• Low CP diets reduce diarrhea risk but may impair growth and welfare. • Balanced amino acid supply is key during immune stress and with low CP diets. • Acute phase proteins have an amino acid profile similar to muscle protein. • Low protein and amino acid levels increase the risk of injurious behaviors. • For piglet diets, the ratio of digestible lysine to CP is important. Weaning is a critical phase in piglet development, marked by abrupt nutritional, immunological, and social challenges. Lowering CP levels is a common strategy to mitigate post-weaning diarrhea, but growth performance may suffer and the risk of injurious behaviors such as biting may arise if dietary amino acids are not balanced. This review addresses how dietary protein, energy, and amino acid balance affect piglet health, behavior, and nutrient efficiency. First, we detail the health implications of CP reduction. During immune stress, piglets consume less feed, and anorexia has been identified as the primary factor affecting piglet growth. Balanced diets are crucial for piglets during immune activation, as muscle catabolism supplies energy and amino acids for acute-phase responses. A beneficial effect of higher dietary methionine + cysteine, threonine, and tryptophan levels has been reported in case of low sanitary conditions, but the potential advantages of supplementing other amino acids are unclear and the overall quantitative amino acid demand for immune function appears modest in comparison to the requirements for muscle deposition. Second, we review piglet behavior and welfare. Low CP diets, even those with balanced amino acids, can increase the incidence of injurious behaviors (e.g. tail and ear biting). Functional amino acids such as tryptophan, tyrosine, and histidine influence neurotransmitter synthesis and showed to modulate stress and aggression in grower-finisher pigs, although evidence in weaned piglets remains limited. Third, we explore the effect of energy, amino acids and protein on performance. In piglet feeding, the standardized ileal digestible ( SID ) lysine: CP ratio is a more informative metric than the SID lysine: energy ratio for optimizing growth and nutrient use efficiency. Energy requirements increase during immune activation but CP often remains the first limiting factor for growth. Excesses or imbalances in amino acids, particularly among branched-chain amino acids, can reduce feed intake and performance. While low CP strategies can improve gut health, they must be carefully managed to avoid compromising growth and welfare. Further research is needed to refine energy and amino acid requirements across states of health as well as to gain more insight into the behavioral effects of dietary interventions in the post-weaning period.
Millet et al. (Sun,) studied this question.