Frequently Asked Questions Concerning Nutrition and Immunity

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What is "immunonutrition"?

Any aspect of nutrition that modulates the activity of the immune system in any way could be termed immunonutrition. However, the term has been most commonly used to describe nutritional interventions that attempt to improve the clinical outcome in critically ill patients through modulation of immunity. There is an optimal diet for every animal in every disease state, and the optimal diets for different animals may differ widely, or be the same for very different diseases. The ambiguity of the term may well have contributed to the overly simplistic thinking about the role of diet in severe inflammatory diseases that has resulted in one-dietfor- all approaches (e.g., PN solutions supplemented with glutamine, arginine, and n-3 PUFA for all septic patients).

What does it mean to "boost immunity"?

For any given immune response, if that response is amplified, exaggerated, or made more efficient, it can be said to be "boosted". However, boosting immunity is not always of benefit to the animal. The normalization of cellular immune responses in malnourished animals with adequate nutrition can be said to be boosting the immune response from deficient to normal. However, in severe sepsis where widespread activation of macrophages and neutrophils contributes significantly to vital organ and vascular damage, increasing the activity of those cells can increase morbidity and mortality. The clearest example of that is with the supplementation of arginine to critically ill humans. At its most extreme, suppression of immunity may be of benefit to the animal if immune-mediated disease is present. The most common reason for supplementing a diet with n-3 PUFA is to reduce inflammation. Finally, it may be that boosting immunity is of neither benefit nor detriment.

Can feeding adversely affect immunity?

Feeding incomplete or imbalanced diets always has the potential to cause immune dysfunction. In addition, feeding in excess of the resting energy requirements of an animal in a systemic inflammatory response syndrome may lead to hyperglycemia and immune-dysfunction.

What is the ideal diet for severe sepsis?

The answer to this question is unknown and there is potential for harmful intervention. For hospitalized patients, the major goals of nutritional intervention are to meet the known requirements, and avoid over-nutrition and dehydration. In cases of severe sepsis, simply meeting the resting energy requirements with a complete and balanced diet via the enteral route when possible are reasonable goals.

Whilst it has not been established what the ideal macronutrient proportions might be for severe sepsis, it is known that all three macronutrients (carbohydrate, protein, and fat) can be detrimental if fed in excess. For cats, carbohydrate might be especially poorly tolerated in severe sepsis, and low carbohydrate (< 20% of energy) high fat diets, fed to not exceed the resting energy requirements, may be ideal.

How could anorexia in sepsis be beneficial?

A potential explanation is that anorexia leads to muscle catabolism and the liberation of essential amino acids and glutamine for optimal leukocyte function. In addition, the increased tissue catabolism increases immunosurveilance through increased presentation of self peptides on MHC-I molecules. Thus, feeding with an imbalanced formulation may impair leukocyte responses and decrease the efficiency of pathogen clearance. However, when the infectious agent is being directly treated, and when supportive care is provided, nutritional intervention supersedes any slight benefit from anorexia.Feeding a highly digestible, moderate to low carbohydrate diet, with adequate glutamine and arginine, supplemented with higher concentrations than required for maintenance of antioxidants (especially ascorbate and tocopherol), and avoidance of over-feeding, the outcome will be greatly improved.

How much fish oil should I supplement to produce immunosuppression in a cat?

As discussed above, there are too few studies on which to make any confident recommendations. In addition, the required amount will depend on the type and severity of the disease, and the fat content of the cat’s diet. However it is likely that sufficient n-3 PUFA will need to be fed to produce a ratio of at most 1.3:1 (n-6:n-3).

Consider this example:

A typical adult maintenance dry cat food contains the following main ingredients: chicken, chicken by-product meal, corn grits, corn meal, chicken fat, dried egg product, fish meal, beat pulp. The total n-6 content of the diet is 2.6%, whilst the total n-3 content is 0.23%.

The diet has an energy density of 4 kcal/g (16.8 kJ/g), and a 4 kg cat, with a daily intake of 200 kcal (842 kJ) will consume a total of 50 g of food, which contains 1.3 g n-6 PUFA, and 0.115 g n-3 PUFA, or a ratio of 11.3 (n-6:n-3).

To reduce this ratio to less than 1.3, an additional 0.9 g n-3 PUFA needs to be added. Salmon oil contains approximately 34% n-3 PUFA, with the rest as saturated, monounsaturated, and a small amount of n-6 PUFA. Therefore, 2.6 g (2.9 mL) of salmon oil is needed to be added to the diet to reduce the ratio to 1.3. That supplement provides an additional 22 kcal (92 kJ), or 11% more than the required intake.

As stated above, it has not been determined what the most important variable for modulation of immunity is, though the single most important ratio in the cat may be the ratio of ARA:EPA. In the absence of evidence, the above calculation serves as an illustration of a starting point, where less than that amount is unlikely to have a significant effect.