Understanding the response to enzyme supplementation on body composition and nutrient balance creates the potential to increase nutrient utilization efficiency and, consequently, maximize growth performance in growing pigs. This study aimed to evaluate the effect of fungal lysozyme supplementation on body composition and nutrient balance in growing pigs. Seventy-two barrows [(40.6 kg ± 2.6; (Yorkshire × Landrace ♀) × Duroc ♂)] were used in this experiment. Pigs were distributed in a completely randomized design with 12 replicates within six treatments (0, 16, 32, 48, 64, and 80 mg of lysozyme/kg of diet). Pigs were group-housed in the same pen, and each pig was equipped with a transponder that enabled feeders to record individual feed intake and dispense feed according to the assigned treatment. Growth performance was evaluated through average daily feed intake (ADFI, kg/d), average daily gain (ADG, kg/d), gain-to-feed ratio (G: F, kg/kg) over 21 days. Body composition, nutrient balance and metabolic changes were measured using dual-energy X-ray absorptiometry and serum sampling. At the end of the experiment, samples of the digesta in the jejunum and cecum, as well as tissue from the jejunum, cecum, and liver, were taken for microbiota analysis and gene expression, respectively. Differences among treatments were analyzed using polynomial contrasts, and adjusted means were obtained with R. Optimal inclusion level was determined by non-linear models' analysis. The ADFI decreased linearly (P ≤ 0.01), while the body weight (BW), ADG, and G: F increased linearly (P ≤ 0.01) as the level of lysozyme in the diet increased. Increased urea and protein deposition and nitrogen -utilization efficiency were observed (P ≤ 0.05). Changes in the gut ecology included decreases in Gram-positive bacteria and inflammatory gene targets. The largest changes were observed in the cecum and were supported by gene expression in the liver. Based on the results of this experiment, the ideal lysozyme inclusion level for growing pigs is 60 mg/kg based on G: F, and 48 mg/kg based on ADG and evenness in jejunum digesta. Thus, dietary lysozyme supplementation improves pig growth performance, likely through immune response modulation initiated in the gastrointestinal tract due to minor changes in microbiota along with decreased expression of genes such as NF-kB1 and TLR2. This likely improved protein metabolism where increases in lysozyme supplementation enhanced nitrogen and amino acid utilization efficiency. Although the ideal dose for lysozyme supplementation will depend on the stage of production and the economic gain achieved from increasing enzyme supplementation, the results of this study support fungal lysozyme supplementation in growing pigs.
