**Background:** High mortality rates in livestock—over 10% of sows die naturally or are euthanized, nearly 30% of pigs do not reach market, and broiler mortality ranges from 2% to 8%—represent major economic and welfare concerns. Current methods for estimating amino acid (AA) requirements rely on growth or protein retention measurements, based on Hammond's hierarchy of nutrient use which posits that survival and reproduction are prioritized over growth. However, evidence suggests AA levels above those required for maximal protein retention may improve key physiological functions. This study used exploratory data analysis (EDA) to investigate whether growth and protein retention measurements alone are sufficient for developing dietary AA recommendations that optimize animal performance, health, and survival.
**Methods:** The EDA involved three components: (1) review of assumptions underlying AA requirement research, (2) data mining of AA dose-titration studies across multiple species to identify response patterns to AA intake above maximal growth thresholds, and (3) a rapid literature review of studies where growth/protein retention data followed the linear-logistic pattern alongside other physiological response measures. The linear-logistic model describes a trend mathematically represented by a linear function minus a logistic function, yielding two inflection points: Rmax (where response is maximized) and Rmin (where response is minimized). CurveExpert Professional (version 2.7.3) was used for model fitting. Studies were selected from PubMed, ISI Web of Science, Science Direct, Scopus, and SciELO, published before August 2021.
**Key Results:** The linear-logistic model was observed across multiple species including rats, fish (Colossoma macropomum), shrimp (Penaeus monodon), poultry, dogs, and humans. In growing pigs, the pattern was observed in dozens of studies spanning from the 1980s to 2020s. Key findings from specific studies include: (1) In Holstein cows, rumen-protected methionine supplementation maximized milk production at AA doses corresponding to Rmin, without decreasing milk quality. (2) In pregnant primiparous sows, SID lysine levels associated with Rmin were related to increased litter size. (3) In broiler breeder hens, Rmin coincided with increased immune response (IgG and IgM). (4) In juvenile Nile tilapia, survivability was maximized at Rmin. (5) In male broilers, dietary arginine-to-lysine ratios corresponding to Rmin (117%) resulted in the lowest mortality. (6) In growing pigs, greater gene expression of zonula occludens protein-1 (ZO-1), a marker of intestinal barrier function, was observed at SID threonine doses associated with Rmin. (7) Multiple studies showed plasma AA concentrations were maximized at dietary AA levels corresponding to Rmin.
**Clinical Implications:** These findings challenge the widely accepted hierarchy of nutrient use proposed by Hammond, suggesting instead that animals prioritize lean tissue deposition over functions associated with long-term survival and reproduction. The results indicate that traditional methods for estimating AA requirements—including the indicator amino acid oxidation (IAAO) technique, nitrogen balance, and plasma AA methods—may be insufficient for optimizing health, reproduction, and survival. The linear-logistic model offers a potential framework for estimating AA doses that optimize these outcomes. For production stages where the goal is reproductive performance and animal survival, AA doses corresponding to Rmin may be considered the requirement; for stages where maximizing growth is primary (e.g., finishing pigs), Rmax may be appropriate. The authors note that genetics, age, dietary fiber, and fermentable crude protein levels influence maximum protein retention but do not alter the dynamics captured by the linear-logistic model. The study is exploratory and the authors emphasize that confirmatory research is needed before clinical application.