The role of nutrition in cancer patients is highly important, not just in preventing development of the cancer but also in avoiding greater weight loss accompanying the cancerous cachexia in the evolution of the cancer. The different ways of treating cancer also involve increased deterioration of the nutritional state. To design a correct nutritional strategy for these patients, it is fundamental to understand the anomalies present at the metabolic level, including those affecting the cancer carrying host and those observed in the tumor itself. The results of various studies carried out on animals and neoplastic subjects using predominantly glucose as the non-protein calorie source suggest a stimulating effect on tumor growth. On the other hand, it appears that there is, at the level of the tumor, an inability to correctly oxidize fatty acids. Enhanced knowledge of the different alterations to carbohydrate and fat metabolism found in cancer patients might aid in a better grasp and design of a nutritional strategy aimed at reducing morbidity and mortality in these patients and improving their quality of life.