WHY IT MAKES THE LIST Assuming the water isn't laced with sewage or polluted with agricultural runoff, it doesn't get cleaner than this. About half your body weight is water (including about two-thirds of your muscle weight), and you can't function without it. Dehydration affects your muscles' ability to contract, your ability to think clearly, and your immune system. A little extra water can help your digestive system function better (especially if you have extra protein and fiber in your diet, as virtually everyone recommends these days). Some research shows a lower risk of bladder cancer and kidney stones with increased fluid intake.

MYTHING LINKS I should add here that I'm as cynical as anyone about the idea that all of us are chronically dehydrated and that the solution is to carry bazooka-size water bottles with you everywhere. Your body has very capable thirst-detecting mechanisms that kick in when your water tank gets a little low. (And I do mean a little; they react when your body loses 1 to 2 percent of its water.) And even if you don't react to that thirst the usual way—by drinking something—your brain has special mechanisms called osmoreceptors that sense dehydration and release anti-diuretic hormone (ADH), which tells your kidneys to hang on to your remaining water for dear life.

Another mysteriously prevalent belief is that any hunger you may feel could actually be thirst disguised as hunger. That makes as much sense as a soldier in combat believing that the bullet that just hit his leg is really an arrow in disguise. How long would we humans have survived if we couldn't tell the difference between hunger and thirst?

Finally, the next time someone says you need "eight glasses of water a day," ask where, exactly, that figure originated. You take in plenty of water; a day's worth of food has several cups of it, and you supplement it constantly. The milk you put in your cereal counts. Coffee and diet soda count (yes, they trigger a diuretic response, as does alcohol, but your body still retains most of the liquid). The "eight glasses of water a day" thing is just something somebody made up at some point.

YOU PROBABLY DIDN'T KNOW . . . One of the "myths" that's regularly debunked is the one about how water helps you lose weight. But it's not really a myth. German researchers showed in a 2003 study that drinking a pint of water increases your metabolic rate by about 30 percent for an hour or so and that drinking colder water is better than room-temperature fluid, since your body has to expend calories to heat the water up in your stomach. The researchers estimated that drinking an extra two quarts of water a day would increase your metabolic rate by about 100 calories. Even better, most of those calories came from increased fat-burning. So, assuming your body wouldn't eventually adjust to that increased water intake (as it adjusts to most changes in routine), you could lose an extra pound of fat every five weeks, or thereabouts, just by flooding your system with fluids.

MAYBE IT'S JUST ME Setting aside the myth of massive dehydration in the most over-hydrated society the world has ever known, and ignoring for a moment the interesting and potentially useful metabolic effects of deliberately drinking too much fluid, I think there's a rarely discussed reason why water helps someone lose weight: If you start each day thinking, "I'm going to drink X glasses of water today," you've started the day by making conscious choices about what you're going to put in your body. I don't know if this is written in any textbooks, or quantified by any published research, but the longer I write about exercise and weight control, and the more experts in the field I meet and pump for information, the more I'm convinced that the real trick lies in planning and awareness. Plan your meals, and be aware of everything you eat and drink throughout the day.

That's the way to ensure that water helps you lose weight.

BEST OF THE BEST Tap water will do, as long as you can trust the source. A green glow is usually a bad sign.

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