Recently, I went to a presentation by Dr. Stacy Sims, a post-doctoral research fellow and exercise physiologist at Stanford University. Dr. Sims was part of an exciting project with Dr. Allen Lim, chief physiologist for Garmin-Slipstream. Their goal was to help the team optimally prepare for the Tour De France, and to create effective thermoregulation and hydration fueling strategies for the race.
Their project focused on a few critical components, namely a pre-race preparation/acclimation phase, the daily nutrition and recovery of the athletes, and the pre-, during and post-event hydration needs. Through the course of the presentation Dr Sims touched on some rather interesting approaches and outcomes.
Hydration and Thermoregulation
First some background on water, hydration and thermoregulation. The human body is 55 to 65 percent water. Water is an essential aid in biochemical and metabolic reactions, it cools the body and helps maintain the acid base balance.
Hydration is the equilibration of total body water (TBW) carried in the intracellular (66 percent of TBW) and extracellular (33 percent of TBW) spaces. Dehydration, medically speaking, is when the body contains insufficient water volume to maintain normal body function.
One of the foundational responsibilities of water is thermoregulation. Thermoregulation is "the ability of an organism to keep its body temperature within certain boundaries, even when the surrounding temperature is very different."
For you and I, that means maintaining homeostasis between 37--40C (92-100F). Thermoregulation is vital to the maintenance of exercise intensity. Find yourself much on either side of that range and you are in for some trouble.
Dehydration's role in thermoregulation and performance is broad and complex. It has been shown that a state of dehydration of as little as 1 percent leads to decreased aerobic endurance. At 3 percent there is a decrease in muscular endurance, while at 4 percent there is decreased muscle strength, fine motor skill and heat cramps. In addition, the maintenance of blood volume is essential for maintaining stroke volume and plasma volume.
Different athletes have different sweat rates, but most will sweat at between 1.5 to 3.0 liters per hour. That means a 150 pound cyclist can reach 3 percent dehydration in as little as 45 to 60 minutes with no fluid intake (1 Liter = 1 Kilogram = 2.2 pounds).
Unfortunately, gastric emptying is typically in the range of 0.8 to 1.3 liters per hour, so you are on the defensive immediately. The more so if you start your race or training session hypohydrated (0.5 to 1 percent dehydrated) as most of us do by some estimates. You simply can't drink enough fluid to offset the loss from sweating.
All of this sweating leads to thirst. There are two kinds of thirst. Hypovolemic thirst is the result of sweating, respiration and/or bleeding. It is a decrease in the extracellular fluid and blood volume. Osmotic thirst is the result of a decrease in the intracellular fluid (e.g. too many solutes). Both depletions must be addressed prior to the onset of thirst.
While there are many products on the market that purport to help with your hydration and electrolyte balance, the truth is that most of them also contain a significant carbohydrate (CHO) load in order to also be seen as a viable fuel source (yet not quite enough to actually be a viable fuel source) and to be palatable.
There are a couple of downsides to this. First, the CHO actually serves to increase core temperature (gotta process that food!). Secondly, it impedes gastric emptying. Often these sports drinks contain too little sodium to effectively replace sweat salt losses, as well. Sodium loss through sweating ranges between 0.8 to 4.0g/hr.
Dr Sims came up with a comprehensive set of solutions to the problems presented above. First, the team undertook a preparation/acclimation phase that included 30-minute bouts in the sauna at 100 degrees immediately after their regular training rides. This was to both increase tolerance of warm temperatures and to systematically dehydrate the athletes to create a natural increase in red blood cell (RBC) volume; a natural ergogenic aid.