The 'ablative' Taste-bud Theory

Everyone has a set of taste-buds for the things their gene-pool has evolved eating.
The taste-buds are matched by mechanisms in the gut that deal with those food items.
When you eat something a small amount of taste-bud goes with the food to identify
it to the gut mechanisms.

Eventually, all of a particular taste-bud has ablated away.
When this happens, that food item no longer appeals.
It is being treated as an 'unknown' by the gut and it is harder to digest.

This explains why old people complain of "nothing tastes good anymore"
and "everything hurts my stomach".