Elsevier

Clinical Nutrition

Volume 26, Issue 1, February 2007, Pages 154-158
Clinical Nutrition

SHORT REPORT
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

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Summary

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

Introduction

In Holland, a working group is busy to prepare guidelines for peri-operative nutrition. In the discussions that are part of the process, it became clear that “just” a critical appraisal of the literature would not lead us to an evidence-based guideline for peri-operative nutrition.

Let us share our dilemma with you: a critical appraisal of the literature upon nutrition in disease states cannot be undertaken unless it is clear what the goal of nutritional therapy is. Is it just influencing morbidity or mortality or is it maintaining or improving a certain parameter and via that way influencing morbidity or mortality. The difference is not trivial, as a negative study with a certain nutritional intervention can mean that this intervention is negative because(1) nutrition has no role in it or(2) the quantity or the components of the chosen food were inadequate. Most studies on this subject pay attention to equal distribution of calories and nitrogen to the different study arms, but almost never justify the chosen amount of nitrogen and to a lesser degree the amount of calories. The results are that nutritional studies on the same subject can have vast differences in composition of the chosen food without justification.1 In other words, it seems that the composition of optimal nutrition in patients is not known or that optimal nutrition does not exist. The result will be negative studies and 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. It also guides the way to future interventional nutritional studies.

The basis of this perspective is the multi-step approach: the necessity to take into consideration all the steps necessary before the conclusion is made that an intervention does not influence morbidity and mortality. An example will be taken from the literature on cardiovascular risk factors:

Step 1: Epidemiological studies suggest a relationship between blood pressure height and cardiovascular complications. It is concluded that anti-hypertensive treatment could be useful.

Step 2: In a series of studies, the best drug (or combination of drugs) to normalize blood pressure is determined

Step 3: The effect of treatment with this (combination of) drug(s) on cardiovascular risk is explored. The most important component of step 3 is measurement of blood pressure. This is essential in a negative study. As otherwise no distinction can be made between insufficient treatment (no normalization of blood pressure) and absence of effect (no diminution of the incidence of cardiovascular complications despite normalization of blood pressure).

In outcome studies of nutritional intervention, almost never are parameters measured that verify that the chosen intervention is of optimal quality. This will be explored more in depth in the following paragraphs.

Section snippets

Considerations about the optimal amount of protein in studies of nutritional interventions in ill patients

If a disease is the direct result of inadequate nutrition, the endpoint of an intervention could be survival. An example is kwashiorkor: from epidemiologic studies, a relationship between poor protein intake and subsequent development of kwashiorkor has been proven. So, augmenting protein intake could be useful to cure kwashiorkor (step 1). However, this association does not prove that existing kwashiorkor is cured by simply providing protein nor does it tell us how much protein should be

Considerations about the optimal amount of protein

In this section, attention is only paid to the amount of protein, administered in the standard well-balanced solution of amino acids or peptides. Discussion of its relative amino acid composition is outside the scope of this perspective, especially as studies on this subject should be judged against the amount of standard protein, optimal for patients.

Under normal circumstances in healthy volunteers, the protein breakdown after an overnight fast is greater than protein synthesis, resulting in a

Optimal energy delivery

Total energy expenditure (TEE) consists of the components’ basal metabolic rate (BMR), specific dynamic action (SDA) or diet-induced thermogenesis, and an activity factor for physical activity (AF). BMR and SDA together are referred to as resting energy expenditure (REE). In circumstances of illness, the energetic requirements of the body to meet the demands inherent of the illness is recalled the “illness factor”.

The golden standard for determining TEE is the double-labelled water method. In

Conclusion

In outcome studies of nutritional interventions in ill patients, the value of those studies should be judged against the concept of optimal nutrition. In studies deviating from this concept, the authors should clarify why they did this.

This could be a first step to (1) harmonize studies on this subject, (2) to clarify the role of short-term nutritional therapy in patients and (3) and hopefully diminish the endless series of confusing studies and by that challenge the existing impression that

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