Drinking Fluids During Marathon

Written by: Prof. Vojko Strojnik, Ph.D.

Marathon run is becoming increasingly popular. More and more people want to overcome the magical distance of 42195 meters. Comparison of the number of participants on the New York Marathon that came to the finish line shows that between 1978 and 2001 the number of participants tripled. For all U.S. marathons numbers are even more impressive. From 25 000 participants that reach the finish line in 1976, the number in 2008 rose to 425 000. The number of marathon competitors is increasing in Slovenia as well. 153 participants ran on the first Ljubljana Marathon in 1996, in 2011 already 1186. However, are we all prepare for it? The results show that by increasing the number of participants prolongs the average run time and a significant number of participants needed fundamentally more time, than the first participants, to overcome the distance. Trends are entirely understandable. The first marathon competitors were generally well trained runners, but now recreationists also attend. They usually have less knowledge about running and less control over their running activity (coach, medical examinations) as athletes in regular workout process.

During running the body strongly overheats. The muscle from the whole consumed chemical energy converts only about one-quarter into mechanical work. The rest is mostly converted into heat. That is why a lot of heat is generated with intensive and long-term movements, which is necessary to get rid of. Sweating is the main way of cooling the body down and that is why we lose a lot of fluids during running. The intensity of sweating also depends on the environment. We perspire more in warm weather. If there is a lot of moisture in the air we will also excrete more perspiration. Clothes also have an effect on the intensity of sweating. Sweat with its evaporation consumes body heat. If it can not effectively evaporate the body responds with increased sweating. However, sweating is not related only with the loss of water from the body, but can cause a lot of problems, also very dangerous ones.

Sweat derives from blood fluid. Intense sweating reduces blood volume and thickens it. Thickening of blood increases its viscosity and with it the force needed to pump blood through the veins. The heart becomes overloaded because the heart muscle has to overcome bigger resistance of blood and therefore gets less blood and less oxygen. So eventually imbalance between the performed work and available energy occurs, which disintegrates with the help of oxygen. Skeletal muscle in such cases begins to produce lactic acid that with discomfort and pain warns of upper disproportion. Even more, due to strong acidity the function of muscles stops before serious problems could develop. Cardiac muscle does not have these fuses. It works until the end without serious warning and then dies. This situation is known as a heart attack.

In order to maintain a sufficient quantity of fluids in the body, it is necessary to substitute the perspired water. It is well known how important it is to drink during movement. This especially applies for marathon. Excessive drinking during movement (over dehydration) can lead to new complications. Measurements of hydration on the finish of New York City Marathon have shown that the best runners come to the finish line slightly dehydrated. Most runners who end up in the lower part of the table come to the finish line over hydrated, which mean too much drinking during running. Their body weight is higher after the marathon not before the start, but normally they should lose 1-2% of body weight during the run.

Excessive drinking of water to replace lost fluids due to sweating during prolonged running causes lack of sodium in the body. Early signs of low sodium include bloating, shortness of breath, nausea, feeling to vomit and headache. These symptoms may also be non-specific and not related to the lack of sodium. Serious and bigger consequences can appear with the increasing shortage of sodium such as encephalopathy as a result of swelling of the brain, behaviour change (confusion, poor orientation, delirium), seizures, pulmonary edema, coma, and death.

In addition to excessive drinking of liquids before, during and after the marathon is a factor of reducing the sodium concentration and decreased urine secretion. Both contribute to increasing amount of water in the body and thereby reduce the concentration of sodium. With sweat salt also excretes, which results in reduced amount of sodium in the body. However, studies show that the loss of sodium with sweating is less important for lowering the sodium concentration than the increase of water in the body. Consumption of isotonic drinks (containing electrolytes) can not prevent lack of sodium in athletes who drink too much. The main reason is that the sodium concentration in these fluids is too low. Increased sodium intake in combination with over dehydration can again be risky. This can increases body temperature which creates new problems...

Risk factors for decreasing concentration of sodium in the body can be divided into those related to athletes and to those related to the event.

Linked to athletes:

  • Excessive drinking of liquids
  • Increased body weight during running
  • Low body weight
  • Female gender
  • Slow running
  • Taking non-steroidal anti-inflammatory drugs

Related to the event:

  • Easy access to liquids during running
  • Marathon time of more than 4 hours
  • Very high temperature of the environment
  • Very low temperature of the environment

So, how to drink during a marathon?

Lack of sodium is primarily caused by excessive drinking of liquids during running. Therefore it is necessary to avoid excessive drinking of liquids before, during and after running. Although it is not possible to give a universal advice how much to drink, monitoring body weight before and after running is a way to determine how much liquid we need. Of course you do not have to do it just before the marathon, but we can do this during the preparations for the marathon to get a sense of the necessary amount of liquid intake. In monitoring body weight it is important to be aware that even maintaining body weight during movement is associated with sodium depletion. During running we consume energy substances (glycogen and fats). Therefore, the goal should be losing weight while running, not its maintenance.