|VAN DER SCHOOR, SOPHIE|
|Burrin, Douglas - Doug|
|VAN GOUDOEVER, JOHANNES|
Submitted to: Journal of Nutrition
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/14/2005
Publication Date: 6/1/2005
Citation: Schaart, M.W., Schierbeek, H., Van Der Schoor, S.R., Stoll, B., Burrin, D.G., Reeds, P.J., Van Goudoever, J.B. 2005. Threonine utilization is high in the intestine of piglets. Journal of Nutrition. 135:765-770.
Interpretive Summary: The digestion and absorption of dietary proteins for growth in young infants is critical for normal growth, especially those born preterm. Most proteins fed to newborn infants are highly digestible, yet the actual amount of amino acids absorbed into the blood stream is affected by gut metabolism. Because the gut is so metabolically active, it consumes much of the dietary protein intake just to meet the needs for normal growth and development. Because it is unethical and impractical to determine the gut metabolism of dietary amino acids in human infants, we use infant piglets as a model system. Our previous studies with young piglets showed that as much as a third to half of the dietary amino acid intake is metabolized by the gut. One amino acid that is especially important for the intestine is threonine, since it is a component of mucus protein layer that protects the intestinal lining from infection and disease. We designed a study to measure the amount of dietary threonine used by the gut in our model of the formula-fed piglet by administering a special form of threonine labeled with carbon-13, which allowed us to trace its metabolism in the gut and whole body. We also fed pigs a low protein diet to see how this would affect gut threonine metabolism. We found that nearly all (85-90%) of the dietary threonine is used by the gut, mainly for synthesis of proteins such as mucin. The complete metabolism of threonine to carbon dioxide was minimal in the normal diet, but increased substantially when pigs were fed a low-protein diet. These results indicate that the gut plays a key role in determining the amount of dietary threonine available for growth in young infants.
Technical Abstract: The whole-body threonine requirement in parenterally fed piglets is substantially lower than that in enterally fed piglets, indicating that enteral nutrition induces intestinal processes in demand of threonine. We hypothesized that the percentage of threonine utilization for oxidation and intestinal protein synthesis by the portal-drained viscera (PDV) increases when dietary protein intake is reduced. Piglets (n = 18) received isocaloric normal or protein-restricted diets. After 7 h of enteral feeding, total threonine utilization, incorporation into intestinal tissue, and oxidation by the PDV, were determined with stable isotope methodology [U-13C threonine infusion]. Although the absolute amount of systemic and dietary threonine utilized by the PDV was reduced in proteinrestricted piglets, the percentage of dietary threonine intake utilized by the PDV did not differ between groups (normal protein 91% vs. low protein 85%). The incorporation of dietary threonine into the proximal jejunum was significantly different compared with the other intestinal segments. Dietary, rather than systemic threonine was preferentially utilized for protein synthesis in the small intestinal mucosa in piglets that consumed the normal protein diet (P 0.05). Threonine oxidation by the PDV was limited during normal protein feeding. In proteinrestricted pigs, half of the total whole-body oxidation occurred in the PDV. We conclude that, in vivo, the PDV have a high obligatory visceral requirement for threonine. The high rate of intestinal threonine utilization is due mainly to incorporation into mucosal proteins.