Drinking Patterns and Rates of Carbohydrate and Fluid Delivery
Exercises 4.4c suggests that increasing the carbohydrate content of the ingested solution beyond about 6% begins to have a rather marked effect on the rate of gastric emptying.
Costill and Saltins (1974) studies were performed in subjects who ingested the solutions only once, and the rate of gastric emptying was averaged over the sampling period, which was usually about 20 minutes. However, more recent studies show that this is an inaccurate method for determining the rate of gastric emptying. Furthermore, during competition athletes ingest fluid repeatedly, and the effect of repeated bouts of fluid ingestion needs to be considered.
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These more recent studies have found that if the solutions are ingested repeatedly during exercise, so that the stomach is kept in a more distended state during exercise, then higher rates of gastric emptying can be achieved, as predicted by Costill and Saltin (1974; Exercises 4.4d). Thus, differences in the rates of gastric emptying between water and solutions with quite high carbohydrate and electrolyte content are minimized when both solutions are ingested repeatedly during prolonged exercise (Davis et al, 1987; J.B. Mitchell et al, 1988, 1989, 1991; Owen et al, 1986; Ryan et al, 1989).
Indeed, for practical purposes, the rates of gastric emptying for water and for carbohydrate solutions at concentrations up to 10% are essentially the same at rest and even during exercise at up to 70% V02max (J.B. Mitchell et al, 1989, 1991; Rehrer et al, 1989a; Sole & Noakes, 1989). However, higher carbohydrate concentrations (>15%) empty significantly slower than does water both at rest (Rehrer et al, 1989a; Sole & Noakes, 1989) and during exercise (Davidson et al, 1988; Davis et al, 1987; Moodley et al, 1991; Rehrer et al, 1989a). However, the rate of carbohydrate delivery to the intestine from these more concentrated drinks increases with increasing carbohydrate content even up to 18% carbohydrate solutions (Davidson et al, 1988; J.B. Mitchell et al, 1989, 1991; Moodley et al, 1991; Rehrer et al, 1989a; Sole & Noakes, 1989). Higher concentrations have not been studied, but there is no reason to believe that the rate of carbohydrate delivery would not continue to rise with further increases in the carbohydrate concentration of the ingested solution.
The reason that frequent drinking effects the rate of gastric emptying can be inferred from the detailed studies of Rehrer et al. (1989a). The critical finding in these studies was that the rate of gastric emptying for any solution is a logarithmic function of the amount of fluid present in the stomach at that time; in essence this means that during any equivalent time period, a constant percentage of the drink that was present in the stomach at the start of that period would have been emptied. They found that approximately 65% of a water solution, 50% of an isotonic 7% carbohydrate solution, and 25% of a 15% glucose or 18% glucose polymer solution would be emptied during successive 10-minute time periods when exercising at approximately 70% V02max.