Low-protein amino acid–supplemented diets for growing pigs: Effect on expression of amino acid transporters, serum concentration, performance, and carcass composition1

Abstract

Pigs fed protein-bound AA appear to have a higher abundance of AA transporters for their absorption in the jejunum compared with the duodenum. However, there is limited data about the effect of dietary free AA, readily available in the duodenum, on the duodenal abundance of AA transporters and its impact on pig performance. Forty-eight pigs (24.3 kg initial BW) distributed in 4 treatments were used to evaluate the effect of the CP level and form (free vs. protein bound) in which AA are added to diets on the expression of AA transporters in the 3 small intestine segments, serum concentration of AA, and performance. Dietary treatments based on wheat and soybean meal (SBM) were 1) low-CP (14%) diet supplemented with L-Lys, L-Thr, DL-Met, L-Leu, L-Ile, L-Val, L-His, L-Trp, and L-Phe (LPAA); 2) as in the LPAA but with added L-Gly as a N source (LPAA+N); 3) intermediate CP content (16%) supplemented with L-Lys HCl, L-Thr, and DL-Met (MPAA); and 4) high-CP (22%) diet (HP) without free AA. At the end of the experiment, 8 pigs from LPAA and HP were sacrificed to collect intestinal mucosa and blood samples and to dissect the carcasses. There were no differences in ADG, ADFI, G:F, and weights of carcass components and some visceral organs between treatments. Weights of the large intestine and kidney were higher in HP pigs (P < 0.01). Expression of b(0,+) in the duodenum was higher in pigs fed the LPAA compared with the HP diet (P= 0.036) but there was no difference in the jejunum and ileum. In the ileum, y+ L expression tended to be higher in pigs fed the LPAA diet (P = 0.098). Expression of b(0,+) in LPAA pigs did not differ between the duodenum and the jejunum, but in HP pigs, the expression of all AA transporters was higher in the jejunum than in the duodenum or ileum (P < 0.05). The serum concentration of Arg, His, Ile, Leu, Phe, and Val was higher but serum Lys and Met were lower in pigs fed the HP diet (P < 0.05). These results indicate that LPAA can substitute up to 8 percentage units of protein in HP wheat-SBM diets without affecting pig performance; nonessential N does not seem to be limiting in very low-protein wheat-SBM diets for growing pigs. Also, the inclusion of free AA in the diet appears to affect their serum concentration and the expression of the AA transporter b0,+ in the duodenum of pigs.

Protein Restriction with Amino Acid-Balanced Diets Shrinks Circulating Pool Size of Amino Acid by Decreasing Expression of Specific Transporters in the Small Intestine

“…Protein retention was not influenced by protein intake which is contrary to previous results showing that protein restriction led to lower body muscle and increased fat content [32, 33]. This discrepancy can be explained by the fact that the diets used in the present study were formulated to better meet the nutrient requirement of pigs recommended by latest version of NRC [15], and moreover, the effects of dietary CP content or protein intake on body composition are less in the growing period than the finishing period [34]. …”

“…The expression of duodenal cationic AA transporter b 0+ has been shown to be enhanced by dietary free lysine [15]. Efficient duodenal absorption of dietary crystalline AA may be responsible for the stability of the size of circulating lysine and threonine pool in pigs fed the 14% CP diet, in which relatively higher proportion of crystalline lysine and threonine were supplemented.…”

“…Deficient AA intake hampers protein anabolism and thus brings out certain health problems [10], which could be remedied by supplementation of crystalline AA now [11–14]. However, the concentration of free AA except for lysine and methionine in the vena cava anterior was decreased when pigs were fed a low-protein diet with increased crystalline AA [15]. Therefore, the influence of protein restriction on postprandial total AA absorption and circulating AA pool size in body remains unclear.…”

“…Free dietary AA are readily available whereas protein-bound AA have to be digested before being released from the dietary protein [15, 16]. More crystalline AA supplemented in low protein diets could bring out changes in AA absorption site and absorption speed, which may alter small intestine morphology, AA transporter expression, as well as AA absorption in the small intestine.…”

“…However, when the diets were also supplemented with branched-chain AA, the expression of jejunal AA transporters was recovered [18]. The expression of b 0,+ , y + L and B 0 was not changed in the jejunum of pigs fed low protein diets with all essential AA (EAA) balanced compared with the normal diets [15]. Based on our knowledge, the influence of dietary protein restriction on the expression of small intestinal AA and peptide transporters remains unclear.…”