Principles of Gastrointestinal Physiology, Nutrition, and Metabolism
Patrick O’NEAL Maynord
Z. Leah Harris
KEY POINTS
Gastrointestinal Tract Anatomy and Physiology
The gastrointestinal (GI) tract serves distinct roles in metabolism, immunity, cell signaling, and hormonal modulation.
Water homeostasis and water recirculation occur throughout the GI tract in the form of secretion and reabsorption.
Bacterial colonization, fermentation, and energy balance are intimately linked.
The Gut as an Immune System
The gut microbiome is a major determinant of nutritional bioavailability.
Susceptibility to infectious complications is directly related to a patient’s nutritional status.
An appreciation of the benefits of immune-enhanced diets for specific patient populations is necessary but it is premature to institute immunonutrition for all critically ill patients.
Probiotic use needs to be studied in greater detail but offers an intriguing mechanism to control gut flora and maintain an individual’s gut integrity.Energy Expenditure and Metabolism
Energy expenditure refers to the amount of energy used by an individual to complete the daily metabolic functions.
Total energy expenditure = basal energy expenditure (resting energy expenditure or basal metabolic rate) + activity energy expenditure (all daily activities).
Indirect calorimetry is essential to measure the true energy requirement of a patient.
Careful attention to macronutrient requirements will help prevent unintended cumulative excesses or deficits of energy and protein in critically ill children.
Both overfeeding and underfeeding are prevalent in the PICU, and their impact on outcomes needs to be studied further in this population.Key Nutrients
Adequate and appropriate nutrition, implemented early, is critical in treating malnutrition in the ICU.
Malnutrition is associated with increased morbidity and mortality in the ICU.
Determination of macronutrient (protein, fat, carbohydrate) composition of the diet is an important aspect in nutrition prescription.
Micronutrients—trace elements, vitamins, electrolytes— are essential and deficiencies may result in clinical deterioration.Body Composition
Malnutrition is associated with worse outcomes from critical illness. The prevalence of malnutrition has remained steady over the past decades.
Body composition is used as a surrogate for nutritional state.
Body mass index has become a proxy for body fat percentage.
Methods of measuring body composition are varied, offer limited agreement, and need to be further studied.
Future studies must examine the association between childhood obesity and clinical outcome in critically ill children and provide optimal nutritional strategies.Enteral versus Parenteral Nutrition
Enteral feeding during critical illness maintains intestinal tropism, stimulates the immune system, reduces bacterial translocation, and is associated with decreased cost.
Further studies are needed to determine optimal timing, site, volume, and composition of feeds in critically ill children. Until then, a uniform consensus-based approach may help achieve nutrition delivery goals in the PICU.Issues with nutrition in the management of critically ill children range from how much to feed (i.e., what is the resting energy expenditure [REE] of a critically ill child?), to what to feed (macronutrient and micronutrient composition), and even how to feed (e.g., route of delivery). Implementation of an evidence-based nutritional management protocol increases the likelihood that critically ill patients receive enteral feeds and is associated with shortened duration of mechanical ventilation (1). However, a comprehensive understanding of the nutritional needs of critically ill children and nutrient delivery goals continue to elude us. Mounting evidence reveals that clinical outcome in the ICU is influenced by genetic variability in energy metabolism, stress response, and inflammation, but the extent of this effect is poorly understood.
Controlling for nutrient intake and distinguishing among the effects of various nutrients and vitamins ingested from food complicate the design and interpretation of studies aimed at assessing nutrient-dependent outcomes in both healthy and critically ill populations. Individual nutrient bioavailability adds a further complexity to interpretation of studies of nutrient supplementation. The identification of single-nucleotide polymorphisms within populations has led to the concept of “personalized designer medicine” (2). The notion that diets can be customized based on individual genetic profiles to decrease the risk of disease underscores two new areas of nutritional research: nutrigenomics and nutrigenetics. The future of nutraceuticals may depend upon translating gene-based differences into health outcome differences.
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