Decreased plasma amino acid concentrations in cats with chronic gastrointestinal diseases and their possible contribution in the inflammatory response

In humans, plasma amino acids (AAs) levels are used as dynamic nutritional markers. Moreover, some AAs are associated with chronic inflammation. In this study, we analyzed plasma AA profiles in cats with chronic gastrointestinal (GI) diseases. Eight healthy controls (HCs) and 12 client-owned cats with chronic GI diseases including chronic enteritis (n = 8) and neoplasms (n = 4) were recruited. Plasma albumin, total protein, and 22 AAs (11 essential and 11 non-essential AAs) levels were estimated. There was no significant difference in plasma albumin and total protein concentrations between the cats with chronic GI diseases and HCs. The plasma concentrations of 7 essential AAs (arginine, histidine, lysine, methionine, phenylalanine, taurine, and tryptophan) and 7 non-essential AAs (asparagine, aspartic acid, glutamic acid, glycine, hydroxyproline, proline, and serine) were significantly decreased in the cats with chronic GI diseases (P < 0.05). Moreover, plasma histidine and tryptophan levels were inversely correlated with severity of symptoms (histidine: rs = −0.7781, P < 0.005; tryptophan: rs = −0.6040, P < 0.05). To examine the contribution of altered AAs levels in the inflammatory response, feline macrophages were stimulated by lipopolysaccharides (LPS) with or without histidine, and the expression of interleukin-8 (IL-8) mRNA was quantified. The expression of IL-8 mRNA was significantly increased in the LPS-stimulated feline macrophages (P < 0.05). Histidine almost suppressed the LPS-induced IL-8 expression in the feline macrophages (P < 0.05). Our findings suggest that plasma AAs levels are more sensitive nutritional markers than albumin and total protein levels in cats with chronic GI diseases. There is a possibility that the decrease of histidine levels in cats with GI diseases is associated with chronic inflammation.