Introduction

In critically ill and hospitalized animals, the role of nutritional support in the overall management of patients is well established. However, nutrition is most often simply regarded as a supportive measure. Recently, further understanding of the underlying mechanisms of various disease processes and the recognition that certain nutrients possess pharmacological properties have led to investigations on how nutritional therapies themselves could modify the behavior of various conditions and improve patient outcomes and this has been dubbed ‘therapeutic nutrition’ (Wischmeyer, Heyland 2010). Nutrients such as certain vitamins, amino acids, and polyunsaturated fatty acids can modulate inflammation and the immune response (Cahill et al. 2010; Hegazi et al. 2011). A major focus of nutrition in critically ill human patients now involves the development of strategies that target or modulate metabolic pathways, inflammation, and the immune system (Hegazi et al. 2011). Exploiting pharmacological effects of certain nutrients to modulate disease processes and patient outcomes has been the subject of various clinical trials in people, however, a similar focus on clinical veterinary patients has of yet not taken place. The use of nutritional strategies in ameliorating animal diseases has been shown to be beneficial in the areas of chronic kidney disease (e.g., protein and phosphorous restriction) (Bauer et al. 1999; Brown et al. 1998), and cardiac disease (e.g., omega-3 fatty acids) (Freeman et al. 1998; Smith et al. 2007). In people, there is mounting evidence that certain nutrients such as glutamine, omega-3 fatty acids, and antioxidants can positively impact both morbidity and mortality in critically ill populations. It is hoped that a greater understanding of how these nutrients impart such beneficial effects may lead to developments of novel strategies for modulating various diseases in small animals. To this end, a review of how nutritional strategies could be used to modulate disease, especially in critically ill animals, is the focus of this lecture and is discussed in greater detail.

Nutritional Management Strategies

Omega-3 Fatty Acids

As inflammation plays a crucial role in many diseases, modulation of the inflammatory response has become an important target of therapy. Inflammation yields several lipid mediators that are involved in a complex regulatory array of the inflammatory process. Lipid mediators are synthesized by three main pathways, namely the cyclooxygenase, 5-lipoxygenase, and cytochrome P450 pathways and they each use polyunsaturated fatty acids (PUFA) such as arachidonic acid (AA), eicosapentaenoic acid (EPA), and gamma-linolenic acid (GLA) as substrates (Mayer et al. 2006). Potent proinflammatory eicosanoids, leukotrienes, and thromboxanes of the 2 and 4 series are produced from arachidonic acid (AA) metabolism. Classically, modulation of inflammation was thought to result from greater substitution of omega-6 fatty acids i.e., AA with EPA and DHA in cell membranes, such that when these PUFAs were cleaved by phospholipases and oxidized by several enzymes led to less inflammatory eicosanoids of the 3 and 5 series (Mayer et al. 2006).

However, it is now clear that the biological anti-inflammatory activities of omega-3 fatty acids are far beyond the simple regulation of eicosanoid production. Namely, these PUFAs can affect immune cell responses through the regulation of gene expression, subsequent downstream events by acting as ligands for nuclear receptors and through control of some key transcription factors (Singer et al. 2008). EPA can also inhibit the activity of the proinflammatory transcription nuclear factor B (NF- kappa B) at several levels, which regulates the expression of many proinflammatory mediators (e.g., cytokines, chemokines) and other effectors of the innate immune response system (Singer et al. 2008). In addition, recent research has revealed that free EPA and DHA also inhibit the activation of Toll-like receptor 4 by endotoxin and, thereby, further inhibit the inflammatory response (Lee et al. 2006). Finally, recent discoveries have identified that EPA and DHA are also substrates of two novel classes of mediators called resolvins and protectins, which are involved in the inhibition and resolution of the inflammatory process, which now appears to be a well-orchestrated, complex, active process involving these mediators (Singer et al. 2008; Willoughby et al. 2000). Therefore, in the context of disease modulation, omega-3 fatty acids help reduce the production of inflammatory mediators and are incorporated in the synthesis of anti-inflammatory and “pro-resolution” factors, which serve to attenuate the inflammatory response and the innate immune response.

In regards to the clinical use of omega-3 fatty acids in critically ill populations, the evidence is exclusively from human medicine. Enteral supplementation of EPA/DHA with concurrent antioxidants has been well established in ventilated patients with acute lung injury (Pontes-Arruda et al. 2008) and more recently, it has been shown to improve outcome in patients with early sepsis (Pontes-Arruda et al. 2011). However, the data is not entirely conclusive, especially when omega-3 fatty acids are administered intravenously via parenteral nutrition. In a recent meta-analysis of studies evaluating supplemental omega-3 fatty acids in parenteral nutrition, no statistically significant benefits were identified in regards to mortality, infection or ICU stay and only weak evidence that such supplementation may shorten overall hospitalization (Palmer et al. 2013). However, the analysis should be considered preliminary as there fewer than 10 trials included in the analysis and 6 of these trials contained fewer than 50 patients and, therefore, the conclusions regarding the utility of parenteral omega-3 fatty acids should be reserved until more data is available (Palmer et al. 2013). It is worth noting that the analysis did possibly uncover that timing of supplementation (i.e., early versus late in the disease process) may have a large impact in results. Many of the trials included in the analysis recruited patients in septic shock and therefore the ability to demonstrate treatment benefit would be extremely difficult (Palmer et al. 2013). The recent results of the INTERSEPT Study (Pontes-Arruda et al. 2011) using enteral EPA/DHA would support this hypothesis as they recruited patients with early sepsis without organ dysfunction and were able to demonstrate various improvements in outcome. Currently, no data is available on the use of omega-3 fatty acids in critically ill veterinary populations. Given the number of potential benefits in modulation inflammation and patient outcome, further research in this area is warranted.

Antioxidants

Similar to inflammation, oxidative stress is also recognized to be a prominent and common feature of many disease processes, including neoplasia, cardiac disease, trauma, burns, severe pancreatitis, sepsis, and critical illness. During various pathophysiological states, particularly those typified by an inflammatory response, cells of the immune system such as neutrophils, macrophages, and eosinophils substantially contribute to the production of reactive oxygen species (ROS) and reactive nitrogen species (RNS). With the depletion of normal antioxidant defenses, the host is more vulnerable to free radical species and prone to cellular and subcellular damage (e.g., DNA, mitochondrial damage) (Manzanares et al. 2012). The degree of antioxidant depletion appears to reflect severity of illness in human patient populations (Alonso de Vega et al. 2002). Oxidative stress is believed to be not only a promoter of inflammation but also a key factor leading to multiple organ failure (Manzanares et al. 2012).

Replenishment of antioxidant defenses attempts to lessen the intensity of the injury caused by ROS and RNS. Antioxidants can be classified in three different systems:

1.  Antioxidant proteins such as albumin, haptoglobin and ceruloplasmin

2.  Enzymatic antioxidants such as superoxide dismutase, glutathione peroxidase, and catalase

3.  Non-enzymatic or small molecule antioxidants such as ascorbate (vitamin C), alpha-tocopherol (vitamin E), glutathione, selenium, lycopene, and beta-carotene

N-acetylcysteine is a powerful progenitor of glutathione and has been associated with some positive results in several patient populations. Treatment with N-acetylcysteine not only scavenges ROS but also enables continual production of glutathione and even blocks transcription of inflammatory cytokines (Manzanares et al. 2012).

In regards to clinical evidence in critically ill people, a number of meta-analyses have indicated that the administration of antioxidant micronutrients (both as monotherapy or in combination therapy or antioxidant cocktails) is associated with a mortality risk reduction, reduced mechanical ventilator dependence but only a trend for reduced infectious complications (Manzanares et al. 2012; Heyland et al. 2005; Visse et al. 2011). It is interesting to note that the effect on mortality reduction was most apparent in populations with expected highest mortality rates but a difference could not be detected when the mortality rate between the critically ill population and control population was less than 10% (Manzanares et al. 2012). However, not all of the data regarding the use of antioxidants in the critically ill is positive. In a recent Cochrane review (Szakmany et al. 2012) of the use of N-acetylcysteine for sepsis and systemic inflammatory responses syndrome (SIRS) in adult human patients, the authors concluded that their analysis casts “doubt on the safety and utility of intravenous N-acetylcysteine as an adjuvant therapy in SIRS and sepsis (Szakmany et al. 2012). At best, N-acetylcysteine is ineffective in reducing mortality and complications in this patient population” (Szakmany et al. 2012). The analysis also highlighted concern that administration of N-acetylcysteine after 24 hours of onset of symptoms could lead to cardiovascular depression (Szakmany et al. 2012). Typically, Cochrane reviews are very conservative in their analytical methods and seldom support novel interventions in critically ill populations. It is clear that further research is required to identify the most appropriate approach in modulating oxidative stress in the critically ill patients.

Despite the clear importance of oxidative stress in various diseases in veterinary species, investigations evaluating the effect of antioxidants on disease processes are limited. Positive results have been demonstrated in experimental models of oxidative stress including in conditions such as:

• Congestive heart failure (Amado et al. 2005)
• Acute pancreatitis (Marks et al. 1998)
• Gastric dilatation-volvulus (Badylak et al. 1990)
• Renal transplantation (Lee et al. 2006)
• Gentamicin-induced nephrotoxicity (Varzi et al. 2007)
• Paracetamol (acetaminophen) toxicity (Webb et al. 2003; Hill et al. 2005)

Supplementation of vitamin E alone did not prevent oxidative injury (i.e., development of Heinz body anaemia), in cats fed onion powder or propylene glycol but the same group of investigators later showed that supplementation of vitamin E with cysteine in cats decreased the production of methaemoglobinaemia following paracetamol (acetaminophen) challenge (Hill et al. 2005).

In naturally-occurring disease such as chronic valvular disease (Freeman et al. 1998) and renal insufficiency (Plevraki et al. 2006) there have also been some positive results that support the need for further evaluation. Unfortunately, the use of antioxidants in the setting of critically ill veterinary patients has not been published.

Immune-Modulating Nutrients

Amino acids fulfill a vast array of functions in the body. They primarily serve as building blocks for protein synthesis and participate in various chemical reactions. Certain amino acids have immune-modulating properties and they help maintain the functional integrity of immune cells, aid in wound healing and tissue repair. They may also serve as an energy source for certain cells; perhaps the most pertinent example being glutamine which is the preferred fuel source for enterocytes and cells of the immune system. During disease states, the body undergoes marked alterations in substrate metabolism that could lead to a deficiency in these amino acids. In the response to stress there may be a dramatic increase in demand by the host of particular amino acids such as arginine and glutamine. In health these amino acids are adequately synthesized by the host. However, during periods following severe trauma, infection, or inflammation, the demand for these amino acids cannot be met by the host, and they become “conditionally essential,” and must be obtained from the diet. Given the importance of these amino acids, the sudden depletion in these important substrates led to the hypothesis that dietary supplementation of these amino acids during disease would improve outcome. In addition, in times of injury and tissue repair and rapid cellular proliferation, nucleotide availability may become depleted and rate-limiting for the synthesis of nucleotide-derived compounds (Hegazi et al. 2011).

Arginine

Arginine is a conditionally essential amino acid that is required for polyamine synthesis (for cell growth and proliferation), proline synthesis (for wound healing) and is a precursor for nitric oxide (signaling molecule for immune cells). Following extensive injury or surgery, immature cells of myeloid origin produce arginase-1, an enzyme that breaks down arginine. The ensuing arginine deficiency is associated with suppression of T-lymphocyte function (Popovic et al. 2011). When steps are taken to replenish arginine along with omega-3 fatty acids, T cell number and function improves. There is also data that demonstrates a significant treatment benefit following supplementation after major surgery. Clinical benefits included fewer infectious complication rates and decreased overall length of stay when compared with standard nutritional support (Hegazi et al. 2011).

The one population where arginine therapy is likely to be contraindicated is patients with severe sepsis (Hegazi et al. 2011). Likely causes of this detrimental effect relate to promotion of excessive nitric oxide synthesis, worsening of cardiovascular tone and decreasing organ perfusion (Hegazi et al. 2011).

Glutamine

Glutamine, another conditionally-essential amino acid, is the most abundant free amino acid in circulation, however, stores are rapidly depleted during critical illness in people. A deficiency in glutamine has been documented to impair several important defense mechanisms of the host. Supplementation of glutamine during critical illness is well accepted to confer beneficial effects on patient outcomes. The evidence had been so strong that nutritional guidelines for critically ill people recommended supplemental glutamine to any patient receiving parenteral nutrition (Wernerman 2011; McClave et al. 2009; Kreymann et al. 2006). The proposed mechanisms by which glutamine improves outcomes involve:

1.  Tissue protection (e.g., heat shock protein expression, maintenance of gut barrier integrity and function, and decreased apoptosis)

2.  Anti-inflammatory and immune-modulation (e.g., decreased cytokine production, inhibition of NF-kB)

3.  Preservation of metabolic function (e.g., improved insulin sensitivity, ATP synthesis)

4.  Anti-oxidant effects (i.e., enhance glutathione generation)

5.  Attenuation of inducible nitric oxide synthase activity (Wischmeyer et al. 2010)

The evidence supporting the use of glutamine in critically ill human patients had been overwhelmingly positive, until the publication of the largest randomized placebo-controlled, double blinded clinical trial evaluating high dose glutamine and antioxidants in severely ill patients (Heyland et al. 2013). In this seminal study, over 1200 critically ill patients, with at least 2 failing organ systems and requiring mechanical ventilation were randomly allocated to glutamine vs. placebo treatment and antioxidants vs. placebo treatment. Unexpectantly, there was a trend for increased mortality associated with glutamine use (Heyland et al. 2013). There was no effect of glutamine on rates of organ failure or infectious complications, and antioxidants had no effects discernable (Heyland et al. 2013). The exact reasons for the observed trend in increased mortality was not identified, but it is worth noting that the dose of glutamine used in this study was much higher than any previous study to date and also, that this study population included patients in shock being treated with nutritional support prior to achieving haemodynamic stability. Most recommendations for nutrition support (both enteral and parenteral) in the critically ill patient, stipulate that cardiovascular stability must be achieved before commencing nutritional support (McClave et al. 2009).

Despite ample evidence guiding treatment recommendations in people, there are no equivalent recommendations in the veterinary literature pertaining to glutamine use. This is likely due to the lack of supporting data and limited availability of parenteral glutamine. To date, there are only a few published veterinary trials that have evaluated the use of glutamine (enteral or parenteral) in dogs and cats. In a trial of cats treated with methotrexate, enteral glutamine offered no intestinal protection in terms of reducing intestinal permeability or improving severity of clinical signs (Marks et al. 1999). Another trial evaluating the effects of enteral glutamine on plasma glutamine concentrations and prostaglandin E₂ concentrations in radiation-induced mucositis showed no measurable benefit (Lana et al. 2003). Possible reasons for the apparent failures in both of these trials could be attributed to inadequate doses used or because of the form used, enteral, was not effective in these conditions. In contrast, a recent experimental canine model of postoperative ileus (Ohno et al. 2009) evaluated the effects of glutamine on restoration of interdigestive migrating contraction in the intestines and they were able to demonstrate a statistically significant reduction in the time to restore contractions in the glutamine treated group. The authors hypothesized that the benefit was derived from glutamine’s ability to maintain glutathione concentration and thereby counteract the deleterious effects from surgical injury, inflammation and oxidative stress. They concluded that administration of glutamine following gastrectomy could shorten the duration of ileus (a major problem postoperatively in critically ill people) and may protect against surgical stress in general (Ohno et al. 2009). Given these positive results, further studies should evaluate the possible beneficial effects of glutamine supplementation in treating ileus and other gastrointestinal motility disorders in dogs with natural-occurring disease.

Most recently Kang et al. demonstrated that the immune suppression induced by high-dose methylprednisolone sodium succinate therapy can be ameliorated by parenteral administration of L-alanyl-L-glutamine (Kang et al. 2011). The study was designed to address a common concern associated with high dose glucocorticoid therapy, namely, immune-suppression. The model employed did demonstrate that such high doses of glucocorticoid can suppress oxidative burst activity and phagocytic capacity of neutrophils. Although the study used an experimental model, it suggests that parenteral glutamine does have immunomodulatory effects in dogs, and that in the future, more clinically applicable uses should be explored. Unfortunately, parenteral glutamine is not routinely available in North America and the majority of studies evaluating parenteral glutamine are performed in Europe and Asia.

Nucleotides

These low molecular weight intracellular compounds (i.e., pyrimidine and purine) are the basic building blocks for the synthesis of DNA, RNA, ATP, and key coenzymes involved in essential metabolic reactions. Similarly to amino acids, nucleotides can be synthesized de novo or can be salvaged and recycled from other molecules. The reason nucleotides are included in this discussion of therapeutic nutrition is that during disease states and injury, the rapid cell proliferation required for tissue healing leads to nucleotide depletion (Hegazi et al. 2011). Dietary supplementation can compensate for such depletions and support cell proliferation and differentiation. As the cell types most affected by shortfall in nucleotides are cells of the immune systems and of the gastrointestinal tract, nucleotide supplementation is often included in ‘immune-enhancing diets.’ The evidence for the beneficial effects of dietary nucleotides is mostly from pre-clinical trials and rodent models, therefore, further research is still warranted (Hess, Greenberg 2012). From the pathophysiological point of view, supplementation of dietary nucleotides may be particularly important in animals with prolonged anorexia as supplementation in rodent models enhance intestinal repair, restore brush-border enzyme activity, and improve gut barrier function (Hess, Greenberg 2012). Additional benefits of dietary nucleotides include positive effects on gut flora, gastrointestinal microcirculation, immune function, and inflammation (Hess, Greenberg 2012). Given the plethora of potential beneficial effects without clear detrimental effects, it is not surprising that nucleotides have been included in some immune-enhancing diet cocktails despite the lack of definitive results. Although results of trials using these immune-enhancing cocktails are encouraging and mostly positive, it is unknown if these effects are synergistic or whether they result from the summation of the individual components. To date, no veterinary studies have evaluated the potential utility of supplementing nucleotides to critically ill patients.

Probiotics

Probiotics are live microorganisms that, when ingested in sufficient amounts, have a positive effect on the health of the host. Some of the benefits purportedly related to probiotics include reduced production of toxic bacterial metabolites, increased production of certain vitamins, enhanced resistance to bacterial colonization, and reinforcing host natural defenses. Probiotics are also believed to shorten duration of infections or decrease host susceptibility to pathogens (Morrow et al. 2012). The proposed mechanisms underlying the positive effects include restoration of gastrointestinal barrier function, modification of the gut flora by inducing host cell antimicrobial peptides (i.e., defensins, cathelicidins) or releasing probiotic antimicrobial factors (e.g., bacteriocins, microsins), competing for epithelial adherence and immunomodulation (Morrow et al. 2012). Probiotics are, therefore, believed to have a role in balancing gut microflora and increasing host resistance to pathogenic bacteria. It is worth bearing in mind that the effects of probiotics are not only dose-dependent but also both strain and species-specific (Petrof et al. 2012). In people, probiotics used include various species of Lactobacillus, Bifidobacterium, and Streptococcus (Morrow et al. 2012). Microorganisms approved for use in animal feeds include strains belonging to the Bacillus, Enterococcus, and Lactobacillus bacterial groups.

The mechanism by which probiotics enhance gut barrier function may involve how certain bacteria e.g., Lactobacillus, stimulate mucin production and thereby inhibit pathogenic bacteria from invading and attaching to the gut epithelium (Morrow et al. 2012). One of the concerns with the use of probiotics is that there is a risk that certain microorganisms, such as enterococci, may harbor transmissible antimicrobial resistance determinants (i.e., plasmids), thus contributing to the problem of antimicrobial resistance. The use of probiotics in the critically ill is controversial and guidelines recommend further safety trials before further use in critically ill patients (Petrof et al. 2012).

In human critical care, probiotics have been used to combat antimicrobial-associated diarrhoea, Clostridium difficile infections, and ventilator-associated pneumonia (Morrow et al. 2012). The probiotic yeast Saccharomyces boulardii apparently produces a protease which degrades C. difficile toxins and may also stimulate IgA secretions against C. difficile toxins (Petrof et al. 2012). The only meta-analysis evaluating probiotics to prevent ventilator-associated pneumonia demonstrated a significant reduction in incidence of ventilator-associated pneumonia and length of ICU stay (Siempos et al. 2010). Thus far, trials evaluating probiotics in critically ill patients have only demonstrated a trend towards reduced ICU mortality (Petrof et al. 2012).

Probiotics have the theoretical risk of transferring antibiotic resistance genes, translocating from intestine to other areas or developing adverse reactions via interactions with host’s microflora. While bacteraemia has not been documented with probiotic use in critically ill people, there are single case reports detailing infections with probiotic strains in patients that are immune-suppressed (Boyle et al. 2006).

In veterinary medicine, there are no trials evaluating the use of probiotics in a critically ill patient population. However, there have been trials in dogs with gastrointestinal signs. A prospective placebo controlled probiotic trial using a canine-specific probiotic cocktail containing 3 different Lactobacillus spp. strains in addition to novel protein diet was able to demonstrate a dramatic improvement in clinical signs after dietary change but no additional benefit attributed to the additional of a probiotic (Sauter et al. 2006). Other studies have documented some positive effects such as improvements in immunological markers or desirable changes in microbiota, however, these trials have mostly been performed on healthy dogs. It is uncertain whether these benefits would improve clinical signs in dogs with critical illness.

Summary

Despite the many pitfalls discussed, nutritional modulation of diseases appears to be a potentially useful strategy for companion animals. However, until trials can elucidate which specific nutrients and what dosages confer beneficial effects to particular patient populations, a certain degree of caution is advised. Of particular concern is the distinct possibility that significant species differences may reduce the usefulness of some of these approaches in veterinary patients. Before general recommendations for the use of immunomodulating nutrients in veterinary patients can be made, many questions must be answered. Central issues of safety, purity and efficacy must be addressed. However, as our understanding of the interactions between nutrients and disease processes grows, we may yet identify specific nutrients that could modulate serious diseases. Based on the progress being made in the area of clinical nutrition, it is quite evident that there should be a greater appreciation for the role nutrients play in ameliorating diseases, and how treatment strategies for certain conditions in companion animals may one day heavily depend on nutritional therapies.

References

References are available upon request.