Newly diagnosed cancer patients are frequently found suffering from a metastatic disease, which poses additional challenges to the delivery of effective therapies. Chemotherapy and radiotherapy are associated with side effects which reduce tolerance to treatment and likelihood of tumour response. Identifying preventable factors of reduced response to therapy would translate into better care of cancer patients. Among other factors, malnutrition, as diagnosed by non-volitional weight loss, and cachexia, as revealed by sarcopenia, are universally recognised negative prognostic factors. Less certainty exists on the role of nutrition therapy in improving cancer patients’ body composition and clinical outcome. The reasons for the lack of convincing evidence are manifold, mostly related to the poor design of nutritional trials. Metastatic cancer patients should receive a quantitatively and qualitatively adequate diet, and in case of reduced tolerance of food, artificial nutrition is indicated. Most importantly, nutritional care should target the underlying mechanisms of reduced food intake/impaired anabolic response, and aim at minimising the impact of catabolic crisis, to maximise the recovery phase. The combined and early use of supplemental energies and proteins, as well as modulators of inflammatory response has been shown to improve nutritional status and may also benefit clinical outcome. When part of early palliative care, nutrition therapy improves cancer patients’ quality of life and may prolong survival at a fraction of the costs of developing new drugs.