implimented we have something already to work with it.
you gotta make this happen, make people really really desire salted meat and scrambled eggs

and if you don't want to give us a free fuel source then just make it waste a mass of waste that we can do nothing with but can rot away on the ground, be dumped into the sea, or burned off this would rock you know it nobody can deny this for it is fact, this needs serious consideration imagine the social dynamic that would develope from this!!!!!!