The single-cell protein from Candida utilis as a functional protein ingredient for broilers.

The search for sustainable protein alternatives to soybean meal (SBM) in broilers diets has highlighted single-cell proteins as a promising candidate. Candida utilis protein (CUP) represents a potential substitute, yet its optimal inclusion level in broiler nutrition remains to be determined. A 42-day trial was conducted with 512 Arbor Acres broilers (1d old, initial body weights 45.00 ± 0.13 g) were purchased and randomly assigned to eight treatment groups. Treatment diets included 0% control group (CON), 1% CUP group (CUP1), 2% CUP group (CUP2), 3% CUP group (CUP3), 4% CUP group (CUP4), 5% CUP group (CUP5), 6% CUP group (CUP6) and 8% CUP group (CUP8). Growth performance exhibited a quadratic response (P < 0.001), with every phase average daily gain and average daily feed intake peaking at 4% inclusion and declining at 8%. Apparent digestibility of dry matter, crude protein and total amino acids was highest at the 3-4% inclusion levels (P < 0.05). Serum immunoglobulins immunoglobulin-Y (IgY), immunoglobulin-A (IgA), immunoglobulin-M (IgM), anti-inflammatory cytokines interleukin-4 (IL-4), interleukin-10 (IL-10) and antioxidant enzyme activities superoxide dismutase (SOD), glutathione peroxidase (GSH-Px), total antioxidant capacity (T-AOC) increased linearly with yeast supplementation (P < 0.001), whereas pro-inflammatory cytokines interleukin-2 (IL-2), tumor necrosis factor α (TNF-α), interferon gamma (IFN-γ), C-reactive protein (CRP) and malondialdehyde (MDA) decreased linearly (P < 0.001). Spleen weight and abdominal fat percentage also rose with increasing inclusion. Microbial community analysis revealed numerical reductions in α diversity with increasing dietary CUP inclusion, whereas principal coordinate analysis demonstrated clear separation of bacterial communities across treatments, with the % CUP group showing the most pronounced deviation from the control group. These results demonstrate quadratic regression analysis using growth performance as the core indicator revealed a theoretically optimal CUP supplementation range of 1.6% to 2.5%. Combined with the measured application effects, the comprehensive recommended optimal inclusion level of CUP in broiler diets is 1.6% to 4.0%, with a recommended level of 4% for large-scale commercial production.