Tumor growth is associated with profound metabolic alterations, which can lead to onset of anorexia-cachexia syndrome. It is a significant cause of morbidity and mortality, occuring in up to 80% of patients with advanced cancer, and responsible for death in up to 20% of cases. Cancer cachexia is a complex disorder characterized by progressive loss of weight, in association with anorexia, asthenia, anaemia and alterations in immune function. Anorexia is defined as the loss of desire to eat, while cachexia results from progressive wasting of skeletal muscle mass and to a lesser extent adipose tissue, occurring even before weight loss becomes apparent. Furthermore, the protein loss that does occur in starvation is split equally between skeletal muscle and visceral protein, whereas in cachexia, visceral protein is relatively preserved. These changes cannot be fully explained by the accompanying anorexia, and nutritional supplementation alone is unable to reverse the wasting process. Despite a falling caloric intake, patients with cachexia frequently show an elevated resting energy expenditure as a result of increase in Cory cycle activity, glucose and triglyceride-fatty acid cycling, and gluconeogenesis. The pathogenic machanisms of cachexia are multifactorial, but proinflammatory cytokines produced by the host immune system and circulating tumour-derived catabolic factors play a significant role. The optimal therapeutic approach should be based on both changes in dietary habits, achieved via nutritional counseling and drug therapy, aimed at interfering with cytokine expression or activity. A polyunsaturated fatty acid, eicosapentaenoic acid, attenuates the action of such catabolic factors and has been shown to stabilize the process of wasting and resting energy expenditure in patients with cancer. It is significant to early recognize and treat cachexia in patients because it is associated with both reduced quality of life and shortened life expectancy. Additionally, cachectic patients also have a lower chance of responding to chemotherapy and are more prone to toxic side effects.