Things that are digested slowly include protein and complex carbohydrates. For comparison, simple carbohydrates are include things like sugar. Complex carbohydrates include whole grains and fresh vegetables. Beware of everything that has ingredients that end in -ose, which denotes a sugar. These include dextrose, glucose, maltose, sucrose, sorbital.
Fruits are sort of in a gray area, with their fructose.
If you have a fruit drink, such as apple juice, then the sugars can be absorbed quickly. If you have applesauce, then the body has to do some breaking down of the applesauce before it can get to the sugar. If you have a whole apple, then the body has a lot of breaking down to do. You have to read labels to see if the products have sugar in them. To help me keep my weight down, a dietician prescribed non fat proteins for me to eat.