Too much fat or protein is a very bad thing for these little
animals. Too much protein can lead to obesity and reduced
activity in your animals. You can remedy this with more --
you got it -- fresh fruit and vegetables.

Too much fat can lead to serious problems regarding breeding.
It seems if the mother has too high a fat content, the babies
will be born with fat rolling in their eyeballs. This can be seen
as small white spots in the babies' eyes. This can be remedied
with an ultra low-fat diet (which you should be feeding them
anyway).