Summary
In malnourished or debilitated individuals, nutritional supplementation (calories and/or protein) has been shown to be beneficial in improving survival, immune function, wound healing, and gastrointestinal function in humans and animal models.2-5 Catabolism of lean muscle during critical illness and the resultant weakness have been the rationale for recommendations to increase amino acid supplementation in critically ill humans.6-10 However, indiscriminate nutritional supplementation in patients without a history of malnutrition has not been clearly linked to improved outcomes and in some studies is associated with increased complications.
Anorexia is part of the acute stress response and may be beneficial during illness as it activates autophagy, a cellular process that results in removal of damaged organelles, protein aggregates, and microorganisms. The autophagic response has been linked to improved outcomes in critical illness.11 Nutrients (particularly amino acids), growth factors, and insulin suppress autophagy and may ultimately have a negative impact on morbidity and mortality. Recent randomized controlled trials have failed to show a clear benefit to early nutritional support (whether caloric or amino acid supplementation) in critical illness, and some have even found potential harm.12-19 In human medicine recommendations are to determine nutritional risk factors in both baseline nutritional status and disease severity, as well as complication risks.18-20 This information is then used to determine route (enteral vs. parenteral), type (standard formulations vs. supplementation of additional components), and rate of nutritional support.21Thankfully, it is rare for large animal veterinarians to have to manage patients with the same severity and types of illnesses and comorbidities found in human intensive care units (ICUs).
Exactly when and how much nutritional supplementation is ideal is not known, but extrapolation from human studies would suggest that meeting complete nutritional needs should not be the goal. However, the combination of an increased catabolism and the altered metabolic processes seen with injury and illness do cause significant loss of lean body mass and can impact the return to performance of a competitive athlete. Recently, research in nutritional therapy in humans and laboratory species has been focused on attenuating the metabolic response to stress, preventing oxidative cellular injury and favorably modulating the immune response with the ultimate goal being to reduce disease severity, complications, and length of stay, as well as to improve outcome.21The goals for this chapter are to provide guidelines to help maintain muscle mass, provide essential nutrition, and decrease morbidity associated with illness. While nutritional support recommendations continue to be debated in the human intensive care unit, some tenets are likely to continue to hold true:
1. Hyperglycemia has been shown to negatively impact immune
function and may increase the risk of infection. Control of blood glucose with a goal of avoiding severe hyperglycemia without risking hypoglycemia is recommended. At times, an insulin infusion may be necessary to prevent severe hyperglycemia, though careful blood glucose monitoring is recommended to avoid hypoglycemia.
2. Meeting 100% of our patients' nutritional needs is unnecessary and often unrealistic, as volume restrictions for enteral supplementation and costs for parenteral nutrition (PN) frequently limit our ability to provide the calculated requirement in large animal species.
3. Nutritional supplementation can have benefits in minimizing the severity of protein loss, providing both essential and conditionally essential amino acids, vitamins, and minerals, and in decreasing morbidity associated with illness.
Lastly, the approach to nutritional support of the large animal patient should be considered on an individual-case basis. This includes evaluating preadmission nutritional status and activity level, comorbidities that could impact recovery and metabolic response to illness, as well as illness severity and likely recovery time.