The high metabolic cost of a functional gut.
OBJECTIVE
Animal studies have shown that more than half of the dietary protein intake is used by the gut and that a large proportion of this utilization is devoted to (glyco-)protein synthesis. Recycling of these secretions may play a critical role in the regulation of overall dietary amino acid bioavailability.


METHODS
Four piglets (age 32 days, 8-10 kg) bearing portal, arterial, and duodenal catheters and a portal flow probe were infused with a complete diet via the duodenum for 12 hours, followed by 12 hours of fasting. The portal balance of glucose and amino acids was measured throughout the 24-hour period. The animals also received duodenal and intravenous infusions of different lysine and threonine tracers. Measurements of intestinal tracer utilization and reappearance in the portal blood were used to calculate intestinal amino acid utilization and recycling.


RESULTS
From 0 to 6 hours, one third of the protein intake appeared in the portal blood. As feeding continued, the portal glucose balance (60% of intake) was constant, but the net amino acid portal balance became progressively more positive. Significant net amino acid absorption continued for at least 6 hours after the cessation of feeding. Over 24 hours, 52% of the dietary protein intake appeared in the circulation and one third of this derived from recycled intestinal secretions.


CONCLUSIONS
Intestinal recycling of amino acids contributes significantly to their systemic availability and may be a critical factor in amino acid nutrition.
Van Der Schoor
Sophie R d
SR
USDA/ARS Children's Nutrition Research Center, Department of Pediatrics, Baylor College of Medicine, Houston, Texas, USA.
Reeds
Peter J
PJ
Stoll
Barbara
B
Henry
Joe F
JF
Rosenberger
Judy R
JR
Burrin
Douglas G
DG
Van Goudoever
Johannes B
JB
D000042:Absorption; D000596:Amino Acids / Q000378:metabolism; D000818:Animals; D000831:Animals, Newborn; D004044:Dietary Proteins / Q000097:blood; D005260:Female; D005947:Glucose / Q000378:metabolism; D006023:Glycoproteins / Q000378:metabolism*; D007413:Intestinal Mucosa / Q000378:metabolism*; D019344:Lactic Acid / Q000378:metabolism; D011168:Portal System; D013552:Swine; D013997:Time Factors
D000596:Amino Acids; D004044:Dietary Proteins; D006023:Glycoproteins; D019344:Lactic Acid; D005947:Glucose