Do you really need 8 glasses of water every day? Should you filter your tap water? Get answers to these questions and more, including if vitamin-spiked water is worth it.

Do I Need 8 Glasses of Water a Day?

The Institute of Medicine says adult men need about 13 cups (3 liters) per day of fluid; adult women need about 9 cups (2.2 liters) of fluid. (You get about an additional 21/2 cups of fluid from foods.)

“But one size doesn’t fit all,” says Leslie Bonci, R.D., C.S.S.D., director of sports nutrition at the Center for Sports Medicine at the University of Pittsburgh Medical Center and dietitian for the Pittsburgh Steelers. Your size and activity level affect your fluid requirements. Simply put, the larger and more active you are, the more you’ll need.

“The easiest thing that anybody could do on a daily basis is monitor their urine color,” says Douglas Casa, Ph.D., A.T.C., who studies hydration at the University of Connecticut. “Lighter urine color—like lemonade—means you’re generally well-hydrated. If it’s darker, like apple juice, you are most likely dehydrated.”

Older adults’ fluid needs don’t change, but they’re more likely to become dehydrated because their sense of thirst declines. Pregnant women and nursing mothers need slightly more water. Some medications, such as antihistamines and certain antidepressants, increase your fluid needs too.

Does Climate Impact My Water Needs?

Yes, in hot weather you typically need to drink more because you sweat more, especially if you’re exercising. Humidity also increases your water needs. When it’s humid and warm—a double whammy—you may need as much as two times more water than when it’s drier. And although we tend to skimp on water when it’s chilly out—don’t. You also lose more water while breathing frigid, dry air. Fluid needs don’t change at altitude (11,500 to 17,500 feet): in fact, your body finds a new normal at 2 percent dehydrated and actually functions better.

Can Foods Help Me Hydrate?

About 20 percent of our fluid intake—or about 21/2 cups daily—comes from food. All foods contain some water—and fruits and vegetables deliver the most.

Do Coffee, Tea or Alcohol Dehydrate Me?

While caffeine is technically a diuretic (it increases water excretion from our bodies), you retain most of the water from caffeinated beverages, such as coffee, tea and soft drinks. Alcohol, on the other hand, particularly at high doses, can cause you to excrete more than you consume. One drink, especially of beer, won’t do much (it’s about 92 percent water), but wine and hard liquor have more of a dehydrating effect because of their higher alcohol content.

Should I Drink Water with Electrolytes?

Sodium, potassium, chloride, calcium and magnesium are essential for vital reactions in your body—all are electrolytes and all are lost in sweat. It’s important that the concentration of those electrolytes doesn’t get too high or low—and that they are replaced when depleted. If your electrolyte balances are out of whack, you won’t properly absorb the water you do drink. Most electrolytes can be replenished simply with regular, healthy meals. “If you’re well-nourished and well-hydrated, then exercise can be at least an hour in length before you need to concern yourself with including electrolytes in the beverage,” says Sam Cheuvront, Ph.D., an exercise physiologist for the U.S. Army Research Institute of Environmental Medicine. Replenishing what you lost could be as simple as having a few pretzels. You could also try a sports drink, because it has some sodium. For a walk in cool weather or a short run, water will suffice.

The More Water, the Better, Right?

Not necessarily. It is possible to overdo it. Water intoxication, or hyponatremia, a serious condition when blood sodium levels drop precipitously, can be caused by sweating excessively over several hours and drinking way too much water (versus a sports drink) while not eating or urinating (which often slows during intense physical activity). This could happen to someone who engages in a long athletic event (e.g., a marathon or multi-day hike). Symptoms include confusion, disorientation, weakness and nausea. Hyponatremia can lead to seizures, coma and death without prompt medical attention.

Can Drinking Water Help Me Slim Down?

Maybe. “If someone chooses water in place of calorie-containing beverages, overall calorie intake is less and they may lose weight,” says Bonci. A 2010 study in the journal Obesity found that adults who drank two cups of water before a meal ate less at the meal and lost more weight over 12 weeks than the group who didn’t drink water before eating.

Should I Filter My Tap Water?

“A filter will give you better water quality,” says Pauli Undesser, director of regulatory and technical affairs with the Water Quality Association. Filters remove chlorine added to disinfect the water and so it may taste better. A filter will also remove metals like lead and copper that may have entered the water supply via underground pipelines or your home’s plumbing, as well as pharmaceuticals, pesticides and other potentially harmful unregulated compounds. Look for a filter labeled with a gold seal by the Water Quality Association or the NSF International mark: both certify water-treatment products to ensure contaminant reduction and product integrity.

That said, American drinking water is quite safe. The EPA sets drinking-water standards for public water supplies and the 1996 Safe Drinking Water Act Amendments require that all community water systems distribute to their customers an annual water-quality report listing contaminant levels detected in the water. If you are one of the 15 percent of Americans with a private well, the EPA has information on how to ensure your water is safe (epa.gov/safewater/privatewells). As for bottled water, the FDA requires that it meet the same standards as tap water.

—Rachael Moeller Gorman