Look for a diet made especially for rats. A pellet or block (essentially a large pellet) type diet is generally considered best. Loose mixes can be very well balanced and complete diets, but only if the rats eat everything in the mix, and many won't. If you feed a block or pellet, they won't be able to pick and choose which parts they are eating. The rat blocks can be available at all times. A commonly recommended diet is Oxbow Hay's "Regal Rat." A rat and mouse diet that meets the same general requirements (e.g low calorie, low fat) is a good compromise if you cannot find a good rat diet. However, hamster, gerbil and other rodent diets are not a good substitute -- their nutritional requirements are different these usually contain alfalfa which is apparently not very digestible to rats.
Rats benefit from a variety of vegetables, fruits and and other fresh foods that help to keep them healthy. Keep in mind that serving sizes are pretty small (as in a teaspoon or half-inch cube) for a rat, so avoid giving large amounts of fruits or vegetables, or diarrhea may result. The following is a list of treats you may wish to try, keeping in mind that feeding a wide variety of foods is the best way to ensure optimum nutrition and health:
- fruit: apples, cherries, grapes, banana, strawberries, other berries, melons, plums.
- vegetables: broccoli, potatoes, peas, carrot, cooked sweet potato, kale, parsley, bok choy, squash.
- cooked liver, other very lean meats (cooked)
- whole wheat pasta and bread
- cooked beans (including soya)
- yogurt (especially with live cultures)
- brown rice
- unsweetened breakfast cereals
- small dog biscuits
- some leftovers from your meals are okay in moderation, but avoid fatty or sugary scraps and items off the list below.
- special treats (given only occasionally): whole nuts in the shell (almonds, Brazil nuts, walnuts), sunflower seeds (high in fat), carob chips
Avoid feeding any of these items to pet rats:
- raw beans
- raw sweet potato
- cabbage, brussel sprouts
- green potatoes
- sweet sugary treats, any other "junk food"
- caffeinated beverages
- carbonated drinks
For the most part, you are better off feeding the fresh foods listed above as treats. Many pet store treats, such as yogurt drops or the treat sticks are quite high in sugar and/or fat and should be given very rarely if at all.
Debbie Ducommun of the Rat Fan Club has published a recipe for a homemade diet claimed to be nutritionally complete and balanced. It is more time-consuming and expensive to produce a good homemade diet, but if you are interested you can find the information on the Rat Fan Club site.