Rats are naturally omnivores. They are renowned for eating a wide variety of food, some of which is not terribly appetising to us!
To ensure a properly balanced diet, it is recommended to feed a pelleted food, rather than a 'muesli' type diet where the rat can pick and choose its favourite bits (similar to humans just eating cakes!). There are two of these diets on the market - Burgess Supa Rat and Supreme Science Selective Rat.
The diet should be supplemented with small amounts of fresh fruit and vegetables.
There are a wide variety of rodent treats available on the market. These tend to be high in calories and are not recommended. You are, however, encouraged to supplement your rat's diet with some fresh fruit and vegetables so these can be provided as training treats as well. Take care not to overdo foods with a high water content and little nutritional value (e.g. lettuce) as this may cause diarrhoea.