Carbohydrate requirementsenergy metabolism

In healthy adults, carbohydrate intake should account for 45 to 60% of total consumed calories. Carbohydrate requirements for patients with burn injuries greater than 25% of the body surface area have been recommended at levels up to 60 to 65% of total energy requirement (4,5). Carbohydrate is the key nonprotein energy source for the patient with burns, in particular. In a study by Hart and colleagues of 14 pediatric burn patients, it was shown that the administration of a high-carbohydrate diet (rather then fat) was associated with an improved net balance of skeletal muscle protein across the leg via an apparent protein sparing effect of the high-carbohydrate diet and a concomitant decrease in protein breakdown (6). Protein is needed in adequate amounts approaching 20 to 25% of the total caloric needs, leaving exogenous fat as the balance of caloric need at 20% or less of total calories due to potential immunosuppressive effects.

Hyperglycemia is a complication of excessive carbohydrate (glucose) provision and must be addressed in reference to the higher percentages of carbohydrate need suggested. Clearly, there is a hyperglycemic effect in the flow phase of burn injury as well as in critically ill patients who may also have a wound. Although short-term hyperglycemia accompanies the stress response to injury, persistent hyperglycemia is a problem that has commonly been associated with poor wound healing and immunity (7). Pediatric burn patients with poor glucose control experienced reduced skin graft take and subsequent mortality. Total caloric intake should be evaluated, as hypercaloric feeding may be associated with hyperglycemia. Further, because carbohydrate is a key substrate in burn wound healing, internal insulin production may be enhanced. Insulin, an anabolic hormone, has positive effects on nitrogen utilization. The use of exogenous insulin has been shown to lead to a decrease in peripheral muscle wasting and an increase in lean body mass and bone mass. Exogenous insulin may be used to control hyperglycemia, but care must be taken to assure that the dextrose or total calorie provision is not excessive.

Van den Berghe studied two different levels of insulin therapy in critically ill adult intensive care unit patients (ICUs) (8). One group received intensive insulin therapy to control blood glucose levels in the range of 80 to 110 mg/dL, and the other group received insulin only when blood glucose levels exceeded 200 mg/dL. A significant difference in intensive care mortality risk (43% reduction), infection (46% reduction in risk of severe infection), and a 35% reduction in prolonged antibiotic therapy requirement were seen in the intensive insulin therapy group. The author concluded that maintaining normoglycemia (blood glucose levels of less than 110 mg/dL) in adult surgical intensive-care patients is associated with positive outcomes. Krinsley utilized an insulin protocol involving intensive monitoring and treatment to maintain blood glucose levels of less than 140 mg/dL in a study of 800 critically ill adult patients and saw improved glycemic control, which was associated with decreased mortality, organ dysfunction, and length of ICU stay (9). Thomas and colleagues demonstrated the benefit of euglycemic hyperinsulinemia with exogenous insulin maintained throughout the hospital course in decreasing muscle catabolism and preserving lean body mass in pediatric patients with major burns (10). These studies demonstrate the importance of careful attention to glycemic control in the ICU population and can be extrapolated to the acute and chronic care setting.

Excessive glucose administration can be as deleterious as inadequate nutrition support for reasons other than exacerbation of hyperglycemia. Burn patients have accelerated gluconeogenesis, glucose oxidation, and plasma clearance of glucose (11).

Excessive glucose intake in the face of a traumatic injury or sepsis is associated with increased carbon dioxide production and hepatic steatosis (12,13). Burke and Wolfe evaluated whole-body protein synthesis and rate of glucose oxidation in 18 patients who were severely burned and determined the optimal glucose infusion rate to be 5 mg/kg/min (13). They concluded that exceeding the maximal infusion rate for glucose does not enhance protein synthesis but is associated with increased deposition of fat in the liver, as noted on autopsy, as well as the above-mentioned increased rates of carbon dioxide production that are associated with ventilatory challenges, including prolonged ventilator dependence. Interestingly, these authors also found that an enteral diet high in carbohydrate and low in fat compared to a diet high in fat and low in carbohydrate resulted in decreased protein degradation, although protein synthesis was unaltered (14).

Respiratory quotient (RQ) can be utilized as an indicator of energy fuel utilization. The RQ is the ratio of oxygen consumed to carbohydrate produced in the Kreb's cycle. In clinical practice, RQs are most often determined during the measurement of energy expenditure using indirect calorimetry (15). The amount of oxygen consumed and the amount of carbon dioxide produced for each major nutrient type occurs at a fixed RQ, ranging from 0.7 for fat oxidation to 0.8 for protein oxidation to 1.0 for glucose oxidation. A diet of mixed nutrient intake — protein, carbohydrate, and fat — will produce an RQ of ~0.85. Net fat synthesis is indicated by an RQ greater than 1.0 (16). RQs greater than 1.0 can occur when carbohydrate (glucose) intake or total caloric intake is excessive. The effect is probably a function of excessive glucose intake. Ireton-Jones and Turner examined the RQs of patients receiving intensive nutrition support to assess the frequency with which net fat synthesis occurred, as determined by RQs greater than 1.0 (15). They found RQs to be significantly lower in patients fed enterally or parenterally with a mixed substrate formulation including glucose (carbohydrate), protein, and fat as compared to glucose and protein alone. This indicated that the route of administration, enteral or parenteral, did not influence energy nutrient utilization determined using RQ. It is important to carefully manage carbohydrate intake, whether given enterally or parenterally, to avoid the deleterious effects of overfeeding, which include hepatic steatosis and increased carbon dioxide production, leading to ventilatory compromise, especially in the critically ill patient.

Many patients who experience wounds are diabetic and are, therefore, predisposed to carbohydrate metabolism abnormalities. As such, their blood glucose concentrations may be significantly above what is recommended as an acceptable range. While these patients are not critically ill, when they experience a wound, and have higher blood glucose levels, their risk for increased infections and delayed wound healing is increased. Animal studies have shown impaired wound healing due to fibroblast dysfunction, which can be assumed to occur in the diabetic human patient. Careful attention to the dietary carbohydrate intake and maintenance of euglycemia through oral and intravenous hypoglycemic agents may be underrated and extremely important in enhancing the healing process. Anabolic agents have been used to attempt to improve wound healing (17). Insulin therapy to maintain euglycemia may also exert an anabolic effect in the nonacute setting.

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