They are usually talking about six to eight glasses of 8 ounces each, for a total of 64 ounces a day.
It does not hurt to drink more, especially if you are exerting yourself. You might weigh yourself before and after a workout and drink 16 ounces of water for every pound you lose.
With a loss of just about 21/2 quarts, you start to have serious problems, like skin shrinkage, muscle weakness and fatigue, irritability, dizziness, and headaches.
Another time to drink extra water is when you are on a diet to lose weight, to help the kidneys flush out the waste products from the extra flesh you burn, and to prevent constipation. Try drinking a glass before and a glass during each meal to control food intake. And at high altitudes, drink up to twenty glasses a day.
What nutritional authorities are talking about is not coffee, soda, or beer, but hydrating liquid, preferably plain water. Other beverages may contain diuretics (especially alcohol and caffeine) that make you lose water, on balance.
And don’t count on your thirst to remind you when it is time to drink.
The body can easily lose a quart or more of water before your thirst may signal you, and when you do take a drink, the feeling of thirst goes away long before you have actually replenished your water supply.