Calculating Nutritional Requirements

4. Calculating Nutritional Requirements

Once the decision to implement nutritional support has been achieved, a stepwise process to calculate the energy requirements and to select the appropriate nutrient profile with respect to protein, carbohydrate, and fat is necessary. In addition to nutrients, the water requirement of the cat also needs to be evaluated.

Energy

The calculation of the energy requirement of critically ill patients has been the subject of some controversy. Direct measurement of patient’s energy consumption is not readily available. Consequently, several equations have been recommended to estimate the requirement. These equations utilize the resting energy requirement (RER), the basal energy requirement (BER), or the maintenance energy requirement (MER). The RER accounts for the energy required by the animal in a resting state and it includes physiologic influences and the assimilation of nutrients (Elliott & Biourge, 2006; Michel 2006). The interspecies formula (1) is most commonly used by the authors.

Formula (2) is an alternative equation that can be used for to estimate RER in cats.
        Formula 1 RER = 70 x (current body weight in kg) 0.73 kcal/day
        Formula 2 RER = 40 x (current body weight in kg) kcal/day

To avoid complications associated with refeeding critically ill patients (see below), the cat’s current body weight should be used for the initial RER calculation, regardless of whether the cat is underweight or overweight. The caloric intake can then be adjusted on a day-to-day basis to ensure the appropriate amounts of calories are administered to maintain current body weight. With resolution of the critical illness, caloric intake is further adjusted to either achieve weight gain in underweight cats, or for obese cats, a healthy weight loss program can be implemented (see Chapter 1).

Some authors have recommended multiplying the RER with an illness factor (0.5 to 2.0) to account for hypermetabolism (Bartges et al., 2004). Other authors suggest that the RER of critically ill dogs, determined with indirect calorimetry, indicates that their energy expenditure is only slightly increased from normal (O’Toole et al., 2004). In addition, feeding excess calories can be associated with gastrointestinal complications, electrolyte imbalances, hepatic dysfunction, or cardiac abnormalities, complications commonly referred to as the refeeding syndrome (Solomon & Kirby, 1989; Miller & Bartges, 2000; Armitage-Chan et al., 2006). Furthermore, overfeeding energy can result in increased carbon dioxide production which can challenge patients with respiratory compromise (Lippert et al., 1993). Finally, a study showed an association between the use of illness factors and the development of hyperglycemia in cats administered parenteral nutrition (Crabb et al., 2006). Therefore, these studies support the recent trend of formulating nutritional support in critically ill patients to meet the RER rather than the more generous illness energy requirements (O’Toole et al., 2004).

Protein

To abolish negative nitrogen balance in a severely hypermetabolic and hypercatabolic patient it may be necessary to supply protein in amounts in excess of normal minimum requirements (Elliott & Biourge, 2006) (Table 5). Although nitrogen balance is often used to determine the protein requirements of critically ill people, this is not commonly measured in critically ill animals. In critically ill cats, protein should reach 30 to 50% of the calories (Chan & Freeman, 2006). Protein requirements are usually estimated based on clinical judgment and the recognition that protein requirements are markedly increased during certain diseases (e.g., peritonitis, draining wounds, severe burns) or require adjustement with other diseases (e.g., uremia, hepatic encephalopathy). The dietary source of protein should be highly digestible and contain all the essential amino acids. Human liquid formulas should be used cautiously, if at all in cats. Human formulations typically do not meet the high protein requirements of the cat, and are deficient in essential nutrients such as arginine, taurine and arachidonic acid.

The branch-chain amino acids (BCAAs) leucine, isoleucine and valine (or their metabolites) may have a regulatory and anabolic role in protein metabolism by either increasing the rate of muscle protein synthesis or by decreasing the rate of protein degradation. Some, but not all, human studies have reported that BCAAs have a positive effect on nitrogen balance in the stressed patient (Skeie et al., 1990). To date, studies to evaluate the benefits of BCCA in critically ill cats have not been reported. However, the metabolism of these amino acids in this species, suggest that BCAA’s could have positive benefits (Elliott & Biourge, 2006).

There are limited studies in critically ill or diseased companion animals supplemented with GLN. Marks et al. (1999) were unable to preserve intestinal function in cats with methotrexate-induced enteritis that were fed a glutamine-supplemented amino acid-based purified diet. However, there are numerous studies evaluating the effects of enteral or parenteral glutamine in critically ill humans. Some studies report positive effects of glutamine supplementation on the gastrointestinal barrier and outcome, whereas other studies report no differences. Summarizing the numerous human studies certainly suggests that glutamine could have positive benefits on gastrointestinal health in critically ill cats.

Carbohydrates

Cats do not have an absolute requirement for carbohydrates other than as an alternative source of energy. However, supplementation with carbohydrates may help to preserve lean body mass by down regulating gluconeogenesis. Excess simple carbohydrates should be avoided in critically ill cats as they can predispose to hyperglycemia (Lippert et al., 1993; Chan et al. 2002; Pyle et al., 2004) (Table 6). The subsequent release of insulin may lead to or exacerbate hypophosphatemia, hypokalemia, and other metabolic derangements (Elliott & Biourge 2006). In addition, cats have difficulty metabolizing large loads of highly digestible carbohydrate. Therefore, carbohydrates as source of energy are not recommended for critically ill cats.

Conversely, the inclusion of fermentable fibers or prebiotics such as beet pulp or fructo-oligosaccharides may have several beneficial effects in critical illness. Fermentable fibers have a positive effective on the mucosal barrier by stimulating the growth of intestinal bacteria such as Lactobacilii and Bifidobacteria. These bacterial species are considered to be beneficial to gastrointestinal health as they decrease the growth of pathogens such as Clostridia and E. coli. In addition, they produce the short chain fatty acids butyrate, acetate and propionate, which provide fuel for colonocytes. Short chain fatty acids enhance sodium and water absorption, increase mucosal blood flow and increase gastrointestinal hormone release. These mechanisms contribute to the trophic role that short chain fatty acids have on the intestinal mucosa, stimulating enterocyte and colonocyte proliferation (Elliott & Biourge, 2006).

Fat

High fat diets (over 40% of calories) have been recommended for critically ill patients because free fatty acids rather than glucose provide the principal fuel in the catabolic patient. The preferential use of fat as a fuel may also help spare protein from catabolic processes for energy generation so that the protein can be used for anabolic processes. In addition, fat provides more than twice the energy density per unit weight than protein or carbohydrates, which helps to make the diet more concentrated (Table 7).

Polyunsaturated fatty acids (PUFA’s) are essential for the maintenance of membrane integrity as constituents of membrane phospholipids and the provision of substrates for eicosanoids synthesis (protaglandins, thromboxanes, and leukotrienes). The eicosanoids regulate the production of several cytokines such as interleukin-1 and TNF-α and are involved in critical inflammatory and immune responses. The long chain omega-3 fatty acids such as EPA (eicosapentanoic acid) and DHA (docosahexaenoic acid) decrease the synthesis of inflammatory mediators (COX-2 inhibitor- like action, PGE2 production inhibition, NF-ÎB nuclear translocation decrease, and cytokines production inhibition), and they have been shown to have clinical benefits in a variety of disease states, including sepsis. Conversely, omega-6 fatty acids have a significant role in immunosuppression, tumorinogenesis, and inflammation (Kerl & Johnson, 2004; Saker, 2006).

Vitamins and Minerals

Vitamins and minerals facilitate complex metabolic reactions and are key components of antioxidant activities (Saker 2006). Electrolytes (phosphorus, sodium, potassium and magnesium) should be closely evaluated in diets formulated for the critically ill patient to prevent the refeeding syndrome (Solomon & Kirby, 1989; Justin & Hohenhaus, 1995; Miller & Bartges, 2000; Armitage-Chan et al., 2006). Zincsupplementation may be beneficial in the critical patient to support the immune system and help promote wound healing. Critically ill cats may also have increased requirements for the water-soluble B vitamins. Vitamin B12 is particularly important for cats with pancreatitis or chronic intestinal disease.

Special Nutrients

The association of malnutrition with reduced resistance to infections has been observed for centuries. Numerous studies have evaluated the clinical effectiveness of specific nutrient supplementation in modulating the immune system (Heyland & Dhaliwal, 2005). Immunomodulating nutrients that have been evaluated include glutamine, arginine, long chain omega-3 fatty acids, antioxidants (such as vitamin C, vitamin E, taurine, caroteinoids), and nucleotides (Chan & Freeman, 2006a). However, the optimal combination and level of immune modulating nutrients to support the immune system of the critically ill cat is not yet known (see Chapter 14).

Free radicals are unstable molecules generated by numerous exogenous and endogenous mechanisms. Hypovolemia, ischemia and reperfusion injury, common components of critical illness, can increase the production of free radicals. Free radicals cause oxidative damage to cellular components, which may ultimately contribute to organ dysfunction. The body counteracts oxidative damage by using free radical scavenging systems such as superoxide dismutase, glutathione peroxidase, catalase, vitamin E, vitamin C, taurine, and carotenoids. However, in critical illness, an imbalance between oxidant production and antioxidant protection can arise. Therefore it is prudent to supplement the diet of the critically ill patient with antioxidants.

In summary, the beneficial effects derived from adequate nutrititional support include: enhanced immune fonction, wound repair, response to therapy, recovery time and survival time (Figure 5).
Figure 5. Nutritional keypoints to improve the speed of recuperation and to improve clinical success.

Water

The water needs of cats reflect their evolutionary status as desert-dwelling carnivorous animals who evolved to obtain most of their water requirements from the consumption of prey. Cats also have a less sensitive response to thirst and dehydration than dogs.

Nevertheless, critically ill cats are generally dehydrated or hypovolemic and restoration of fluid and electrolyte balance, and circulating blood volume typically requires intravenous support. However, consideration should also be given to critically ill cats to ensure adequate intake of free water, which can be administered enterally or parentally.