Why Dehydration is Important on Marathon Runners

on January 23, 2023

Water accounts for 50 to 60% of the body mass in most people, and it resides between lean tissue like muscles, the heart, or the liver, which has a higher water percentage than fatty tissue.

A lean, healthy man weighing 70kg has roughly 42 liters of water, or 60% of his body weight. However, a lean woman weighing 70kg has 35 liters or 50% of her body weight. Women tend to be lighter than men regarding muscles but have a higher proportion of fat.

Most athletes are conscious of dehydration, and such can produce a deterioration in performance. But even then, most do not follow a proper strategy to prevent it during training and competition. If you feel related to this, keep reading this article to enhance your performance in your next marathon and take it to its maximum potential.

Our organism can thermoregulate itself, which means that if it detects an increase in temperature in the body during exercise or not, it utilizes a mechanism to regulate the body´s temperature and get it back to normal. One of these mechanisms is sweat, the primary method of releasing heat from the body.

For each liter of oxygen consumed during the exercise, we produce 4 kcal of heat and only use 1 kcal to perform the mechanical work. In athletes, the oxygen consumption is 4 liter per minute during exercise or 918Kcal/hr.

Just a tiny portion of the heat produced during exercise is released through the skin; the rest passes directly to the core via the connective flow that circulates back to the heart.

During an intensive workout equivalent to 80-90% of VO2 max, the heat production of a person can exceed 1000W, while during rest, this is about 70W. It means an increment of 1º C every 4 to 5 minutes if there is no change in the dissipation of body heat. A 70kg man with 15% body fat has a capacity of specific body heat of 3.21 kg/Kg. Therefore, 1000W per minute produces 14,3Kcal (1000Kjx60s=60.000j) of calorific energy, raising body heat to 0,27ºC. per minute (70Kg x 3,21Kj/Kg / ºC). This means that the core temperature could reach dangerous levels at 12-15 minutes of exercising, forcing the runner to stop due to fatigue produced by such a degree of hyponatremia (Low sodium levels).

It is important to highlight (the loss of sodium through sweat) and injuries due to heat not only occur when performing exercises at high body temperatures. The production of body heat is directly related to the intensity of the such workout. Hence, if such is an extenuating activity, the same fatigue symptoms will arise without considering if the runner finds himself at low temperatures.

The dehydration level affects the velocity of gastric emptying from the fluids ingested during the exercise. A study performed by Neufer, Young, y Sawka in 1989 demonstrated that a slowdown of 20 to 25% of gastric emptying occurred when the athletes reached a level below 5% of their body weight due to dehydration.

Consumption of liquids should begin in the early stages of warm-up, not only to minimize the dehydration levels but also to optimize the bio-ability of ingested fluids. 

Dehydration sets a hazardous toll on our health, increasing the risk of heat fatigue and heat strokes that cause cramps and endanger our life.

For such mentioned reasons, I recommend the following:

- It is essential to know the amount of liquid you lose during exercising, and for this, you should use the following formula:

- Weigh yourself before and after the exercise, ideally without clothing

- Calculate all ingested liquids during the exercise in LITERS

- Weigh before exercise (in Kg) + liquid ingestion (Lt) - weigh after exercise (in Kg)

- Example: a 70Kg man before exercise that consume 0.5Lt of liquids and weigh 69Kg after exercise lost 1.5Lt of liquids during its training (70Kg + 0.5Lt – 69Kg).

  1. In relationship with the previous calculation, try to replenish the lost weight during the exercise to try and end it with, at most, a loss of 1%.
  2. It is vital to control your urine color during the day in such a way as to have the same precise color before beginning the training.
  3. Make sure to cover 50% of your losses in the first hour of your training, consuming 2 cups of water at least 15 minutes before going to run and then every 20 to 30 minutes of exercising. 
  4. Do not feel thirsty to drink water and when you drink, try not to do it in large quantities all at once as this could cause gastric discomfort.

Besides these tips, it is advisable to maintain yourself adequately hydrated during the day before your training or race and to carry a small towel to dry out the sweat to prevent your body from rising in temperature more than it should.

Lastly, if you would prefer to be 100% sure of all you should be doing, go to a professional sports nutritionist who can guide you in your liquid requirements for your day-to-day activities, training, and competition.