Q: What are the common symptoms of dehydration?
A: In most cases runners experience only mild dehydration, 1-4%, and have no symptoms. Early signs can include dry mouth, thirst, and lack of saliva. In general, tachycardia (fast pulse), low blood pressure, increased aggression, and loss of discipline occur at high rates of dehydration.
Serious signs and health risks are likely to occur only at 15-20% dehydration, a level never seen in endurance runners.
Dehydration can however lead to performance deficits, even when mild. Thus, it is prudent to remain well hydrated in all cases.
Q: Following a long marathon, I crossed the finish line and after stopping running I fainted. The doctor at the finish line thought I was dehydrated and had heat stroke, and started an intravenous. I had drank very well throughout the race. How much do I need to drink to prevent dehydration and collapse?
A: In general, although the popular conception is that dehydration leads to collapse at the finish line, recently this thinking has been revised.
In general, although most endurance runners will sweat more than they drink, dehydration is usually mild, around 1-4%, and rarely has any serious health effects. Furthermore, dehydration itself does not likely lead to heat stroke.
In fact, the collapse that occurs in runners at the finish line, is likely due to low blood pressure while standing (postural hypotension). While running, particularly in hot weather, the veins of the legs expand. During the running process, the contraction of the muscles of the legs squeezes these veins, and acts as a pump to return the blood to the heart. When the runner stops, the blood tends to accumulate in these expanded veins - the amount of blood returning to the heart decreases - the blood pressure falls - the athlete faints.
In general, as in fainting of most causes, the body treats itself. When the runner collapses, the blood in the legs is able to return to the heart, and the blood pressure comes back up. The athlete can simply stay lying down, the legs are raised above the level of the heart to facilitate drainage. Intravenous fluids are not usually necessary, since the volume of fluid in the body is normal, just in the wrong place. The veins will contract normally in time.
No study has ever shown that runners collapsing after a race are dehydrated, or that they have a high temperature.
It is important to note however, when athletes collapse during the race, especially if they lose consciousness, something else may be wrong. In this case, the athlete should see a doctor to check for heat-stroke, heart difficulties, or dehydration.
Q: How much should I drink while running? Should I drink as much as I possibly can, or can I drink too much?
A: Although the exact answer to this question is not understood with 100% certainty, it is likely possible to drink too much. It is postulated that the human intestine can only absorb about 750-1000 ml of water per hour.
If an athlete drinks more that this amount, the water may stay in the intestine. In most cases this is not a problem, however, the water may in some cases draw sodium from the body into this intestinal water. If too much sodium leaves the body tissues, a condition called hyponatremia may result. This, in some cases, can lead to severe repercussions such as seizures.
In general, this condition is unlikely to occur unless drinking continues above 1.5 liters/hour for an extended period of time.