Effects of dietary branched ‐ chain amino acid supplementation on growth and skeletal muscle cellularity in Japanese amberjack Seriola quinqueradiata

fibers [1]. Skeletal muscle fibers increase in size as the body increases in size, until they reach their maximum diameter [2]. Skeletal muscle growth includes the proliferation and differentiation of cells, which are responsible for hyperplasic and hypertrophic growth, respectively, in fish [2]. Skeletal muscle growth and cellularity are hormonally regulated. Insulin-like growth factor (IGF) is an anabolic hormone that consists of a growth hormone/IGF axis, which is an important system for body growth in fish [3]. IGF1 regulates skeletal muscle growth [4]. Feeding increases IGF1 levels in the skeletal muscles [5], and IGF1 treatments stimulate the differentiation of myoblasts to myotubes in rainbow trout Oncorhynchus mykiss [6]. This molecule may be important for growth because IGF1 expression levels in hepatic tissues and skeletal muscles correlate with growth in Japanese amberjack [7]. Skeletal muscle cellularity is also influenced by nutritional manipulations in fish, such as restricted feeding [8], dietary protein content [9], protein sources and amino acid profiles [10], lipid sources [11], and vitamin content [12]. However, research on skeletal muscle cellularity has not been extensively examined in Japanese amberjack, and a determination method for skeletal muscle fiber diameters has not been established in this fish species. Amino acids have many beneficial functional properties in fish growth, feed intake, and immunity, among other functions in aquaculture [13]. Branched-chain amino acids (BCAAs), which are composed of valine, leucine, and isoleucine, have physiological impacts, such as affecting gene transcription and protein anabolism [14]. Dietary supplementation with a mixture of crystalline BCAAs (a 1:2:1 mixture of valine:leucine:isoleucine) in a fishmeal-based extruded pellet diet significantly increases body weight and postprandial hepatic IGF1 and IGF2 expression levels in Japanese amberjack [15]. In Japan, only valine is permitted Abstract The present study investigated a determination method for skeletal muscle fiber diameters in Japanese amberjack Seriola quinqueradiata. We also investigated the effects of two supplements, a mixture of branchedchain amino acids (BCAAs) and valine alone, on body growth and skeletal muscle cellularity. The minor distances between two points on randomly selected individual fibers were measured and termed fiber diameters. Using growing fish (ranging from 4.8 g to 194.0 g in body weight), body weights and median fiber diameters were significantly correlated. A BCAA-supplemented diet (valine:leucine:isoleucine, 1:2:1) significantly increased the body weight after 105 days. The distribution frequency of fiber diameters between 61 and 81 μm was significantly higher in the 1 % BCAA group (P < 0.05) than in the control. However, dietary valine supplementation (1 %) had no effect on growth or skeletal muscle cellularity. These results indicated that hypertrophic growth occurred in the skeletal muscle of Japanese amberjack and was affected by dietary supplementation with a BCAA mixture.