Nutrition in the acutely ill

Administration of preoperative TPN must be geared to the needs and reactions of the individual patient. While some patients will respond to refeeding (parenteral or oral) within a week, others (particularly stressed patients) may require as much as 6 weeks of nutritional therapy before elective surgery. A key predictive factor in determining a given patient's response to preoperative TPN appears to be the serum albumin level, as an indicator of ECF status. We now know that there are specific patients for whom the side effects of excessive glucose infusion may offset the physiologic benefits of TPN. This category consists of patients who are under significant metabolic stress as a result of injury or infection. For these people, delivery of the maximum glucose dosage possible may not be an acceptable therapeutic goal. In such circumstances it is essential to recognize the cost at which lean body mass is being preserved. Recent findings indicate that during the metabolic stress, matching glucose dosage to glucose need may be preferable to supplying an overabundance of glucose. Fat emulsions, previously prescribed solely for their ability to prevent essential fatty acid deficiency, have gained acceptance as auxiliary caloric substrates and may provide the answer to satisfying increased energy requirements in the presence of impaired glucose utilization. As usage of these products increases, some of the metabolic complications associated with glucose loading may be avoided. While there is clearly a need for some glucose during stress, the question of whether to supply the balance of required nonprotein calories as glucose of fat remains to be answered. What is certain is that prevention of essential fatty acid deficiency necessitates the administration of at least 5 per cent of the total energy requirement in the form of fat. Although we usually supply at least half of the nonprotein caloric load as fat, the composition of the TPN regimen is ultimately governed by the patient's clinical condition and responses to nutritional support. Putting the discussion of caloric substrate aside, we arrive at the issue that is probably most crucial to the success of nutritional support-that of timing. It is clear that nutritional support in any form becomes progressively less effective as the severity of the stress response increases. Consequently, if such supportive therapy is to benefit the patient, it must be instituted early in the course of illness. If a patient has been previously well nourished, the goal should be prevention of major protein losses while complications are minimized. Under these conditions, nutritional support may be given via a peripheral vein, as a combination of nutrients that approximates metabolic expenditure. If the stress is expected to continue for longer than 5 to 7 days, a full central venous support regimen should be instituted that aims for, as a minimum, N equilibrium. In the nutritionally depleted patient, an attempt to achieve positive N balance should be made as early as is practical. New horizons in metabolic support are extremely exciting and will make use of the pharmacologic effects of nutritional support to modify fundamental physiologic processes.