The question: I've always heard caffeine is a diuretic, so does it count for or against my eight-a-day?
The expert: Nutritionist Tara Gidus, M.S., R.D., C.S.S.D., L.D.N.
The verdict: Coffee addicts are more than caffeinated. They're also hydrated.
While caffeine is a mild diuretic and can cause your body to lose some water, in the grand scheme of things, the fluids lost are negligible, Gidus says. And, in the end, isn't coffee largely water?
The reasoning holds up. In one 2014 study, when University of Birmingham researchers examined the fluid levels of 50 men who regularly downed three to six cups of coffee each day, they determined that coffee hydrates about the same as water does.
"Most people are well adapted to caffeine," explains Gidus, who says coffee and tea can contribute to most peoples' daily fluid intake. Your goal, though, shouldn't exactly be eight glasses of water or coffee a day.
The Institute of Medicine recommends men consume 120 ounces of fluid and women, 90 ounces. While that's a lot more than the 64 we always assumed we needed, if your diet is rich in water-logged foods like fruits and veggies, you can count on eating about 20 percent of your fluid needs each day, Gidus says.
The rest—or about half your body weight in fluid ounces—should come straight from sucking down drinks. And, yes, coffee counts. So, for example, a 180-pound guy should aim to drink 90 ounces, or 11 cups, a day.
In the end, though, your hydration levels depend on tons of factors—everything from your exercise habits to your city's humidity levels. The best way to check your hydration levels? Look at your pee: If it's pale yellow or approaching clear, you're good to go. If it's dark or stinky (and no, the asparagus smell doesn't count), you should probably start pushing fluids.