The metabolism of the small intestine: physical properties, oxygen uptake and L-lactate formation along the length of the small intestine of the sheep

Lactate, fructose and glucose oxidation profiles in sports drinks and the effect on exercise performance

Exogenous carbohydrate oxidation was assessed in 6 male Category 1 and 2 cyclists who consumed CytoMax™ (C) or a leading sports drink (G) before and during continuous exercise (CE). C contained lactate-polymer, fructose, glucose and glucose polymer, while G contained fructose and glucose. Peak power output and VO₂ on a cycle ergometer were 408±13 W and 67.4±3.2 mlO₂·kg⁻¹·min⁻¹. Subjects performed 3 bouts of CE with C, and 2 with G at 62% VO₂peak for 90 min, followed by high intensity (HI) exercise (86% VO₂peak) to volitional fatigue. Subjects consumed 250 ml fluid immediately before (−2 min) and every 15 min of cycling. Drinks at −2 and 45 min contained 100 mg of [U-¹³C]-lactate, -glucose or -fructose. Blood, pulmonary gas samples and ¹³CO₂ excretion were taken prior to fluid ingestion and at 5,10,15,30,45,60,75, and 90 min of CE, at the end of HI, and 15 min of recovery. HI after CE was 25% longer with C than G (6.5±0.8 vs. 5.2±1.0 min, P<0.05). ¹³CO₂ from the −2 min lactate tracer was significantly elevated above rest at 5 min of exercise, and peaked at 15 min. ¹³CO₂ from the −2 min glucose tracer peaked at 45 min for C and G. ¹³CO₂ increased rapidly from the 45 min lactate dose, and by 60 min of exercise was 33% greater than glucose in C or G, and 36% greater than fructose in G. ¹³CO₂ production following tracer fructose ingestion was greater than glucose in the first 45 minutes in C and G. Cumulative recoveries of tracer during exercise were: 92%±5.3% for lactate in C and 25±4.0% for glucose in C or G. Recoveries for fructose in C and G were 75±5.9% and 26±6.6%, respectively. Lactate was used more rapidly and to a greater extent than fructose or glucose. CytoMax significantly enhanced HI.

Nutrient metabolism by gut tissues

The gut uses a disproportionate amount of energy (about 25% of total oxygen consumption) for the size of the tissue (about 6% of body weight). Therefore, knowledge of metabolites used in gut tissues is important to assess nutrient needs. The VFA produced intraruminally are used to varying degrees by rumen epithelial tissue. A review of net portal absorption data essentially confirms earlier studies that show acetate, propionate, and butyrate metabolism by stomach tissue to be 30, 50, and 90%, respectively, of ruminant production. Glucose also is used by rumen epithelial tissue; however, glutamine appears to be a poor substrate for energy production. Tissue of the small intestine uses glutamine, glucose, and ketones but does not use VFA very well for energy. Cecal and colonic tissues can use glucose and glutamine for energy, but butyrate is the preferred substrate in both tissues.