Muscle catabolism is a major problem in cancer patients undergoing surgery as it negatively affects post-operative recovery. Recent evidence exists that protein metabolic changes are already apparent in cancer before muscle wasting is being present. In line, patients with breast cancer, generally characterized by a normal nutritional status, were recently found to be arginine deficient. Arginine deficiency in cancer can be explained by: 1) Reduced arginine availability, due to exhaustion of endogenous (muscle) sources of arginine 2) Enhanced arginine catabolism, due to conversion of arginine by arginase, which is abundant in tumors. Protein is the most important endogenous source of arginine. Arginine deficiency will lead to a negative feedback loop in cachexia by promoting protein breakdown in an attempt to restore plasma arginine levels. We hypothesize that pre-operative arginine supplementation in breast cancer patients diminishes the occurrence of muscle wasting after surgery by 1) normalizing arginine availability pre-operatively, resulting in conservation of protein, 2) diminishing the catabolic effects of surgery by supplying exogenous arginine for the post-operative response, 3) enhancing the anabolic capacity to feeding through supplying substrate for protein synthesis.