Impact of a Ketogenic Diet Intervention During Radiotherapy on Body Composition

Type of study: Pilot / phase I trial Study purpose To assess the impact of a dietary intervention during radio(chemo)therapy (RCT) on body composition changes Trial Treatment Patients will be split into a control group and intervention group 1. If willing to undertake a ketogenic diet for the duration of radiotherapy, patients are entered into intervention group 2. * Group 1: On irradiation days irradiation after overnight fast + ketogenic breakfast consisting of 50-250 ml betaquik® (vitaflo, Bad Homburg, Germany) and 10g MyAmino (dr. reinwald healthcare gmbh + co kg, Altdorf, Germany). * Group 2: Complete ketogenic diet plus 10g MyAmino/day * Control: No dietary intervention. * All groups: Weight measurements and bioimpedance analysis (BIA) once per week, routine blood parameters and quality of life (QoL) questionnaire before, during and after RCT Endpoints Primary: * Feasibility of the dietary intervention during RCT, measured by dropout rates * Changes in body weight * BIA phase angle and quantities derived from BIA variables Secondary: * QoL * Toxicities * Blood parameters * Grade of regression at time of surgery in case of rectum carcinomas Inclusion criteria * One of the following tumor entities: * Breast carcinoma * Rectum carcinoma * Head \& Neck Cancer * Histological confirmation of malignancy * Signed written informed consent * Karnofsky index ≥ 70 * Age between 18 and 75 years * BMI between 18 and 34 kg/m\^2 Exclusion criteria * Palliative patients, in particular with metastasis * Type I diabetes * Pregnancy * Pacemaker and other metallic parts within the body * Known defects in enzymes necessary for ketogenesis, ketolysis, fatty acid oxidation or gluconeogenesis * Unable to speak or understand German * Cognitive impairments or psychological disorders * Renal insufficiency Planned accrual * 15 patients with colorectal and mammary tumor plus 5 patients with H\&N cancer in intervention group 1 * 15 patients with colorectal and mammary t