This review aims to highlight the synergies between clinical nutrition, body composition and cancer treatment. Body composition is already a powerful tool to diagnose cachexia and determine response to nutritionnal intervention. It may be used in the future to fine tune body surface area (BSA) based drug dose determination thanks to its capacity to predict chemotoxicity. The overall aim of nutritionnal intervention is to optimize the oncological care by reducing treatment interruptions and improving the quality of life. However, to achieve this goal, nutritionnal intervention has to be very accurate as most of the failures result from inappropriate intervention.