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.
Highlights
Dietary protein restriction largely resembles dietary restriction in being beneficial to longevity, stress resistance [1, 2], chronic kidney disease, and age-related pathologies in humans [3]
Crypt depth in the jejunum was not influenced by the crude protein (CP) content of the diet (P > 0.05)
Villus length as well as the ratio of villus length to crypt depth in the jejunum were decreased (P < 0.05) when dietary CP content was reduced from 18% to 14 or 16% CP
Summary
Dietary protein restriction largely resembles dietary restriction in being beneficial to longevity, stress resistance [1, 2], chronic kidney disease, and age-related pathologies in humans [3]. The role of protein restriction in nutrient metabolism and health keeping attracts intensive interests both from the fields of human clinical nutrition and animal nutrition. The small intestine plays a major role in digestion and absorption of protein, which subsequently influences the health, growth, development, reproduction, and sustaining life of the organism [5]. Body protein synthesis depends on the amount of intestinal AA absorption [8] and is regulated synchronously with the sensing of the concentration of extracellular AA [9]. 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]. The influence of protein restriction on postprandial total AA absorption and circulating AA pool size in body remains unclear
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