**Background:** Guanidine acetic acid (GAA) is an endogenous precursor of creatine biosynthesis, synthesized from glycine and arginine. Dietary GAA supplementation can spare arginine and glycine for other metabolic functions and increase creatine levels in the body. Previous research has shown that GAA improves growth performance in broilers and pigs, as well as meat quality. However, no studies had characterized the effects of GAA on tissue-bound amino acid profiles in finishing pigs. This study aimed to investigate the effects of dietary GAA on carcass traits, plasma biochemical parameters, tissue antioxidant capacity, and tissue-bound amino acid contents in finishing pigs.
**Methods:** Seventy-two 140-day-old crossbred pigs (Duroc × Landrace × Large White) with an initial body weight of 86.59 ± 1.16 kg were randomly assigned to four treatments with six replicate pens and three pigs per pen. Pigs were fed basal diets supplemented with 0%, 0.05%, 0.10%, or 0.15% GAA for 42 days. At slaughter, blood, heart, liver, spleen, pancreas, kidneys, and longissimus thoracis muscle (LM) were collected. Carcass traits were measured, and plasma biochemical parameters including glucose, albumin, creatine kinase, GAA, and creatine were analyzed. Antioxidant indices (SOD, T-AOC, GSH-Px, MDA, protein carbonyl) were measured in plasma and tissues. Hydrolyzed amino acid profiles were determined in all tissues using an automatic amino acid analyzer. Statistical analysis used one-way ANOVA with Tukey's test and linear/quadratic contrasts.
**Key Results:** Loin muscle area showed a linear increasing trend with GAA supplementation (p = 0.084). Plasma glucose decreased linearly and quadratically (p < 0.05), while creatine kinase activity (linear p = 0.002), GAA (linear p = 0.003, quadratic p = 0.045), and creatine (linear p = 0.011) increased. Dietary GAA linearly increased creatine content in LM (p = 0.014) and heart (p = 0.041). Antioxidant enzyme activities (SOD, T-AOC, GSH-Px) increased linearly in plasma and multiple tissues (p < 0.05), while MDA and protein carbonyl decreased linearly (p < 0.05). In the heart, GAA linearly increased proline (p = 0.036), alanine (p = 0.039), aspartic acid (p = 0.039), and NEAA (p = 0.049), with quadratic effects on methionine (p = 0.016), isoleucine (p = 0.022), leucine (p = 0.033), tyrosine (p = 0.032), and EAA (p = 0.044). In LM, GAA had significant linear and quadratic effects on threonine, methionine, isoleucine, carnosine (linear p < 0.001), and anserine (linear p = 0.004). No significant differences in amino acid content were observed in liver, kidney, pancreas, or spleen.
**Clinical Implications:** This study demonstrates that dietary GAA supplementation improves oxidative status and enhances bound amino acid profiles in the heart and LM of finishing pigs, without adversely affecting renal function (no increase in plasma creatinine). The increased creatine content in muscle and heart, along with enhanced antioxidant capacity, may improve energy metabolism and meat quality. The elevation of carnosine and anserine in LM further supports antioxidant benefits and potential shelf-life extension. These findings support the use of GAA as a feed additive to improve the nutritive value of pork and visceral products, particularly in markets where pig viscera are consumed. However, the study was limited to viscera and LM; further research is needed on gastrointestinal tract, brain, and different muscle types.