Toxicity of protein excess

FIGURE 4.3 Relationship among flutamate, proline, and ornithine, with the coupling of urea and the tricarboxylic acid (TCA) cycle.

improved nitrogen sparing does not occur when protein intake exceeded this level.54 Thus, the tracer kinetic findings have provided further quantitative evidence to support the notion that an adequate amount of protein intake for these patients should be about 1.5g/kg/day.

Protein overfeeding is not only unnecessary, but may lead to potential complications. In critically ill patients, the major complication of protein overfeeding is acidosis and azotemia.55 In surplus protein feeding, the nitrogen of the amino acids is deaminated and produces ammonia and then urea. Increased renal urea excretion requires obligatory water excretion. Overfeeding protein with inadequate water can produce a state of hypertonic dehydration or "tube feeding syndrome."

Increased gluconeogenesis from amino acids in patients who are severely burned also contributes to the overload of urea nitrogen. Ureagenesis in the liver consumes up to 1000 mmol of HCO3/day in humans as a result of 2NH+ + 2HCO3 => urea + CO2 + 3H2O. In a recent study, Hosch et al.56 revealed that in experimental human subjects, the rate of ureagenesis was correlated negatively with plasma HCO-concentration over a wide range. Therefore, ureagenesis is an acidifying process. Overproduction of ureagenesis in critically ill patients may contribute adversely to the homeostasis of the acid-base balance of the patients.

Therefore, along the course of providing nutritional support to patients, a comprehensive evaluation of the clinical parameters to assess tolerance, weaning and ventilator parameters, organ function, visceral proteins, volume status, glucose tolerance, and other biochemical markers such as blood urea nitrogen (BUN), ammonia and urinary urea nitrogen, or even indirect calorimetry measurements are necessary to adjust feeding regimens to adequate levels. For patients with a tendency toward or early signs of impaired renal function, the responses to protein intake should be more closely monitored.

In summary, improving whole body protein nutritional status is an important factor in supporting the wound healing process. While this has been observed clinically, recent in vivo studies confirmed the metabolic relationship between whole body nutritional status and wound protein synthesis. Providing adequate nutrition plus using certain anabolic factors are an important aspect in taking care of the wounds of surgical patients.

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