It seems I hit a nerve with my last post on “Why No Gels”. There is a nuance to be highlighted with regards to performance nutrition: the separation of fatigue due to a drop in blood volume and fatigue due to a lack of carbohydrate. This Part 2 will focus on the basic physiology to give insight on Part 1 (as well as some solutions!)
Open any exercise physiology textbook and the first factor of fatigue is a drop in blood volume, with the second factor of fatigue decreased carbohydrate availability. Why? Simplistically, you can rectify low circulating carbohydrate pretty effectively by eating something and feeling the effects within minutes. A drop in blood volume is more complex, and it takes hours to rectify (as it involves the kidney regulatory hormones aldosterone and arginine vasopressin).
Blood volume is the red cells and plasma in circulation. When discussing exercise and fluid shifts, the term “plasma volume” is often used, as this refers to the watery component of blood. With the onset of exercise, there is a shift of blood flow to supply the working muscles and to the skin, to divert the increased heat produced (from muscular contraction). With the increased demand for blood to the muscles and skin, there is hypoperfusion of other organs, including hypoperfusion of the GI system.
As exercise continues and plasma volume is lost through sweating, breathing, and gastrointestinal water usage, available circulating blood diminishes (there is less overall water in the blood, thus it is “thicker”). An endurance athlete will feel the drop in blood volume as “muscle fatigue”. As the viscosity of blood increases, the competition between the skin and muscles becomes fiercer; with the skin winning over muscle perfusion. Heat is a large threat to the body, the actual window of survival core temperature perturbation is quite narrow: 37-39.5°C (98.6-103.1ºF – Athletes often push themselves to a core temperature of 40°C before true adverse effects of heat stress are seen.). As heat storage of the body increases and more blood is shunted to the skin, there is less blood for muscle metabolism- nature’s purpose: shut down the threat. By this, I mean, less blood for muscle metabolism means fewer contractions, thus less heat produced (the temperate of the muscle itself contributes to fatigue as well by denaturing the contractile proteins). This is muscle fatigue – a drop in power. In a traditional mentality, this is when an athlete will typically reach for a gel or other quick hit of carbohydrate, thinking they are low in carbohydrate; but it is water deficit of the blood that forces the fatigue.
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