Sunday, October 23, 2005

Devany on Water

Follow Up to "Water Bottles,..."
April 19, 2005 09:39 AM
To follow up on my post on the overzealous promotion and consumption of water.
Your thirst over the course of a day, not within a brief span of hard work, is an excellent guide to your water requirements. Research in the Journal of Physiology shows that you don't need the 8 glasses of water "authorities" recommend (a lot of this advice comes from bottled water and sports drink manufacturers).
You need water when you are thirsty. Just realize that your thirst is a bit slow to come on line when you are doing hard work. You can anticipate this effect by sipping before you become thirsty if you are on a long hike or working in hot weather. Even then, there is little risk your body will fail to become thirsty in time to keep your water balance. How could it be otherwise when our ancestors survived the African Savanna for millions of years?
A bit of evolutionary reasoning tells us: 1. that thirst is an adaptive mechanism evolved over thousands of generations of humans; it can't be all that wrong. 2. the urgency of thirst is compelling, another evolved adaptation that keeps us from ignoring this important signal. 3. humans are the only carnivore that can operate at high daytime temperatures; wild cats can't because they lack the thermoregulatory system and sweating systems of humans. This adaptation gave humans a niche in which to operate where there was less competition from other, lethal (to us) carnivores. 4. watering places were dangerous; prey (including other humans) attracted carnivores, so it would have been maladaptive for humans to have required water on a continuous basis, as some trainers practice and recommend for their clients. To do so would have entailed continuous exposure to carnivores. 5. hunter gatherers obain a good deal of their water from their consumption of plant food as well as the blood of prey. Similarly, we modern humans get a lot of it in our food. I hope this makes clear the need to think beyond proximate or mechanism-based explanations (you get thirsty to manage your water balance) to evolutionary explanations that give a deeper understanding of why the proximate mechanism evolved and how it works in a broader scheme.
Unlike your hunger mechanisms, which go awry at the low energy flux sedentary individuals live at, your thirst mechanism is pretty much spot on. There are some individuals whose sense of thirst is over-developed even to the point of craving water well beyond physiological requirements. This "water intoxication" may stem from constant water binging.
You need water when you feel thirsty and not otherwise. Excess water bloats you and is hard on the kidneys. Water does help to cleanse the kidneys, but too much puts a burden on them. When you carry extra water you carry extra weight (which slows you down and increases your work rate), you dilute the minerals in your blood which changes the chemical gradient across cell membranes (intefering with metabolism and thermal control), and your blood volume can rise to a level that stresses your heart. Exess blood volume builds up in the circulatory system and leaks through the vessels into the interstices of the body (the same thing that happens with congestive heart failure).
I could go on, but a more general point is lurking here. A lot of fitness advice is just repeated rather than thought through and it is probably true that most manufacturers go beyond the pale in marketing their products.

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