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Title: Extensive gut metabolism limits the intestinal absorption of excessive supplemental dietary glutamate loads in infant pigs

Author
item JANECZKO, MICHAEL - BAYLOR COLLEGE MED
item STOLL, BARBARA - BAYLOR COLLEGE MED
item CHANG, XIAOYAN - BAYLOR COLLEGE MED
item GUAN, XINFU - BAYLOR COLLEGE MED
item Burrin, Douglas - Doug

Submitted to: Journal of Nutrition
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/27/2007
Publication Date: 11/1/2007
Citation: Janeczko, M.J., Stoll, B., Chang, X., Guan, X., Burrin, D.G. 2007. Extensive gut metabolism limits the intestinal absorption of excessive supplemental dietary glutamate loads in infant pigs. Journal of Nutrition. 137:2384-2390.

Interpretive Summary: Glutamate is a major constituent of dietary protein and is also consumed in many prepared foods as a flavor additive in the form of monosodium glutamate (MSG). Previous studies with adult humans and our work with young pigs indicated that glutamate is the major energy fuel for the gut and that dietary glutamate is extensively metabolized by intestinal cells. Glutamate also is important for brain function, but can be toxic when consumed in excessive amounts. The importance of glutamate as a gut fuel makes it a good candidate for improving function of the infant gut, which has a high growth rate. The aim of the current study was to measure the capacity of the gut to metabolize MSG when it is fed in excess of the normal dietary intake to infant pigs. We fed young pigs MSG at the normal intake (100%) and at 200% and 300% intake and compared route of administration directly into the stomach or small intestine. We also infused a MSG labeled with heavy carbon atoms (13C) to trace the metabolism in the body. We found that even when MSG is fed at higher (3-fold) than normal dietary quantities, the majority (~70%) of MSG is either used for energy or converted to other amino acids by the gut tissue. We also found that at high dietary intakes the rate of MSG absorption into the blood is higher when given directly into the stomach compared to the intestine. These findings imply that the gut has a large capacity to use MSG for energy and that absorption of MSG directly from the stomach in addition to the intestine may be important.

Technical Abstract: Glutamate (Glu) is a major intestinal oxidative fuel, key neurotransmitter, and may be a useful dietary supplement to augment health of the infant gut. We quantified the metabolic fate of various supplemental dietary Glu intakes in young pigs surgically implanted with vascular, intraduodenal (ID), or intragastric (IG) catheters and a portal blood flow probe. Piglets were acutely fed a range of dietary Glu intakes using a basal milk formula (100%) supplemented with varying amounts of monosodium Glu (up to 400%) via ID or IG routes. We quantified the gastrointestinal metabolic fate of dietary Glu using [U-13C] Glu tracer. The Glu net absorption in the basal 100% group was low in both ID and IG groups, ranging from 13 to 17% of intake. Enteral Glu supplementation significantly increased the absolute absorption rate and arterial concentration of Glu. In both the ID and IG groups, enteral [13C]Glu absorption was limited (<5% tracer input) at the basal Glu intake (100%) but increased nearly 4-fold (~20% input) in the 300% intake group. A substantial fraction (33–50%) of the enteral [13C]Glu input was oxidized by the gut to 13CO2 in both the 100 and 300% intake groups. We conclude that extensive gut metabolism limits the absorption of supplemental dietary Glu even at excessive intakes.