You have your choice of guidelines: Drink eight glasses a day. Drink four cups an hour during exercise. Weigh yourself before and after workouts. Whip out a urine color chart when you go to the bathroom. Or even the scandalously simple: Drink when you're thirsty. How much water do you really need? And is it possible to get too much?

First of all, let's talk about what counts as water. The Institute of Medicine's guidelines make very clear that the water your body needs can come in any form: on its lonesome, as part of a Gatorade or a beer or a chocolate milk, or even in food. You may have heard that caffeinated beverages cause you to lose more water than you take in; while caffeine does have a diuretic effect, it's short-lived, and doesn't mess up your fluid balance in real terms. Coffee counts as hydration. So already we see the "eight cups" rule begin to break down: it's not eight glasses of cool clear spring water you need, but an adequate amount of h2O from any source.

Eight isn't a bad number, in general terms. The IOM describes the typical person's requirement as about nine cups for females, and thirteen for males. They calculate we usually get 20 percent of our fluids from food (think about soups, fruit, and anything that doesn't have the word "dried" or "astronaut" in its name), so this means about seven and 10 cups, respectively.

But, of course, exercise changes that. In fact, anything that changes how much you sweat is going to influence your water needs. That includes body size, genetics (basically, if you're a heavy sweater or not), how efficient you are at the movements you're doing, and—if it's hot out—how acclimated you are to the heat.

It's hard to put a number on some of those factors, so estimates are fuzzy. Most guidelines focus on how to stay hydrated in the heat, but that's less important if you're inside with the AC, or braving the cold for some sweat-free winter fun (like when I breezed through a snowy 12-miler on just a half cup of water). Given the fuzziness, can we drop the guidelines and place our trust in thirst?

Thirsting for knowledge

Timothy Noakes is a sports scientist who trained as a marathoner in an age when drinking during the race was discouraged. He eventually rebelled against that advice, singing the praises of hydration in the 1990s when everybody else was, only to settle on an approach that is ferociously middle-of-the-road: Drink when you're thirsty.

He's an outspoken defender of thirst, both in his scientific publications and in the myriad interviews he did when his book Waterlogged came out a few years ago. He told Outside magazine: "We have a 300 million year developed system that tells you with exquisite accuracy how much you need to drink and when you need to drink. It’s called thirst."

Let's back up for a second and look at this theory's competition. The American College of Sports Medicine is in favor of drinking ahead of thirst, weighing yourself, and checking the color of your urine. When the Institute of Medicine updated their guidelines to say that "the vast majority of healthy people adequately meet their daily hydration needs by letting thirst be their guide," the ACSM shot back with a press release that said thirst "is not the best indicator" of fluid needs. Who knew there was a hydration controversy, and that it was so cutthroat?

The idea of weighing yourself goes like this: If you know how much you weigh at the start of your workout, and you weigh less at the end, then you must have lost water through sweating. Since a pound of water is about two cups, you can figure out how many cups of water you were missing. Lost two pounds? That's a quart you should pack next time – about two small water bottles.

The ACSM, and many textbooks, maintain that you should never lose more than 2 percent of your body weight during exercise. This is more of a concern for endurance athletes (think marathoners) than it is for lifters, but anyone who does long workouts, especially in heat, can end up losing water weight. Proponents of this "fluid replacement" protocol point to studies where athletes who dropped more than 2 percent of their body weight saw their performance take a nosedive. But Noakes doesn't buy it, arguing that the body can safely lose water weight as long as the athlete is drinking to thirst, which allows blood chemistry to stay balanced. One of his own papers documented heat-acclimated soldiers doing a 15-mile desert march in 110-degree heat. They averaged close to a 3 percent loss of body weight, but their blood chemistry and body temperature stayed stable. He concluded in an interview with iRunFar: "I do not see any evidence that weight losses in excess of two percent impair performance."

What about urine color? An oft-repeated rule is that your pee should be almost clear if you are fully hydrated, and that dark urine is a danger sign. That's true in theory, but reality is more complicated. (Isn't it always?)

If you're dehydrated and start chugging water, your pee will be clear fairly soon – before your body has really rebounded. On the other hand, after hard exercise you might have dark urine or even be unable to pee, but that can be a temporary result of the anti-diuretic hormone that helps you conserve water. In other words, you can have light urine even if you're dehydrated, and dark urine even if you're not. So even though urine color is generally associated with hydration status, don't let a post-workout color chart (or a urinalysis, which the ACSM prefers) overrule what your thirst is telling you.

What's the harm? Why not drink a ton of water? In short workouts there are no health risks to drinking a lot, but performance can be impaired by overhydration.

In multi-hour efforts, it's possible to drink so much you get hyponatremia—diluted sodium in your body fluids. In its extreme form, hyponatremia can be fatal. Who is most at risk? Small people who drink a ton of water and are exercising for more than four hours. This describes many back-of-the-pack marathoners; one study found that 13 percent of Boston Marathon finishers were showing symptoms of hyponatremia, and there's at least one report of a case that resulted from tennis and weightlifting in the heat.

In a paper examining drinking policies, Noakes and colleagues point out that the fluid-happy ACSM accepts funding from the Gatorade Sports Science Institute, which is owned by Gatorade and in turn PepsiCo. This group does a lot of great research on sports nutrition and hydration, but one suspects they are biased toward conclusions that will help their parent company sell more drinks.

The bottom line: You have to decide for yourself, but I side with Noakes and organizations like the IOM: Thirst seems to do a fine job of telling us when and how much to drink, and there is no evidence showing that mild dehydration during a gym workout is problematic. If you're doing serious endurance work, though, you may want to discuss your hydration plan, and the competing hydration guidelines, with an experienced coach.