Small intestinal starch, dextrin, and glucose digestion in steers

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Introduction
Previous experiments have been conducted at Kansas State University (Dairy Day, 1987) to evaluate small intestinal starch digestion in steers. In those experiments, 1) increasing the level of abomasal glucose infusion increased net portal glucose flux and 2) increasing levels of corn starch and corn dextrin infusion increased small intestinal disappearance. However, net portal glucose flux became maximal when they were infused at 20 gjhr. The present experiment includes the abomasal infusion of glucose, corn starch, and corn dextrin at a level known to exceed the digestive capacity of the small intestine (66 gjhr). Thus, the objective of this experiment was to evaluate the effect of carbohydrate complexity on small intestinal disappearance and net portal glucose flux.

Procedures
Three Holstein steers (930 Ib) were fitted with an elevated carotid artery, hepatic portal and mesenteric venous catheters, and abomasal and ileal cannulae. Steers were fed daily alfalfa hay at 1.5% of body weight. Treatments included the continuous infusion of water, corn starch (66 gjhr), corn dextrin (66 gjhr), or glucose (66 gjhr) into the abomasum. Treatments were randomized in an incomplete Latin square design, using three steers and eight infusion periods. Steers were infused with 250 ml of solution per hr, and each infusion period lasted 10 hr. The infusate contained tap water, carbohydrate, and Cr:EDTA as a fluid flow marker. Between hours 4 and 10 of infusion, ileal digesta samples were collected, and disappearance of carbohydrate within the small intestine was determined. Simultaneous blood samples were collected from the hepatic portal vein and carotid artery, and glucose flux across the small intestine was calculated. Portal plasma flow was determined by a primed continuous infusion of para-aminohippuric acid into the mesenteric vein catheter.

Results and Discussion
Results are summarized in Table 1. Steers consumed daily 14 Ib of alfalfa hay (dry matter basis) during the carbohydrate infusions. Fluid flowing past the ileum was similar . ; t 'Department of Surgery and Medicine, Deceased. among treatments and ranged from 1.6 to 2.0 l/hr. Small intestinal disappearance of corn dextrin and glucose was larger (P<.05) than that of corn starch. A portion of the infused corn • starch and corn dextrin flowing past the ileum occurred as glucose (10-15%). This indicates limited active transport of glucose at distal sites of the small intestine.
Yolatile fatty acids (YFA) flowing past the ileum were highest when starch was infused (P<.IO). The increased YFA presumably resulted from microbial fermentation. Since starch infusion resulted in the highest amount of infused carbohydrate passing the ileum, perhaps the increased substrate availability increased microbial fermentation at distal sites within the small intestine.
No significant changes in portal plasma flow occurred across treatments. Net portal glucose flux was large and negative (-15.2 g/hr) when steers were infused with water. This indicates that glucose was metabolized by the gut for maintenance of normal functions. Net portal glucose flux became positive when each of the carbohydrates was infused (P<.05), was largest during glucose infusion (P<.05), and was similar for starch and dextrin infusions. An increased net portal glucose flux also was associated with an increased arterial glucose concen tration.
Small intestinal carbohydrate disappearance accounted for by net portal glucose flux was 52, 54, and 72% for corn starch, corn dextrin, and glucose, respectively. The carbohydrate disappearance unaccounted for by glucose flux might be attributed to microbial fermentation or changes in gut metabolism. The relative importance of these processes may be critical in our overall understanding of small intestinal starch utilization in ruminants. aWater vs others, P < .05 bstarch + Dextrin vs Glucose, P < .05 cStarch vs Dextrin, P < .05 dstarch vs Dextrin, P < .10.