Sports drinks cause more gastrointestinal complaints than water during distance running, without improving performance, according to a new study from The Netherlands (‘The effect of two sports drinks and water on GI complaints and performance during an 18-km run’, Int J Sports Med 2005; 26:281-285).
These unexpected results emerged from a field study comparing the effects of three different drinks – non- carbonated mineral water, a carbohydrate-electrolyte solution (CES) and a CES containing added caffeine – on GI complaints and performance in a competitive 18k run. The findings contradict the evidence of previous laboratory-based studies that sports drinks boost performance without affecting gastrointestinal function.
Ninety-eight well-trained subjects (90 men and eight women) performed the run three times within eight days under the three different conditions, each consuming 150ml of the relevant drink at the start of the race, after 4.5k, 9k and 13.5k and then ad lib after the race. The incidence and intensity of upper and lower GI complaints were determined by questionnaire.
Contrary to the researchers’ hypothesis, there were no significant differences in performance between the three conditions, with mean run times for all three varying by no more than 20 seconds, at around one hour 18 minutes.
Equally unexpected was the finding that the use of carbohydrate- containing sports drinks led to a higher incidence of each of 11 different GI complaints studied than water, with significant differences recorded for the incidence of reflux and flatulence. However, there were no significant differences in the intensity of these complaints.
‘Although we know from previous studies carried out in our laboratory that sports drinks, with or without caffeine, do not affect GI parameters but do increase performance, it appears that these data cannot be extrapolated to a field situation,’ the researchers write.
The key factor may be that whereas most laboratory studies of endurance exercise use cycle ergometers, the current study used running, which is known to pose higher risks of GI complaints than cycling.
Why then should sports drinks produce more GI problems than water? The researchers speculate it may be ‘that, if the athletes perform at their maximal capacities, running leads to a larger decrease in intestinal blood flow than cycling, leading to malabsorption of the ingested carbohydrates’.
While they do not rule out the possibility that sports drinks may confer greater benefits in events of longer duration, they conclude that mineral water induces fewer GI complaints than sports drinks without limiting performance in running events lasting less than 1.5 hours in moderate environmental conditions.