Gastric emptying, absorption, and carbohydrate oxidation during prolonged exercise

Abstract

This study was designed to examine aspects of digestive function that may limit assimilation of water and oxidation of orally ingested carbohydrate (CHO) during exercise. Eight males completed a crossover study in which each cycled on four occasions for 80 min at 70% maximal O2 consumption. Beverage was consumed at 0, 20, 40, and 60 min. Beverages were water, 4.5% glucose (4.5G), 17% glucose (17G), and 17% maltodextrin (17MD). CHO beverages contained 20 meq/l NaCl and were 13C enriched to measure exogenous CHO oxidation. Gastric (beverage) volume was measured at 80 min. Water uptake was estimated by including 2H2O in the beverage and measuring 2H accumulation in blood. Jejunal perfusion tests were conducted at rest with the same subjects and beverages. In 60 min, 1,294 +/- 31 (SE) ml were ingested; at 80 min, volumes emptied with H2O (1,257 +/- 32 ml) and 4.5G (1,223 +/- 32 ml) were greater than with 17G (781 +/- 56 ml) and 17MD (864 +/- 71 ml; P less than 0.05). Total CHO oxidized was similar with all beverages, but there was a greater increase in exogenous CHO oxidation over time with 17G and 17MD than with 4.5G; 54, 19, and 18% of the CHO ingested with 4.5G, 17G, and 17MD, respectively, was oxidized. This represents 57, 32, and 27%, respectively, of the CHO emptied from the stomach. 2H accumulation in the blood was more rapid with H2O and 4.5G than with 17G or 17MD. Net jejunal water absorption was greater from 4.5G than from water. Net water absorption was also observed from 17MD, whereas net secretion was observed with 17G.(ABSTRACT TRUNCATED AT 250 WORDS)

Fluid and fuel intake during exercise

“…It is recommended that carbohydrate be ingested at a rate of 30-60 g × h 71 during exercise, recognizing that ingesting more does not increase oxidation rate yet ingesting more can produce gastrointestinal discomfort in many people (Rehrer et al, 1992;Wagenmakers et al, 1993). Therefore, carbohydrate feeding can be counterproductive when ingested in amounts ( 460-90 g × h 71 ) or concentrations ( 47-8%) that are too large (Febbraio et al, 1996a;Galloway and Maughan, 2000).…”

Influence of Carbohydrate-Electrolyte Drinks on Muscle Metabolism and Endurance Running Performance in Man

“…Studies using a wide range of concentrations of carbohydrate solutions during prolonged exercise have shown little differences in total and exogenous CHO oxidation rates (Rehrer et al 1992;Maughan et. al.…”

“…The more concentrated solutions will empty slower but the amount of glucose delivered to the small intestine will be greater than that of less concentrated solutions (Rehrer et al 1992;Vist and Maughan, 1993).…”

“…1992), it has been suggested that its absorption rather than gastric emptying rate determines its delivery to the muscles (Hawley et al 1992). However, absorption rate of glucose has been shown to increase with increasing delivery of glucose into small intestine (Rehrer et al 1992) suggesting that intestinal absorption might not be a limiting factor in determining the rate of exogenous glucose oxidation. Indeed, it was shown that the latter was relatively the same after ingesting a 4.5% and a 17% CHO solutions despite a 3.3 times faster absorption rate of CHO from the latter compared with former solution (Rehrer et al 1992).…”

“…However, absorption rate of glucose has been shown to increase with increasing delivery of glucose into small intestine (Rehrer et al 1992) suggesting that intestinal absorption might not be a limiting factor in determining the rate of exogenous glucose oxidation. Indeed, it was shown that the latter was relatively the same after ingesting a 4.5% and a 17% CHO solutions despite a 3.3 times faster absorption rate of CHO from the latter compared with former solution (Rehrer et al 1992). Thus, although CHO solutions may have slower gastric emptying rate than plain water, their absorption in the small intestine is higher due to the stimulating effect of glucose and sodium (Murray, 1987).…”