Applying accurate nutrition and hydration principles are of great benefit to an athlete's training and performance. An athlete who understands their digestion and absorption of nutrients and fluids is more likely to develop optimal methods of maintaining blood volume (a critical issue for performance), without inducing nausea and vomiting (GI distress).
For any fluid to be beneficial during exercise, it must first empty from the stomach, and then be absorbed into the bloodstream from the intestines. A number of factors influence the gastric emptying rate, including hydration status, concentration of the liquid, volume, caloric density (concentration of the fluid), temperature of the liquid, as well as external temperature and exercise intensity.
The most common causes of GI distress are thus a spin off from these influences:
Hydration status
A delayed gastric emptying response results from dehydration. Therefore, drinking when in a dehydrated state can cause gastrointestinal distress. Moreover, drinking at this point is unlikely to adequately hydrate the muscles. A common mistake made by many athletes is waiting to feel thirsty before hydrating. Furthermore, if lost fluids and electrolytes are not replenished, fatigue and heat illness can result.
Thirst is a sign that dehydration has already set in and performance is already being reduced. Thirst also tends to cause consumption of higher volumes of liquid at one time -- and with the delayed gastric emptying, GI distress is almost guaranteed. Remember that dehydration is cumulative.
To overcome this risk, don't wait until you feel thirsty; maintain fluid hydration regularly throughout training. Continuous sipping -- if appropriate -- is recommended, or else hydrate with small volumes every 20 to 30 minutes.
Fluid concentration
The speed a beverage travels from the stomach to the small intestine (the gastric emptying rate) depends on the energy content (calories) and volume of the beverage consumed. A small concentration of carbohydrate will encourage rapid absorption, but too much carbohydrate will slow gastric emptying and can result in GI distress.
Research has consistently found that beverages with a carbohydrate concentration of six to eight percent take longer to empty from the stomach than either water or lower concentrations of carbohydrate. Thus, carbohydrate/electrolyte drinks are often better than water for endurance training. Sports drinks may also aid in replenishing glycogen stores in working muscles and will balance and replenish electrolytes.
Exercise intensity
Stomach and intestinal distress tend to increase during high-intensity training. Stomach fullness is also directly related to gastrointestinal discomfort levels during intense sporting activity. Different intensities also result in different carbohydrate utilization.
For instance, in endurance running and intermittent stop-and-go sports, there's a reduction in the rate of muscle-glycogen depletion when carb drinks are consumed. However, for strenuous cycling, the rate of muscle-glycogen depletion hasn't been affected.
Keeping in mind that gastric emptying is optimal with sports drinks at six- to eight-percent carbohydrate, consuming these fluids regardless of activity may help maintain athletic performance and prevent over-volumizing with water.