Perspective: How to evaluate studies on peri-operative nutrition? Considerations about the definition of optimal nutrition for patients and its key role in the comparison of the results of studies on nutritional intervention.

Different nutritional outcome studies on the same subject can have vast differences in composition of the chosen food without justification, suggesting that the composition of "optimal" nutrition in patients is not known or that optimal nutrition does not exist. The result will be negative studies which reinforces the existing impression that nutritional intervention is of limited value in every day's patient care. This perspective will put arguments forward that optimal nutrition exists and that the definition of optimal nutrition should be the base of future nutrition intervention studies. This perspective aims at providing a definition of optimal nutrition and consequently a basis to critically appraise the literature upon nutritional interventions in disease states.