The Institute of Medicine’s 2004 report on water and electrolytes set adequate total daily water intake at 2.7 liters for adult women and 3.7 liters for men. Total means everything: plain water, coffee, tea, and the water inside your food, which supplies roughly 20 percent of the total. Their actual recommendation was that healthy people should drink to thirst.
The eight glasses a day rule is a different thing. Heinz Valtin went looking for its source in a 2002 review in the American Journal of Physiology and could not find one. It appears to be a misreading of an older recommendation that counted water from food, then stripped that part out. So the rule asks for roughly double what it was originally derived from.
The bodyweight formula, half your weight in pounds as ounces of water, has the same problem. For a 140 pound woman that is 70 ounces, about 2.1 liters of drinking water, which is not unreasonable. But the formula ignores climate, training volume and what you eat, which are the three variables that actually change fluid needs. It is a number that sounds precise and is not.
water is 60 percent of you, which is not an argument
Total body water is roughly 50 to 60 percent of body mass in adults, lower in people with more fat tissue since fat holds less water than muscle. That is a real number and it gets cited constantly as a reason to drink more, which does not follow. Your body is also about 15 percent protein, and nobody argues from that to a protein quota.
What matters for performance is the size of a deficit. Dehydration of around 2 percent of body mass begins to measurably impair endurance performance, particularly in heat. For a 140 pound woman that is roughly 2.8 pounds of fluid lost, which is a hard, hot, long session. Most one hour indoor workouts do not get close.
Which means if you feel sluggish 20 minutes into a normal gym session, dehydration is probably not the reason.
the more likely explanations
Four things account for most mid workout energy crashes, and only one of them is fluid.
Sleep debt is first. It degrades perceived exertion before it degrades actual power output, so the same weight feels heavier and you quit earlier while your maximal strength is nearly unchanged. Nothing you drink fixes this.
Under eating carbohydrate is second, especially for people doing higher volume work. Muscle glycogen is the primary fuel above moderate intensity, and a long gap since your last real meal shows up as a flat, heavy feeling in the third or fourth set.
Third is simply accumulated training fatigue, which usually means the program has no deload and you are three weeks into pushing every session.
Fourth, and only then, hydration, which matters most in hot rooms, long sessions and anyone who sweats heavily.
coconut water versus a sports drink
Coconut water is genuinely high in potassium, about 470 mg per cup, and low in sodium, about 30 mg per cup. A typical carbohydrate electrolyte sports drink inverts that, with roughly 110 mg of sodium and 30 mg of potassium per cup.
Sodium is the electrolyte you actually lose in meaningful quantity through sweat. Sweat sodium concentration varies widely between people but typically runs in the range of 20 to 80 millimoles per liter, while potassium losses are far smaller. So for replacing what sweat takes, coconut water is the worse formulation, despite being the one marketed as natural rehydration.
A 2012 study in the Journal of the International Society of Sports Nutrition by Kalman and colleagues compared coconut water, sodium enriched coconut water, a sports drink and bottled water in trained men after dehydrating exercise. It found no significant differences between the beverages in rehydration or subsequent treadmill performance, though the coconut water conditions produced more reported bloating and stomach upset.
The practical read: coconut water is a pleasant drink with a decent potassium content. It is not a superior recovery beverage, and for sessions under an hour neither it nor a sports drink beats water plus food.
flavoring water, which does work
The reason to put fruit in a pitcher is behavioral, not nutritional. Infused water contributes negligible vitamin content. If you slice a lemon into a liter of water, you get a small fraction of the vitamin C in that lemon, because most of it stays in the pulp you discard.
But people drink more of a beverage they like. Cucumber and mint, strawberry and basil, citrus and ginger, sliced peach and raspberry. Let it sit in the refrigerator for a few hours rather than overnight for citrus specifically, since the peel oils turn bitter after about 12 hours. Cucumber and melon soften and get slimy after a day. Pull the solids out at 24 hours.
Cold water empties from the stomach slightly faster than warm, and most people voluntarily drink more of it, which is a bigger practical effect than the emptying rate.
the simplest test
Urine color in the morning is a crude but usable indicator. Pale straw generally suggests adequate hydration, dark amber generally suggests a deficit, though B vitamins from a multivitamin will turn it bright yellow regardless and some medications alter color too.
For training specifically, weigh yourself before and after a hard session. Each pound lost is roughly 16 ounces of fluid. If you are consistently dropping more than 2 percent of your bodyweight, drink more during the session rather than trying to catch up afterward. If you are losing half a pound, your hydration is not the thing holding your workouts back.