GlyNAC Supplement Shields Chicken Gut and Liver from Heat Stress Damage

Heat stress is a growing concern in poultry production and a useful model for studying oxidative stress biology more broadly. When ambient temperatures rise, animals accumulate excess reactive oxygen species (ROS), depleting glutathione (GSH) — the body's primary intracellular antioxidant — and triggering inflammation and tissue damage in the liver, intestine, and muscle. GlyNAC, a combination of glycine and N-acetylcysteine, provides both precursors needed for GSH biosynthesis: NAC supplies cysteine for the first enzymatic step, while glycine completes the final ligation step. This study is among the first to test GlyNAC specifically against heat stress in poultry.

Sixty-four male Cyan-shank partridge chickens (60 days old) were randomized into four groups in a 2×2 factorial design: control diet under thermoneutral conditions (25°C continuous), GlyNAC diet under thermoneutral conditions, control diet under cyclic heat stress (32°C for 4 hours daily), and GlyNAC diet under cyclic heat stress. GlyNAC supplementation consisted of 0.5% glycine plus 0.5% NAC added to the basal diet. The 7-day intervention ran from day 60 to day 67 of age, with rectal temperatures measured on days 1 and 5, and tissue samples collected at endpoint for histology, Western blot, and qRT-PCR.

Cyclic heat stress successfully elevated rectal temperatures on both measurement days (P < 0.05), confirming the thermal challenge model worked. However, neither heat stress nor GlyNAC supplementation significantly affected growth performance metrics — initial and final body weight, average daily feed intake, average daily gain, or feed conversion ratio — nor did they alter relative organ weights of heart, liver, or spleen. Serum malondialdehyde (MDA) levels were also unaffected, suggesting the 7-day exposure was insufficient to produce measurable systemic lipid peroxidation.

Despite the absence of growth effects, GlyNAC supplementation significantly increased GSH content in serum, liver, and pectoral muscle (P < 0.05). Notably, jejunal GSH levels were unaffected by either heat stress or GlyNAC, suggesting compartment-specific GSH regulation. Heat stress reduced the villus height-to-crypt depth (V:C) ratio in the jejunum — a marker of intestinal absorptive capacity — while GlyNAC supplementation restored this ratio (P < 0.05). In the liver, heat stress increased the inflammatory infiltration score (IIS) assessed by NAFLD activity scoring, and GlyNAC supplementation significantly reduced this score (P < 0.05).

At the molecular level, heat stress downregulated hepatic protein expression of caspase-3 and the glutamate-cysteine ligase modifier subunit (GCLM), as well as GCLM mRNA. GlyNAC supplementation upregulated both hepatic caspase-3 protein and GCLM mRNA expression (P < 0.05). The upregulation of caspase-3 by GlyNAC in the context of reduced inflammation is noteworthy and may reflect a shift toward regulated apoptotic clearance of damaged cells rather than necroinflammatory pathways. These molecular findings align with the histological improvements observed.

For the longevity and clinical community, this study reinforces the mechanistic rationale for GlyNAC supplementation — already being explored in human aging trials — as a strategy to replenish GSH and protect tissues from oxidative and inflammatory insults. The poultry model provides a clean, controlled system to isolate these effects. Caveats include the short 7-day duration, a single species and sex, and the absence of direct ROS quantification in tissues. Translation to human heat stress or metabolic stress contexts requires further investigation.