In cancer patients treated with radiotherapy to the abdominopelvic region, dietary modifications and the use of functional foods (fortified food with added ingredients to provide specific health improving benefits, such as antioxidants, omega-3 fatty acids, and glutamine), may contribute to the improvement of the toxic effects of treatment, including nausea, diarrhea, and constipation, among others. With the aim of analyzing which coadjuvant foods benefit these patients, scientific evidence was gathered by a group of experts. For these patients, the authors recommend a diet that includes sufficient foods rich in antioxidants and polyphenols instead of supplements. Docosahexaenoic and eicosapentaenoic acids have proven useful for the management of anorexia/cachexia in pancreatic cancer patients. Probiotics composed of Lactobacillus spp. and Bifidobacterium spp. are regarded as safe even in patients with neutropenia and have been proven to decrease gastrointestinal symptoms. Several factors should be considered before probiotic supplementation, these include the stage of the disease, radiation dose, and symptomatology of each patient. There is no demonstrated clear benefit to the use of glutamine, so it is not recommended due to its high cost.