Rats are scavenger animals by nature. They can eat pretty much anything we can eat. However this does not mean that they should. Many foods are too high in sodium, fat, or protein. Too much fat or sugar in a diet can lead to an overweight or diabetic rat. Too much protein can lead to dry, itchy skin.

Some foods to avoid are as follows;
Blue Cheese.
Licorice.
Dairy Products.
Raw Dry Beans or Peanuts.
Raw Sweet Potato.
Raw Red Cabbage.
Brussel Sprouts.
Raw Artichokes.
Green Bananas.
Green Potato Skin and Eyes.
Wild Insects.
Rhubarb.
Raw Bulk Tofu.
Oranges/Orange Juice.
Raw Onion.
Carbonated Drinks.
Hay/Grass.

Some foods to feed with caution are as follows;
Chocolate. It contains stimulants that can lead to heart failure or neurological poisoning in high quantities. A small bit of dark chocolate is okay once in a while and may actually alleviate respiratory distress.
Peanut Butter. This can cause rats to choke. If you want to feed it to your rat, mix it with jam or water to make it less sticky.
Avocados. These are high in fat and good for underweight rats. However, the pit, rind, skin and leaves of avocados are toxic. The part of the fruit in contact with the pit has a higher concentration of toxins.
Dried Corn. It can have high levels of fungal contaminates which can lead to liver cancer. A little fresh corn is fine.

Things to watch out for are as follows;
Mold
. This can be toxic. Remember that grains can go moldy too.
Chlorinated Tap Water. Chlorine isn't good for us or our pets.
Household Plants. Here's a list of toxic plants.
Bug Poison/Spray. It was made to kill. You wouldn't eat it, would you?

Healthy foods for occasional treats are as follows;
Yogies. (any flavor; store-bought; these may also be found in the baby food aisle)
Unsalted crackers.
Whole wheat bread w/ peanut butter.
Whole wheat bread w/ olive oil.
Cheerios.
Oats.
Corn Flakes.
Blueberries.
Strawberries.
Apples w/ peanut butter.
Peas.
Broccoli.
Grapes.
Carrots.
Corn (fresh/frozen).
Romaine lettuce.
Unsalted nuts and seeds.
Dry, uncooked pasta.

Bananas.
Green peppers.
Baby food.
Baby cereal.
Plain yogurt.
Cottage cheese.
(very small amount)
Dark chocolate. (very small amount, size of 1 chocolate chip)
Plain brown rice (cooked).
Cooked chicken w/ bone.
(only a tiny bit of meat)
Fully cooked fish.
Turkey
Plain popcorn.
(no salt, no butter)
Keep in mind that all of these are to be used as treats and should be given in moderation. Try to avoid sugary, salty, greasy, and spicy foods.

Homemade Healthy Treat Mix
:)
1/2 Box (6oz) Weetabix Cereal (or Cornflakes)
1/3-1/2 Box Multi-Grain Cheerios
1 Bag Sunflower Seeds
2 Bags Dried Fruit (any kind)
1 Box TriColored Rotini (dry, uncooked)
6-9oz Dry Rolled Oats (oatmeal)

This mix makes for a great treat and a great supplement to your rats' staple diet. This can be fed 3-4 times a week (or every other day). Some rats may gain too much weight if they are getting this mix throughout the week. If your rat is gaining too much weight, I'd suggest only feeding this once per week.