The severe impairment of the nutritional state, which usually accompanies malignant diseases, heavily contributes to the high morbidity and mortality rates observed in cancer patients. Nevertheless, the utility of an artificial energy supply to these patients is still controversial because the nutrients given to replete the host may also stimulate tumor growth. Consequently, a correct nutritional approach for cancer patients should be based upon a well-defined understanding of tumor as well as host-metabolic needs. In this regard, the most typical metabolic abnormalities observed in cancer patients and experimental animals are examined. Specific modifications of the plasma levels of different groups of amino acids--including glucogenic, aromatic, sulphur-containing and branched-chain amino acids--have been observed in cancer patients independently of the their degree of malnutrition, glucose tolerance and tumor diffusion. This may reflect a series of specific modifications induced by the neoplastic tissue on host's protein turnover. Little information is available regarding the protein metabolism in the neoplastic tissue. A number of attempts have been made to reduce tumor growth by withholding single amino acids considered essential to the tumor; nevertheless, the results obtained are still controversial. The two major abnormalities of carbohydrate metabolism observed in cancer patients are an increased glucose turnover and an impaired glucose tissue disposal. The former seems to be due to an increased glucogenesis, whereas the latter may be attributed to an insulin resistance in contrast to the high anaerobic glucose utilization observed in the neoplastic tissue.(ABSTRACT TRUNCATED AT 250 WORDS)