Dehydration and Fluid Intake
The athlete body is constantly processing and controlling body temperature and for the release of excess heat. When the ability to maintain the body's core temperature is challenged (hot or cold) performance imbalances occur. In some instances, these changes can markedly slow the athlete and even become life-threatening at either end of the spectrum in instances such as, hypothermia (low body core temperature) and/or hyperthermia (elevated body core temperature).
A common misconception is that training improves the ability to work in a dehydrated state. Even though there is fitness, cellular and heat and cold tolerance adaptations the body cannot adapt to dehydration no matter the individual. Life and sport performance and especially, the endurance athlete must maintain adequate levels.
One of the advantages humans have is the ability to remove heat from the body through evaporation. Through this process endurance athletes are capable of extraordinarily long durations of exercise. However, because of this capacity to sweat and evaporation and heat removal the athlete must also replace fluids at regular intervals and with the appropriate amounts.
As an athlete exercises sweat secretion is needed to help lessen increases in body temperature. Similarly, in cooler climates while sweat secretion is less there are still fluid requirements to maintain homeostasis (the body's equilibrium) and optimal performance - It just is not the same amounts.
At the same time, sweat rates vary individually as a result of fitness, duration, intensity and ambient conditions. So, it can be complex in terms in how much to ingest, when to do so and the type of fluids chosen. However, there are well-established ranges and for most athletes these work very well. The important point to NOT under or over ingest fluids above or below these ranges and to learn in training what is the right amount for the given conditions and your fitness.
The Limits of Gastric Emptying
There is a limit to how much fluid can be emptied from the stomach and re-absorbed. Generally, the guidelines are based upon every hour of exercise and the duration, intensity and so on. Even in the most extreme temperature conditions the athlete cannot match fluid loss. It's not as simple as weighing oneself to determine sweat rates (and always applying this same formula) because losses change with conditions, effort, fitness and durations of exercise.
There's a lot more going on as it's not only fluid lost, but body mass changes too affecting total weight loss. Again, there are ranges of intake to help make your decision less of a challenge.
Dehydration stresses the athlete through elevations in body temperature, heart rate (lowering the hearts stroke volume - lessening blood flow to working muscles), increases health risk and most certainly, elevates the energy cost for exercise. Developing a consistent and individual plan for rehydration on a daily basis should be a key point for the athlete. Still, some dehydration with endurance sports cannot be avoided, but replacing all fluid losses isn't possible either.
While guidelines exist for replacing fluids these ranges are affected by individual differences. It is not sufficient to merely replace the fluids based upon sweat rate tests as recommended by some sources. There are limits to the amount of fluids that can be absorbed and emptied, but there body mass changes from other than fluids that affect loss of weight. So, it isn't as simple as working out and weighing oneself to determine sweat rate and fluid losses.
Body mass changes are coupled to the stored carbohydrates, fats and protein oxidation and use during exercise. Those changes in weight aren't completely connected only to fluids ingested or sweat. Therefore, a loss in body mass (weight) is expected (about 2-3%) and not all due to dehydration, but from the use of energy from stored carbohydrate, fats and proteins.
It's no surprise to most that dehydration affects an individual's capacity to exercise, but every individual processes nutrients differently and training is the time and place to determine needs (within recommended ranges of intake). There is a definite decrease in the capacity to perform when body fluids are out of balance by even by the smallest of amounts. However, there are individual differences and many elite athletes actually finish and win events with body mass losses of 2-3%.
What is occurring is that their body weight (mass) is lower, but body water actually may increase due to releases of water from stores in muscle glycogen. Therefore, fitter athletes may have an advantage in fluid replacement strategies. The key is establishing individual differences and by replacing fluids within a range of guidelines and practicing in training over a variety of durations and environmental conditions. It is NOT a "one" set rule of thumb, but a rather, a smaller array of fluid ingestion that is neither, too low or too high in quantity and/or sugars.
To maintain and enhance training, performance and promote recovery the athlete should consider a plan of fluid intake before, during and after exercise. Most athletes cannot keep up with the sweat loss during exercise it still important to take in enough fluids, but not more than can be emptied from the stomach.
The general recommendation is no less than 400mL (14 ounces) and no more than 800mL or 28 ounces for each hour. Elite athletes will be on the lower end and less experienced on the higher end.
FLUIDS + NUTRITION NEEDS FOR EVENTS AND TRAINING
For all athletes, fluid replenishment should be a primary task before, during exercise, following exercise and especially, within the context of the density (how close are the workouts), duration (length of time) and intensity of workouts in addition to environmental conditions.
Fluid management and maintaining a balance between sweat losses and reestablishing fluid balance can make for much improved performance and of course, the quality of workouts and for enhancing recovery.
Marc Evans was the first USA Triathlon head coach for the inaugural Olympic-Distance World Championships, and coach of two-time Ironman champion Scott Tinley. He has written three books on endurance training and is the patent holder for the bestselling SPEEDO Contour and Swim-Foil training paddles. Marc was presented the "Award of Excellence" from the American Medical Association for his pioneering work in triathlon. He conducts 1-on-1 sports performance coaching from his studios near Lake Tahoe and is available for clinics, workouts for individuals and groups around the world.