Nutritional Needs of Sick or Injured Large Animals
Energy Requirements
While the energy requirements of large animal species of various ages, stages of growth, and activity levels have been determined and equations are available to calculate these needs, the energy and protein requirements for the hospitalized, sick patient are not known and likely vary depending on disease state, environment conditions, and level of fitness of the individual.
Historically, multipliers have been used to estimate the energy requirements in certain conditions including severe sepsis, trauma, and burn injuries. However, the increased metabolic demand of illness or surgical trauma and recovery is likely to be balanced by the inactivity and decreased metabolic demands of digestion of the patient during hospitalization. Consequently, these multipliers likely overestimate the caloric requirement of certain illnesses. Studies in the human ICU and in sick neonatal foals suggest that the metabolic energy needs may fall more closely within the range of the resting to maintenance energy requirements of the normal, healthy animal.22-24 The exceptions to this are individuals with extreme trauma, burns or severe sepsis, surgical conditions that require intestinal resection, and patients with large areas of devitalized tissue (such as clostridial myositis patients undergoing multiple fasciotomies). When estimating the energy requirements of the majority of other patients, resting energy requirements (DEr) are an acceptable target. Resting energy requirements are defined as the amount of energy needed to maintain an individual (no weight gain or loss) in a thermoneutral environment without the metabolic demands of digestion. Maintenance energy requirements (DEm) include the demands of digestion in this equation and are approximately 30% higher than resting energy requirements. Resting energy requirements will vary slightly depending on the size of the horse but can be estimated using approximately 22 to 23 kcal/kg/day for the average full-size horse. Maintenance energy requirements can be estimated using 30 to 35 kcal/kg/day. If a patient tolerates nutritional supplementation at resting energy requirements, the feeding rate (i.e., calories/kg/day) can be gradually increased if needed.Protein Requirements
The maintenance protein requirement of the healthy adult horse is estimated at approximately 0.5 to 1.5 g/kg/day. The needs of a growing foal are higher and may approach 7 g/kg/day during maximum growth periods. In addition, the lysine requirements of growing horses are higher than for mature horses.25 During critical illness or severe injury, protein catabolism and utilization of amino acids as a source of fuel continue despite the presence of other energy stores. Consequently, when calculating protein requirements in human patients, it is recommended to provide the higher end of the estimated need.22,26 Another consideration is supplementation of nonessential amino acids that may improve outcome in illness. Glutamine is considered a conditionally essential amino acid and is used as a fuel for enterocytes and other rapidly dividing cells.27 In periods of inappetence the majority of glutamine is produced by mobilization of muscle stores.28 Glutamine release during critical illness may act as a cell signaling molecule regulating the inflammatory and immune response; however, in periods of illness when glutamine requirements may actually increase, endogenous sources may be inadequate, resulting in relative glutamine deficiency.29 Glutamine has been shown to be critical in maintaining gut wall integrity, immune function, and antioxidant supplies.29 In a large-scale review of published meta-analyses and systematic reviews of glutamine supplementation’s effect on outcomes, glutamine supplementation was associated with decreased infection complications, length of hospital stay, and mortality, suggesting it may be worth adding to supplementary diets.30 Other recommendations for amino acid supplementation include branched chain amino acids and arginine supplementation.
Arginine, a nitric oxide precursor, is an important vasodilating agent, it upregulates immune function, and may reduce ischemia reperfusion 31-32injury.31 32
The amount and ideal route of glutamine or arginine supplementation has not been determined in the horse. In one in vitro study, glutamine supplementation improved mucosal restitution in an oxidant-injured equine colon model.33 Currently the American Society of Parenteral and Enteral Nutrition guidelines do not recommend addition of immune-modulating amino acid supplements due to a lack of evidence for outcome improvement in patients in the medical ICU and concerns about cardiovascular risks associated with modulation of nitric oxide.9 In actuality, many commercially available parenteral and enteral products contain nonessential amino acids. When feeding by the enteral route, protein needs can be met by feeding a complete feed (which generally contains nonessential or conditionally essential amino acids) or by addition of specific amino acids to a component diet. Alternatively, glutamine can be purchased separately and added to a diet, though recommendations for amounts are currently not available.