The past 4 decades have been a time of dramatic advances in knowledge of feline nutrition, especially the relationships between protein metabolism and numerous disease states. Blood amino acid concentrations have been used for years to aid in the assessment of nutritional and protein status of cats and have a pivotal role in the diagnosis of specific medical conditions. The ability to measure whole blood and plasma taurine concentrations in cats aided in the discovery that taurine deficiency was a major cause of central retinal degeneration1 and dilated cardiomyopathy2 in cats. These discoveries have saved many cats from these debilitating and potentially fatal diseases. Without the ability to analyze blood amino acid concentrations, these important connections may never have been made. Amino acid analysis has also been of benefit in investigating many other disease processes, including liver disease,3 diabetes mellitus,4,5 heart disease,6 and even brain injury,7 in humans and other animals, but comparatively little of this research has focused on cats.
Blood amino acid concentrations are dynamic and can be reflective of the most recently consumed diet when samples are obtained during the immediate postprandial period.8,9 Alternatively, they can reflect the mean amino acid concentrations in protein-malnourished animals that eat a constant diet, thus allowing detection of severe and chronic amino acid deficiencies.10
Despite a long history of the use of amino acid concentrations for diagnostic and research purposes, there are few reports of typical amino acid concentrations in plasma and whole blood of adult cats eating commercial diets. Although plasma amino acid concentrations for adult cats eating commercial diets have been published in 2 studies,6,11 neither of those studies was designed to establish representative reference ranges; therefore, data were reported for only small numbers of healthy cats (29 and 24 cats, respectively) used as control animals in those studies. Additionally, neither study specifically addressed the diets being fed to the cats.
Currently, published reference values for feline amino acid concentrations have come primarily from studies12–15 in which investigators evaluated amino acid requirements and protein metabolism in growing kittens consuming purified diets. Those diets were formulated with crystalline amino acids or protein concentrates (such as casein and soy protein), and the bioavailability of amino acids in such diets is extremely high.16,17 In contrast, commercial foods undergo processing that may negatively impact the bioavailability of some amino acids, such as lysine, tryptophan, methionine, and cysteine.10,18–20 Additionally, those studies12–15 used only small numbers of growing kittens that were closely related and lacked the genetic diversity inherent in the general feline population. The potential problem with the use of data obtained from genetically similar animals consuming purified diets to develop reference ranges and nutritional recommendations was addressed in 2006 in the recent version of an NRC publication.10 That publication acknowledged the differences between diet types and provided nutrient requirement recommendations based on the availability of nutrients in ingredients commonly used in foods commercially available for pets.
Our intent in the study reported here was to establish reference ranges for plasma amino acid and whole blood taurine concentrations in cats. We hypothesized that there would be associations between amino acid concentrations and dietary protein concentrations and ingredients. The first objective was to obtain samples from a large number of cats to facilitate the creation of new reference ranges to more accurately reflect the general feline population. A second objective was to analyze the collected data for relationships between plasma amino acid concentrations and signalment, body weight, BCS, dietary protein, and dietary ingredients. Other studies have not adequately addressed how sex, age, body weight, and BCS affect plasma amino acid concentrations, and such information allows for better interpretation of plasma amino acid values in clinically affected animals. In another study21 conducted by our laboratory group, we detected a relationship between dietary ingredients and plasma amino acid concentrations, particularly taurine, methionine, and cysteine, in dogs. This relationship may also be relevant in cats because there has been a shift toward increasing the use of plant-based protein sources in commercially available pet foods. These ingredients can have lower concentrations of essential amino acids that may also be preferentially impacted by processing, which can result in a decrease in digestibility and bioavailability.22–25
Body condition score
National Research Council
Biochrom 30 amino acid analyzer, Biochrom Ltd, Cambridge, England.
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Pion PD, Kittleson MD, Rogers QR, et al. Myocardial failure in cats associated with low plasma taurine: a reversible cardiomyopathy. Science 1987;237:764–768.
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