As kids we learn about the consistency of blood from the inevitable cuts and bruises we suffer while growing up. Not as thin as water, not as thick as oil, blood's viscosity is a reflection of its composition: serum (the liquid part), diluting the red cells, white cells, and platelets (the solid part). Blood needs to be thin enough to go through the microscopic capillary vessels of the lungs, kidneys and the rest of the body. Any substantial loss of fluids during exercises makes the blood thicker and its flow more difficult, with consequent repercussions to the body unless continuous replenishment of the lost liquid normalizes the viscosity. The heart initially compensates for the decreased fluid and the thicker blood by working harder and pumping faster; eventually dehydration causes a collapse of the circulatory system, dropping the blood pressure enough to cause fainting, or worse.
Not only do one's heart, lungs, and muscles become less efficient when there is significant fluid loss, but the kidneys can't filter properly either, a grim prospect considering that strenuous exercises can release substantial quantities of myoglobin, a common muscle protein, into the bloodstream. Myoglobin needs plenty of fluid to allow its filtration and elimination by the kidneys, at the risk of clogging the microscopic filters responsible for the production of urine. Marathon runners and professional athletes are familiar with the occasional passing of red tinted urine, colored by myoglobin, following an exhausting competition.
As a rule of thumb, try to drink a few ounces of liquid before you start your workout; don't wait till you become thirsty to start replenishing fluids since it is common for a "pumped-up" exerciser to disregard the body's initial dehydration warnings. This becomes even more vital if you're doing a strenuous workout in a hot environment, when the body may lose water faster than it can absorb. Remember that if you do become dehydrated, you can develop nausea and vomiting, making it difficult or impossible to regain the lost volume of fluid by ingestion alone.
Plain water is the liquid of choice to replenish the body fluids in all but the most strenuous activities. Gatorade or similar beverages may have an advantage in limited circumstances, like during some long, arduous, sun drenched workouts, because it replenishes both lost electrolytes and calories.
Despite a flood of ads that promote specially formulated light beer for "physically active people," alcohol is an obviously poor choice as an exercise drink and should be avoided for at least a couple of hours before or after exercising due to its dehydrating properties and negative effect on coordination. Dieters should note that alcohol contains seven calories per gram, almost fifty percent more than pure sugar!
The jury is still divided as to whether coffee has any negative effect on exercising. Limiting caffeine consumption to no more than 100-200 mg. is sensible and probably OK for most people, despite some increase in heart rate.
Tip: Older people may have an impaired sense of thirst and can easily become dehydrated because they "forget" to replenish the liquids they lose. The ingestion of six to eight glasses of fluid distributed throughout the day is a reasonable preventive measure; still, the most reliable way to detect dehydration in an otherwise healthy person is when there is reduced urine production, such as when Grandma has not used the bathroom "all day long."
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