Simple SummaryAcidifiers such as lactic acid, formic acid, acetic acid, propionic acid, and citric acid are a class of feed supplements used in fish diets. Acidifiers play a role in the growth and development of intestinal villi and increase the absorption surface. Additionally, various studies have shown that these dietary supplements are utilized as nutrients by beneficial intestinal bacteria, which increase the populations of these bacteria and ultimately decreases the intestinal pH. The present study found that dietary 5 g/kg LA benefits common carp feeding to improve the growth rate, antioxidant capacity, and intestinal health. Such effects may be mediated via alterations in intestinal microbial communities.AbstractThe present study investigated the effects of dietary lactic acid (LA) supplementation on the growth performance, intestinal digestive/antioxidant enzymes’ activities, gene expression, and bacterial communities in common carp,Cyprinus carpio. Four diets were formulated to contain 0 g/kg LA (control), at 2.5 g/kg LA (2.5LAC), 5 g/kg LA (5LAC), and 10 g/kg LA (10LAC) and offered to the fish over a period of 56 days. The results showed that dietary 5 g/kg LA supplementation improved growth performance and feed efficiency in the fish. All LA treatments exhibited significant elevations in the intestinal trypsin and chymotrypsin activities, whereas the intestinal lipase, amylase, and alkaline phosphatase activities exhibited significant elevations in the 5LAC and 10LAC treatments. All LA treatments exhibited significant elevations in the intestinal heat shock protein 70, tumor necrosis factor-alpha, interleukin-1 beta, and defensin gene expressions, and the highest expression was observed in the 5LAC treatment. Additionally, dietary LA treatment significantly increased the lysozyme expression andLactobacillussp. population in the intestine of the fish, and the highest values were observed in the 5LAC and 10LAC treatments.Aeromonassp. andVibriosp. populations decreased in the LA treatments, and the lowestAeromonassp. population was observed in the 10LAC treatment. The intestinalmucin2andmucin5expressions, and the hepatic reduced glutathione content, significantly increased, whereas hepatic glutathione peroxidase, glutathione reductase, and malondialdehyde significantly decreased in the 5LAC and 10LAC treatments. In conclusion, dietary 5 g/kg LA is recommended for common carp feeding to improve growth rate, antioxidant capacity, and intestinal health.