Muscle buffering capacity of yellowtail fed diets supplemented with crystalline histidine

Juvenile yellowtail Seriola quinqueradiata (initial body mass of 22 g) were fed either a commercial diet (control, diet 1) or diets supplemented with histidine (diet 2), histidine+β‐alanine (diet 3), or histidine+β‐alanine+thyroxine (diet 4), for 6 weeks. The dietary treatment did not affect the final body mass. Free histidine levels of white muscle in the fish fed the diets supplemented with histidine (diets 2‐4) were significantly higher (>62 mmol kg−1 of wet tissue) than that of control group (42 mmol kg−1 of wet tissue). Dietary supplementation of β‐alanine (diet 3) or β‐alanine+thyroxine (diet 4) failed to increase muscle anserine (β‐alanyl‐π‐L.‐histidine) level. Muscle buffering capacity of the range from pH 6·0 to 7·5 of the fish fed the diets 2‐4 (41·6‐42·7 mmol NaOH pH−1 kg muscle−1) reflected the increase of muscle histidine level, having slightly but significantly intensified compared to control fish (36·6 mmol NaOH pH−1 kg muscle−1). Most of the free amino acids other than histidine were significantly lower in the fish fed the diets 2‐4 than in control fish. Thus, crystalline histidine supplemented to diets appears to be deposited in muscular tissue, and consequently enhance muscle buffering capacity in this species.