Chemo is long and tiring and physically hard on the body. You need to make sure you are in the best possible weight before you start because it is going to take a lot of effort to get through. If not, you will not have enough strength and - to be blunt - you risk dying. During chemo you will be nauseous and will not want to eat. You will be irritable, sleepless and physically weak.
Have you spoken to your specialist about diet? If you had part of your intestines removed, food will move through you a lot quicker and therefore your body will not absorb the necessary vitamins and calories. You need to eat a special diet (and avoid some foods!) and eat in a particular manner when you have has this operation.
Try eating 6 small meals a day instead of 3 large ones.