Exercising in a dehydrated state can hurt performance. That really is nothing new. However, what is not known is how dehydration can effect substrates being used by the muscle during exercise, particularly in women. So, this one is for you ladies.
Introduction: Did you know that a 2% loss in body mass because of dehydration can elevate HR, core temperature, and the osmolarity of blood plasma? Did you also know that it is said that women thermoregulate less effectively because of a higher core temperature during the same exercise load as men? In fact, females usually experience a quicker rise in core temperature during exercise. Depending on core temperature during exercise, the body can switch between using muscle glycogen or fat. Therefore, you could say that depending on hydration status (which effects core temperature) the body will switch between these two fuel sources as well. But which one? The hypothesis in this study states that women will rely more heavily on whole body carbohydrate oxidation as well as the breakdown of glycogen from muscle during dehydrated exercise.
Methods: Nine women underwent cycling at 65% VO2peak for 120 min. Some received fluids during the exercise and the others did not. It is important to note that before the exercise trial, both groups were properly hydrated. Thus, this study is just examining the consequences of not drinking water during prolonged endurance exercise.
Results: One way to measure whether or not you are using carbohydrates or fat during exercise is by a method called indirect calorimetry, which can provide you with a RER. RER, as I’ve described before, is the respiratory exchange ratio. A RER of 1 means you are using primarily carbohydrates and a RER closer to 0.75 means you are using primarily fat. With that said, the RER of the dehydrated group was significantly higher than the hydrated group, meaning that they were using more carbohydrates during exercise. Likewise, carbohydrate oxidation and total body carbohydrate oxidation was higher in the dehydrated group whereas fat oxidation was lower. The dehydrated group had a significantly higher core temperature and heart rate as well.
Discussion: At this point you would probably like to know why being dehydrated makes the body rely more on carbohydrates rather than fat. There currently is no answer to that; however, the authors suggest three theories behind it.
- An augmented nervous system response from the adrenal glands leading to activation of an enzyme that uses glycogen.
- Low energy levels that are sensed in cells
- Higher intramuscular temperature (which appears to be the primary mechanism)
My input: It is now being understood a little more why dehydration causes performance deficits. Clearly, if you are going to tap into your muscle glycogen faster, you will not be able to perform as long as if you were using primarily fat, which is very energy rich. Still, more needs to be done to understand the exact mechanism for the switch.
Logan-Sprenger et al Med Sci Sports Exerc. 2012 Oct;44(10):1949-57.