Severe muscle depletion (sarcopenia) is a common issue among cancer patients and adequate nutrient intake is pivotal to maintain muscle mass. Yet, optimal amounts of protein are undefined for preventing or treating sarcopenia in people with cancer as most methodological approaches to assess protein requirements are expensive, burdensome and unfeasible for this population. This study's overarching goal is to investigate the efficacy of diets of different protein content on: 1) muscle mass maintenance (primary endpoint); and 2) improvements on physical function in cancer patients capable of oral nutrition support. Additional study measurements include: energy metabolism (including total body), body composition using multi-compartment models, blood biomarkers, tolerance to treatment, quality of life and overall prognosis.