Energy sources, stores, and hormonal controls.
Energy is required by humans for such vital activities as metabolic processes and muscular contraction. The support of protein synthesis is critical in clinical nutritional support; approximately 25 per cent of energy expenditure is devoted to the support of protein synthesis. Energy is stored as carbohydrate, fat, and protein. Stored carbohydrate is rapidly exhausted, and the utilization of protein for energy is detrimental. Fat storage is usually abundant but cannot be mobilized and oxidized sufficiently to meet energy requirements, particularly in injured or septic patients. The provision of energy sources with amino acids in clinical nutrition permits the most efficient use of amino acids for protein synthesis. The majority of non-protein calories may be provided as either glucose or fat emulsion in patients with mild to moderate degrees of stress, but fat emulsion may be ineffective in severely stressed patients. A reliance on fat emulsion for energy may allow the use of peripheral venous infusion but is more expensive then glucose.