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

Abstract

Dietary protein restriction is not only beneficial to health and longevity in humans, but also protects against air pollution and minimizes feeding cost in livestock production. However, its impact on amino acid (AA) absorption and metabolism is not quite understood. Therefore, the study aimed to explore the effect of protein restriction on nitrogen balance, circulating AA pool size, and AA absorption using a pig model. In Exp.1, 72 gilts weighting 29.9 ± 1.5 kg were allocated to 1 of the 3 diets containing 14, 16, or 18% CP for a 28-d trial. Growth (n = 24), nitrogen balance (n = 6), and the expression of small intestinal AA and peptide transporters (n = 6) were evaluated. In Exp.2, 12 barrows weighting 22.7 ± 1.3 kg were surgically fitted with catheters in the portal and jejunal veins as well as the carotid artery and assigned to a diet containing 14 or 18% CP. A series of blood samples were collected before and after feeding for determining the pool size of circulating AA and AA absorption in the portal vein, respectively. Protein restriction did not sacrifice body weight gain and protein retention, since nitrogen digestibility was increased as dietary protein content reduced. However, the pool size of circulating AA except for lysine and threonine, and most AA flux through the portal vein were reduced in pigs fed the low protein diet. Meanwhile, the expression of peptide transporter 1 (PepT-1) was stimulated, but the expression of the neutral and cationic AA transporter systems was depressed. These results evidenced that protein restriction with essential AA-balanced diets, decreased AA absorption and reduced circulating AA pool size. Increased expression of small intestinal peptide transporter PepT-1 could not compensate for the depressed expression of jejunal AA transporters for AA absorption.

Intestinal IMINO transporter SIT1 is not expressed in human newborns

“…Even when an adequate amount of calories is supplied in the blood, as for instance with parenteral nutrition, morphological and functional alterations of the small intestine mucosa are observed; the small intestine absorptive surface area appears to be reduced because of a decrease in cell proliferation, differentiation, and migration along the crypt-to-villi axis. That luminal amino acid availability also affects intestinal amino acid transporter expression has been shown in different animal models (2,20,27,28). In our study, we observed a reduction in cryptal cell proliferation in the intestinal area distal to the obstruction not receiving amniotic fluid.…”

Integrated Remodeling of Gut–Liver Metabolism Induced by Moderate Protein Restriction Contributes to Improvement of Insulin Sensitivity

“…Most previous studies focused on extremely protein restrictive diets, in which protein levels were reduced by about 60–100%, or on specific essential amino acid (EAA), such as methionine and branched‐chain AA, which were restricted by 50% or more, far below the estimated average requirement for human and rodents, likely unsustainable and unhealthy due to the severe deficiency of EAA . We demonstrated that moderate PR diets, whose protein levels were reduced by about 20%, shrank circulating AA pool size and profoundly altered the pathways concerning immune function in the small intestine even with the supplementation of crystal EAA in diets . However, the impact of moderate PR without malnutrition on insulin resistance remains unclear.…”

“…In Experiment 3, 12, 60 days old, castrated male growing pigs ( n = 6) with 22.7 ± 1.3 kg BW were housed individually. After 3 days of adaption, pigs were equipped with catheters in portal vein and cared as described previously . Subsequently, pigs were pair‐fed the same amount of the control (18% CP) and moderate PR diet (14% CP), respectively.…”

The involvement of NF-κB/P38 pathways inScutellaria baicalensisextracts attenuating ofEscherichia coliK88-induced acute intestinal injury in weaned piglets

“…Protein expression levels of TLR4, MyD88, NF-κB, TNF-α, IL-1β, occludin, zonula occludens-2 (ZO-2) and P38 in both jejunal and ileal mucosa were measured by Western blot (21) and normalised by the protein expression of β-actin. Briefly, about 100 mg of frozen jejunal or ileal mucosa were powdered in liquid nitrogen and lysed in RIPA buffer containing 50 mM Tris-HCl (pH 7•4), 150 mM NaCl, 1 % NP-40 and 0•1 % SDS, plus a Halt protease inhibitor cocktail (Thermo Fisher Scientific).…”