What matters for fruit and vegetables is the ripeness of the product (unripe bananas have a much lower GI than the ripe ones). Also, different varieties of the same plant may have different glycemic indexes. For instance, basmati rice has a GI of 55, while while rice – of 70.
The degree of food processing matters as well; processes such as grinding, mincing, rolling, mashing or kneading (no matter whether in a factory or at home) increase the GI of the products because they become easier to digest and absorb.
Thermal processing of the food also has a significant impact on the GI levels, in particular when it comes to products containing starch, which swells while cooked and becomes gelly-like. As a result, it is easily decomposed by the digestive enzymes. This leads to increased blood sugar levels.
High-fibre products are usually low GI foods; fibre delays digestion, hence blood sugar concentration increases slowly and gradually. The same goes for fats which also delay digestion. Don't be afraid of fats, just pay attention to their caloric value.
Proteins play a key role in the diabetic diet, especially those of animal origin. They reduce the post-meal blood glucose levels, i.e. the glycemic index.
Blood glucose is very much affected by carbohydrates, however, their influence is quite complex because most high-carb products are eaten as part of a meal rather than separately. The composition of the whole meal also has a significant impact on the post-meal blood glucose levels. For example, potatoes go well with meat and vegetables. If high-carb products are accompanied by products rich in fats or proteins, the digestive system slows down, which lowers the total GI value.