Fat rats live shorter lives, are prone to tumours, are more susceptible to infection, and less likely to recover from surgery. Does should be sleek and lean, and bucks muscular; neither should feel soft and squashy, nor should they feel bony.
Like people, rats often enjoy foods that are bad for them, and like children, rats will often choose fatty or sweet foods over healthy ones.
It is up to you to make sure that your rats eat healthily, and you may find it better to save treats for hand-feeding after your rats have eaten their healthy food.
A healthy balanced diet, regular exercise (at least an hour outside the cage every day), and large, clean living conditions will insure that your rats’ lives are lived to the fullest.