Managing the Stress Response with Nutritional Support
Jean-Charles Preiser, MD, PhD
Centre Hospitalier Universitaire du Sart-Tilman
Liège, Belgium
References
The healthcare community’s understanding of the metabolic alterations associated with stress response in the intensive care unit (ICU) has changed greatly throughout the last decades. Several scientific discoveries have been followed by a close, unique and fruitful collaboration with the nutraceutical industry, resulting in important changes in the composition of the solutions used for nutrition support.
The metabolic changes in the stress response are viewed as adaptive events designed to redefine the hierarchy of metabolic priorities. During prolonged and intense stress, this physiological response rapidly leads to the depletion of body stores, especially in malnourished patients. Therefore, readily usable energetic substrates and components necessary for the synthesis of proteins and for the replication of cells in visceral tissues are the basis for appropriate metabolic and nutritional support. The administration of a sufficient amount of exogenous energetic substrates via nutritional support could slow the catabolic response, prevent the wasting of energy at the level of metabolic cycles, and provide sufficient amounts of energy and nitrogen for anabolic processes. However, the absorption, metabolism, utilization and disposal of macronutrients are impaired in critically ill patients.1,2 During the early ebb phase, nutrients cannot be utilized efficiently as a source of energy. In contrast, during the ensuing flow phase, nutrients incorporated into nutritional support solutions are more efficient energy sources. Interestingly, the gut absorption of carbohydrates, the most efficient energy source, can be reduced by impairments in the jejunal lactase enzyme in the critical care population. Therefore, the use of lactose-free enteral solutions is now a standard of care. Blood glucose concentrations typically are increased in critically ill or injured patients, so the novel concept of intensive insulin therapy to restore normoglycemia also will change daily practice profoundly, although the optimal target blood glucose level and the practical requirements of intensive insulin therapy must be defined more precisely.3,4
The use of lipids as efficient energetic substrates is another concept that has been challenged. The systematic finding of enhanced plasma levels of free fatty acids reflects the enhancement of lipolysis that cannot be attenuated by carbohydrates, in contrast to fasting. Moreover, in acute conditions, especially when tissue hypoxia is present, the fatty acids are not used efficiently.5 Therefore, the proportion of lipids incorporated into commercial formulas was dropped from 60%-70% to 30%-40%. Simultaneously, the anti-inflammatory roles of omega-3 fatty acids and the lower sensibility of monounsaturated fatty acids to oxidation (compared to polyunsaturated acids) led to the incorporation of fatty acids extracted from fish, borage or olive oils.
For amino acids, an increased rate of protein breakdown and significant loss of muscle mass are standard findings in critically ill or injured patients. The best way to attenuate this loss of lean body mass is to provide a sufficient amount of nitrogen and to provide the calories required for efficient protein anabolism. The currently recommended range of calorie-to-nitrogen ratio (100:1-150:1) is matched in most formulas.6
Glutamine deserves special attention here. This conditionally essential amino acid is released in large quantities from skeletal muscle, as the increase in efflux exceeds its rate of synthesis. Glutamine is used avidly by the gut, kidney, immune system and wound tissue as a major fuel for rapidly dividing cells and as a precursor for gluconeogenesis, nucleotide synthesis, ammonia excretion and glutathione formation.7 Therefore, several glutamine-enriched nutritional support solutions were developed and marketed. The systematic addition of glutamine in parenterally fed critically ill or injured patients is supported by several clinical studies.8
Finally, the ongoing understanding of the physiological and detrimental roles of oxidative stress and of the imbalance with endogenous antioxidants led to the development and marketing of solutions and supplements enriched with trace elements and vitamins. In contrast to the current recommendations based on the dietary reference intakes, the amounts of antioxidants and vitamins for critical care patients may be much higher than those for the general population. The composition of several nutritional support formulas designed for critically ill and injured patients, a population “at high risk” of oxidant-related damage, was altered accordingly. The clinical advantage, in terms of decreases in the rates of organ failure and mortality, has been confirmed in some clinical trials.9
Management of the stress response is improving, thanks to the ongoing advances in the understanding of the associated metabolic changes and the translation of these discoveries into changes in the mode of administration and the composition of nutrition support solutions.
