Water makes up about , and without enough of it, can set in, causing dry mouth, tiredness, and headaches (no wild partying required). Another reason to drink up: Getting the right amount fluids daily could be the difference between feeling drained and running like a well-oiled (read: hydrated) machine.

H 2 the Izz-0—Why It Matters

To stay hydrated, it’s essential to replace the water our by breathing, perspiring, and— ahem— using the bathroom. to flush toxins, carry nutrients around the body, and protect sensitive tissue, like the nose and mouth Buyckx, M.E. Director, Nutrition and Health Sciences, The Beverage Institute for Health and Wellness, The Coca-Cola Company, P.O. Box 1734, Atlanta, GA. Journal of the American College of Nutrition, 2007 Oct;26(5 Suppl):533S-4S.. For adult males, that means about 3.7 liters of fluids per day (nearly 16 cups), while ladies need roughly 2.7 liters (or 11 cups) per day, . But not all fluids need to come from water or even liquids. Everything from and even coffee, tea, and soda can contribute to our daily fluid intake, though water is still the best bet for avoiding excess calories. On average, of our daily fluid intake, but it can account for more if we reach for water-rich foods like fruits and veggies (think lettuce, oranges, tomatoes, etc.). So taking into account that 20 percent from food, in a temperate climate, men need to drink 3 liters (about 13 cups) of fluids daily, and women need 2.2 liters (about 9 cups) Meinders, A.J., Meinders, A.E. St. Antonius Ziekenhuis, afd. Interne Geneeskunde, Nieuwegein, The Netherlands. Nederlands Tijdschrift voor Geneeskunde, 2010;154:A1757..

Just Add Water—The Answer/Debate

But not everyone’s hydration needs are the same. , climate, activity level, and illness —and how much we’re each affected by those factors varies too. Some sedentary humans in climate-controlled environments need not worry about . Studies show thirst may be a better guideline than numbers, since just by letting our conscience—err, mouths—be our guide. And there’s no need to overdo it. Drinking more water than we need doesn’t necessarily have any added benefits (unless frequent runs to the bathroom could be considered a benefit) Meinders, A.J., Meinders, A.E. St. Antonius Ziekenhuis, afd. Interne Geneeskunde, Nieuwegein, The Netherlands. Nederlands Tijdschrift voor Geneeskunde, 2010;154:A1757.. But don’t be fooled into thinking only hot temps require . When the weather outside is frightful, all those layers can cause sweating, and dry, heated indoor air can sap skin of moisture. No matter what the weather, , drink one to two cups pre-workout and an additional half to one cup every 15 to 20 minutes. Post-gym, continue to replace any lost fluids and electrolytes (that means 16-20 ounces per pound of body weight lost during the workout). When in doubt, urine color (yeah, we went there) can be a good indicator of whether the body’s getting enough H20 Mentes, J.C., Wakefield, B., Culp, K. University of California, Los Angeles School of Nursing, CA. Biological Research for Nursing, 2006 Jan;7(3):197-203.. When well hydrated, urine should be —though we don’t recommend calling in a pal for a second opinion (that’s what doctors are for, right?).

The Takeaway

On average men need 3.7 liters of fluids per day, and women need 2.7 liters. All fluids count, including milk, juice, coffee, tea, and even water-rich foods. Thirst and urine color might be better guidelines for staying hydrated. Weather, activity level, illness, and age can all impact water needs.

READ THIS NEXT: Things That Happen When You Actually Drink Enough Water