Challenges and Methodologies to Assess Protein Requirement and Quality Across Different Life Stages in Dogs: A Review.

Determining protein requirements (PRs) for dogs remains a longstanding challenge. During growth, the rapid rate of protein deposition increases the demand for amino acids. In adult dogs, differences in overall diet digestibility and lower energy requirements of domestic dogs have led to discrepancies between the minimum crude protein (CP) value proposed by the National Research Council (NRC; 80 g of CP/kg of diet) and the 180 g of CP/kg of diet proposed by the European Pet Food Industry Federation (FEDIAF) and the Association of American Feed Control Officials (AAFCO), although most commercially available adult dog feeds offer protein levels that exceed both recommendations. In elderly dogs, physiological changes such as sarcopenia and reduced energy intake indicate a potential increase in PR, although evidence remains scarce. A similar gap exists for pregnant and lactating bitches, since most recommendations rely on extrapolations from growth studies. Classical PR recommendations were based on body weight gain and nitrogen balance (NB), methods that present important limitations. Due to this, stable isotope methods-including 13C-leucine, 15N-glycine, and 13C-phenylalanine-have emerged as precise methodological tools, enabling a detailed and dynamic assessment of whole-body protein metabolism, protein quality, and more accurate determination of PR and recommended allowance across different life stages.