You need to drink more water if:
You’re exercising. Pre-hydrate yourself by drinking at least two glasses (16 ounces) of water an hour or so before you work out. As you work up a sweat, take frequent sips of water, as dehydration makes muscles tire more easily. After exercising, top off with two more glasses of water to rehydrate yourself.
You’re sick. Bodies lose a lot of water with illnesses that cause fever, vomiting, or diarrhea. Becoming dehydrated makes you feel doubly sick.
You’re pregnant. Blood volume increases by 40 percent during pregnancy, and extra fluids are also needed for the ongoing manufacturing of amniotic fluid. Water also helps maintain overall well-being during pregnancy. Drinking lots of water helps move along and dilute the body’s waste products, lessening problems with constipation and reducing the risk of urinary tract infections. Pregnant women need to drink at least eight 8-ounce glasses of fluid a day to keep body and baby well-hydrated. If you don’t like to drink that much plain water, yet are worried about the weight gain from extra juice, flavor your water with just a teaspoon of frozen juice concentrate. Space this fluid intake evenly throughout the day, consuming larger, more frequent drinks along with smaller, more frequent meals. The swelling of ankles, feet, and hands during pregnancy are not signals to cut back on water. Some of this is the normal swelling that occurs with pregnancy, or it may be a sign that your body is retaining too much salt, in which case drinking more water will actually help decrease the swelling.
You’re breastfeeding. Enjoy an extra glass of water before each nursing. Milk is the baby’s water source, as well as his source of nutrition.
You’re hot. During hot weather or while working in the hot sun, bodies perspire more. Drink several glasses of water before going outside and drink more water afterward.
You’re thirsty. Obviously, you should drink water when you’re thirsty. Thirst means your body already has a water shortage. Best to drink enough water so you don’t get thirsty. If thirsty, quench your thirst and then drink two more glasses of water. This is especially important for senior citizens, since the thirst signal declines with age.