Iced tea, lemonade, water. Everyone I met in California’s record-breaking heat last week offered something to drink. And there really is a physiological explanation for all that hospitality.

The recipe for a human body calls for 60 pounds of water for every 100 pounds of person. Water is the medium for all the chemical reactions in cells that produce energy. Water bathes each nook and cranny with nutrients and helps wash out waste. Even our bones rely on water to stay strong and flexible.

Water comes in from food and beverages. It goes out in urine, feces, sweat and as we breathe. Keeping the balance is our goal.

How much do we need? “The vast majority of healthy people adequately meet their daily hydration needs by letting thirst be their guide,” according to the most recent guidelines established by the National Academy of Sciences Institute of Medicine. “Higher intakes of total water will be required for those who are physically active or who are exposed to hot environments.”

Don’t let these general guidelines lull you into dehydration. What the NAS estimates to be an “adequate intake” for women is about 2.7 liters or 91 ounces or 9 cups of water from food, water and other beverages each day. Men (who carry more muscle, which contains more water) need more – about 3.7 liters (125 ounces or 13 cups) each day. Yikes.

About 80 percent of our fluid comes from water, coffee, tea, milk, juice and unsweetened or sweetened beverages. (Alcohol doesn’t count.) The other 20 percent is in food. Fruit, vegetables and yogurt are mostly water, for example.

Contrary to what we thought in the past, caffeinated beverages such as coffee and tea do count toward meeting our daily water needs. Caffeine is still a diuretic that can cause some water loss, but most experts agree the effects are minimal unless we take in more than 400 milligrams of caffeine a day. (One 8-oz. cup of coffee contains about 150 milligrams of caffeine while one cup of tea contains about 50 milligrams.)

If bodies were cars, they would register “hot engine” when fluid levels dipped just 2 percent below normal and body temperatures begin to rise. One of the first symptoms of dehydration is fatigue… muscles don’t work well when water is low. Neither do dried out brain cells. Headaches, confusion and clumsiness are all symptoms of dehydration.

Children are especially vulnerable to the effects of heat and too few fluids. They can experience serious symptoms with just 1 percent drop in fluid level.

Symptoms of dehydration include thirst (duh) and dark-colored urine. For athletes who work out every day, a weight drop of more than 1 pound in 24 hours can indicate you are not adequately replenishing fluids from day to day.

Heat stroke is the dangerous buildup of heat in the body when the body is unable to cool itself effectively in hot weather. High humidity makes the problem worse. Drink more fluid in hot humid weather, say experts. Wear light clothes that allow for sweat to evaporate off the skin. And get out of the heat or at least sit in the shade when you can.

Although pure water is still the gold standard for most of our fluid needs, there really is a condition called “water intoxication” – drinking too much water – that is life-threatening. Best to balance our water needs with a variety of water-containing foods and beverages.

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