To investigate the effects of individual essential amino acids (EAA) on growth and the underlying mechanisms, EAA individually supplemented a low-protein (LP) diet fed to young rats in the present study. Treatments were an LP diet that contained 6% crude protein (CP), a high-protein (HP) diet that contained 18% CP, and 10 LP diets supplemented with individual EAA to achieve an EAA supply equal to that of the HP diet. The CP concentration of the LP diet was ascertained from the results of the first experiment, which examined the effects of dietary CP concentrations on growth rates, with CP ranging from 2% to 26%. Weight gain was increased with the supplementation of His, Ile, Lys, Thr, or Trp as compared to the LP diet (p < 0.05). Feed intake was greater for the His-, Lys-, and Thr-supplemented treatments as compared to the LP group (p < 0.05). Protein utilization efficiency was lower for the HP group than other groups (p < 0.01). The supplementation of Leu, Lys, and Val led to reduced protein utilization efficiency (p < 0.05), but the supplementation of Thr and Trp led to greater efficiency than the LP group (p < 0.05). Compared to the LP group, plasma urea concentrations were elevated with individual EAA supplementation, with the exception of the Thr addition. The added EAA resulted in increased concentrations of the corresponding EAA in plasma, except for Arg and Phe supplementation. The supplementation of Arg, His, Leu, Lys, and Met individually stimulated mTORC1 pathway activity (p < 0.05), and all EAA resulted in the decreased expression of ATF4 (p < 0.05). In summary, the supplementation of His, Ile, Lys, Thr, or Trp to an LP diet improved the growth performance of young rats. Responses to His and Lys additions were related to the activated mTORC1 pathway and feed intake increases. The improved growth performance resulting from the addition of a single EAA is not solely attributed to the increased plasma availability of EAA. Rather, it may be the consequence of a confluence of factors encompassing signaling pathways, the availability of amino acids, and other associated elements. The additivity of these factors results in independent responses to several EAA with no order of limitation, as is universally encoded in growth models for all production animal species.

This study evaluated the effects of supplementing rumen-protected methionine, threonine, isoleucine, and leucine to low-protein diets on lactating dairy cow performance. Sixty Holstein cows were assigned to one of four dietary treatments in a 9-week randomized complete block design: positive control (16% crude protein diet; 16% CP), negative control (12% CP), 12% CP plus the four essential amino acids (12% CP + EAA), and 14% CP supplemented with the four EAA (14% CP + EAA). The milk protein yield was significantly decreased in the 12% CP group compared to the 16% CP group but was restored to comparable levels with EAA supplementation of both the 12% and 14% CP diets. Dietary nitrogen intake and urinary nitrogen excretion both increased with higher dietary CP levels. Nitrogen utilization efficiency in milk was significantly improved by EAA supplementation, with the highest efficiency observed in the 12% CP + EAA treatment (39.0% vs. 33.3% in the 16% CP diet). Plasma urea levels increased with elevated dietary CP and EAA supplementation. Moreover, EAA supplementation significantly elevated venous methionine levels and showed a tendency to increase venous leucine levels. Additionally, compared to the negative control, EAA supplementation increased concentrations of glucagon and prolactin (p < 0.05). EAA supplementation of low-protein diets, particularly the 14% CP diet, improved the dietary protein efficiency of lactating cows without a concomitant decrease in milk protein yield.