This study aimed to investigate the effects of adding 360 mg/kg niacinamide (NAM) to diets on nutrient metabolism, providing insights into how dietary NAM supplementation enhances nitrogen utilization and growth performance in pigs. Forty growing–finishing pigs were randomly assigned to one of four experimental diets as follows: basal diet + 30 mg/kg NAM (CON), basal diet + 360 mg/kg NAM (CON + NAM), low-protein diet + 30 mg/kg NAM (LP), and low-protein diet + 360 mg/kg NAM (LP + NAM). Results showed that supplementation of both the CON and LP diets with 360 mg/kg NAM resulted in decreased urea nitrogen concentrations and carbamyl phosphate synthetase-I activity (p < 0.05). The pyruvate dehydrogenase activity in the serum and liver, as well as the activity of pyruvate dehydrogenase, citrate synthase, and glutamate dehydrogenase 1 in the ileum mucosa, was increased by supplementing the LP diet with 360 mg/kg NAM (p < 0.05). The LP diet with 360 mg/kg NAM increased the villi length to crypt depth, mRNA expression of glucose transporters 1 and 2 and alanine-serine-cysteine transporter 1, and mRNA expression of mechanistic target of the rapamycin 1 in the ileum (p < 0.05). Additionally, 360 mg/kg NAM supplementation in the LP diet reduced ileal Lactobacillus abundance (LDA > 4) and increased ileal microbial nucleotide and purine metabolism (p < 0.05). Our findings suggest that addition of 360 mg/kg NAM to the LP diet reduced urea production in the liver, enhanced glucose and amino acid absorption and transport in the ileum, and improved glucose metabolism.

This study aimed to systematically evaluate the effects of low-protein (LP) diets and their associated nutritional interventions on growth performance and intestinal health in weaned piglets through a meta-analysis. Following the PRISMA guidelines, 21 studies were selected from an initial set of 1720 to construct a database. A random-effects model was employed to assess the growth performance and intestinal health of weaned piglets, comparing LP diet with control diet, and LP diet + additive with LP diet alone. Heterogeneity was assessed using the I² statistic, and its sources along with publication bias were explored through meta-regression, subgroup analysis, and Egger’s linear regression. The results show that LP diets (crude protein < 18%) reduced final body weight (p < 0.001), average daily gain (p < 0.001), and gain-to-feed ratio (p < 0.001) of weaned piglets. Additionally, forest plot data show that LP diet (crude protein < 18%) decreased villus height and crypt depth in the jejunum and ileum of weaned piglets (p < 0.05). To mitigate these adverse effects, plant extracts, amino acids (AAs), fatty acids, vitamins, enzymes, and carbohydrates are commonly used as additives. Among these, the results show that LP diets supplemented with AAs and plant extracts increased average daily feed intake (p < 0.001) and ileal crypt depth (p < 0.05). Network meta-analysis further identified AAs and plant extracts as the most effective additives for improving growth performance of weaned piglets. In conclusion, dietary protein levels below 18% negatively affect the growth performance of weaned piglets, and supplementation with AAs or plant extracts represents a promising strategy to counteract these adverse effects.