This week NYC temperatures shot into the 80s. A handful of friends originally from CA commented that it finally felt like spring -- spring?? Pshaa! Spring was a month ago when the high was 60. I acclimate to cold very well, but to hot... yea, not so much. I don't sleep well (or very much at all), I sweat all the time, and I end up with light sensitive headaches because maintaining actual hydration becomes difficult while I continuously melt onto the floor. I ended up geeking out yesterday over hydration analysis.
An interesting take on hydration that seems physiologically grounded rather than assumption grounded appeared lately on Marathon Talk, a UK based weekly podcast. They've had a 3 part series with guest Dr. Mark Hetherington (BSc, PhD), a visiting senior research fellow at the Institute of Membrane and Systems Biology at the University of Leeds. The second part was of most interest regarding how fluid that is sweat out by the body correlates with appropriate fluid intake. That is, Hetherington describes how the common assumption that any loss of body weight after prolonged exercise means that exact amount of fluid must be replenished. I.e., if you end a run having lost 2 kg (or 4.4 pounds), then the assumption is that you lost 2 kg of blood volume and thus should drink back that same 2 kg (or 2 liters, since 1 kg = 1 liter) of water to regain proper hydration. According to Hetherington this is wrong on two counts, and this is why so many distance runners are in danger of hyponatremia, a state of abnormally low levels of salts/electrolytes in the blood.
The first point deals with the origination of sweat, which comes from two sources. One source is from the blood, like most would assume, which secretes the fluid through the skin in order to cool you down during/after such physical exertion. But apparently this only accounts for 50% of the sweat created by physical activity. The second source is is a byproduct of cellular metabolism of glycogen/glucose and fats. That means the sweating out of this byproduct water has no affect on blood volume -- your body creates the water as you burn fuel during exercise (same as it produces CO2), the water goes into the blood stream to then join in the fun as sweat (just as CO2 is transported by blood to then be exhaled through the lungs). Herherington breaks it down into three sub-categories: of the total fluid sweated out (including true blood volume fluid) 25% is produced by releasing glycogen (i.e. concentrated glucose stores) from storage in the muscles, 16.7% is produced by burning of glycogen (breaking it down into glucose) and fat, and 8.3% is from oxidizing glucose -- thus contributing to 50% of the fluid lost via sweat.
What does this mean? If this hypothetical runner lost 2 kg during the run, then only 1 kg was from blood volume, necessitating that only 1 kg of fluid be taken in. Otherwise you'd be taking in twice the fluid you need, and if this is straight water then you are at risk of essentially diluting yourself to death. This is Hetherington's second point -- that the composition of sweat is not pure water, it is an isotonic fluid. So if you drink plain water you'd have to consume food with the requisite electrolytes. Otherwise your blood will be diluted while cells will retain their normal isotonic properties, and this will cause osmotic flow and retention of water with cells. When this happens to your brain, this is when things get really bad really fast.
I also found an article described by exercise phys site called Sweat Science that publishes the results of current literature -- "Effect of exercise-induced dehydration on time-trial exercise performance: a meta-analysis" by Eric Goulet, published in April of this year in the British Journal of Sports Medicine. Comparison of 5 studies that met analysis criteria yielded that those who drank according to their thirst lost up to 4.0% of normal body mass (average 2.2%) , and yet these subjects performed better in a self-paced time trial than those who drank nothing or drank to maintain a near-perfect body mass (losing only an average of 0.44% of normal body mass). The premise is the latter category was over-hydrated despite maintaining "normal" body mass and thus hindered their performance.
So that means I don't need to freak out as much when comparing my fluid intake with others, or with temporarily dropping a few pounds after a really long run. I just wonder how I can apply this to non-running life in hopes of surviving the real summer once it gets here.
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