You've probably heard the old adage: Drink eight glasses of water each day. You may wonder if that's because human bodies need to process a certain amount of water in a certain period of time. Humans do need water, because the human body is about 60 percent water. But the body processes only so much, and you can drink either too much or too little water.
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Water is contained in the food you eat and the fluids you drink. Your body processes all of it. A healthy body can process a maximum of 27 ounces to 33.8 ounces per hour, but the ideal amount of fluid you should drink depends on your gender, weight, diet, exercise and the weather.
Why Do You Need Water?
Water is the main ingredient in the human body. With 60 percent of the body composed of water, water is in every cell, tissue and organ. It's the kidneys that process much of the water you take in, but there are many ways the body sheds water, according to the Mayo Clinic.
You lose water through your breath, perspiration, urine and bowel movements. In order for your body to work the way it should, it needs to continually replace all that lost water. You replace those fluids any time you drink liquids or eat any food, like many fruits, that contain water. Drink plain water to replace fluids, according to the Centers for Disease Control and Prevention, because it has no calories.
The Mayo Clinic reports that the rate at which you lose fluid depends on what you are doing, so there's no exact rate at which your body processes water. You may lose water quickly through perspiration if you're outside on a hot, humid day, especially if you're working or exercising.
How Much Water Is Needed?
Typically, the CDC follows the 2004 National Academy of Sciences, Engineering and Medicine recommendation that men take in 15.6 cups per day of fluid, while women take in 11.4 cups of fluid per day. This recommendation — which is still used not only by the CDC but the Mayo Clinic in its 2017 report, along with the National Kidney Foundation (NKF) — says most healthy people meet their recommended water intake each day.
The maximum amount of water people with normal kidney functioning should drink per hour is 27 ounces to 33.8 ounces, according to a June 2013 study in the Annals of Pediatric Endocrinology and Metabolism.
The old advice was to drink eight glasses of water per day. The NKF says "Eight is great, but not set in stone." That recommendation, the organization says, became popular as a way to remind people to keep drinking fluids throughout the day.
Read more: Healthy Body Water Percentage
When to Drink More
You should drink if you're thirsty, the NKF says, unless you have kidney failure. Those patients must restrict their water intake. If you have kidney problems or any chronic disease, always follow your doctor's recommendations.
Common sense calls for you to determine your own ideal daily fluid intake. Eight glasses is not bad advice, considering you get some of your fluids from food, the Mayo Clinic says.
Some reasons you may need to drink more, according to the Mayo Clinic, include:
- Exercise. Breaking into a sweat means you're losing water. Whether you're running, walking your dog or gardening, if you're sweating, you're losing water. That's why water tastes so good during a workout. The rule of thumb is that if you're exercising hard for over an hour, you need not only water, but electrolyte replacement.
- Weather. Heat and humidity will make you sweat more, so drink extra if it's hot or humid. Remember, if you live in, or are visiting, a high altitude, it's easier to get dehydrated, so you may need even more fluid than your intake at lower altitudes.
- Health. Your physical condition has a lot to do with how many ounces of water your body can absorb in an hour. If you're sick, especially if you have a fever, are vomiting or suffer from diarrhea, your body may be losing fluids rapidly. You should follow a doctor's directions when trying to rehydrate. You may need to replace not only water, but electrolytes. Bladder infections and kidney stones may also cause you to lose water.
- Pregnancy or breastfeeding. If you're pregnant or breastfeeding, you'll need more water. The Office of Women's Health recommends you drink 10 cups of fluid daily if you're pregnant, and while breastfeeding, drink a glass of water each time you nurse your baby.
How Much Is Too Much?
There are times when people drink more water than their bodies can process, according to the NKF. Hyponatremia occurs most often to athletes who drink too much fluid and in people with certain chronic diseases. This is a very dangerous condition that requires immediate medical attention.
It occurs when the sodium level in the blood drops below normal, according to a December 2014 study in the Journal of Clinical Medicine. Your body needs sodium for fluid balance, blood pressure control, nerves and muscles.
The normal blood sodium level is 135 to 145 milliequivalents/liter, or mEq/L, the NKF says. When your mEq/L goes below 135, hyponatremia can occur. What that does is cause extra water to go into your cells and cause them to swell. The cells in your brain have nowhere to go because the brain can't expand beyond the skull.
Hyponatremia, the NKF says, can occur in anyone with kidney failure, congestive heart failure, anyone who's on pain medicine or antidepressants, or from severe vomiting. It can also occur from excessive drinking during an endurance athletic event. It's treated with IV fluids and other fluids containing sodium.
Read more: 10 Myths About Salt Debunked
Marathon Runners and Hyponatremia
Exercise-associated hyponatremia (EAH) is thought to occur when endurance athletes take in too much fluid during a long-distance run or another athletic event, according to a March 2017 study in Frontiers in Medicine. It was first noticed in the 1980s during Ironman triathlons and ultramarathons, especially in hot climates.
Since then, there have been cases in shorter-distance races and some other sports, even in yoga classes. According to the Frontiers in Medicine study, despite more publicity about EAH, athletes continue to die from this condition. High school football players, a soldier training to be an Army Ranger, a policeman in a 12-mile bike ride, and a college student doing calisthenics for a fraternity all died from EAH, the study reported.
The common thread is that all of these athletes were drinking beyond thirst, researchers said. The Frontiers in Medicine study also noted that a highly fit soldier died during a 31-mile training march from both EAH and heat stroke. Despite better awareness, the study reports that fatalities, case reports and incidence rates have spread into a wider variety of sports.
Read more: Is Alkaline Water Extra Healthy or a Hoax?
Safe Hydration Levels
Water helps your kidneys remove waste from your blood and keeps your blood vessels open. To get enough, but not too much, check your urine, says the NKF. Drink enough to keep your urine light yellow or colorless. If your urine is dark, you're not getting enough water.
When participating in an endurance athletic event, drink according to the dictates of thirst during and immediately following exercise, according to the March 2017 Frontiers in Medicine study.
- Harvard Health Publishing: "How Much Water Should You Drink?"
- Mayo Clinic: "Water: How Much Should You Drink Every Day?"
- Centers for Disease Control and Prevention: "Get the Facts: Drinking Water and Intake"
- Frontiers in Medicine: "Exercise-Induced Hyponatremia: 2017 Update"
- The National Academies of Science, Engineering and Medicine: "Dietary Reference Intakes: Water, Potassium, Sodium, Chloride, and Sulfate"
- National Kidney Foundation: "6 Tips To Be 'Water Wise' for Healthy Kidneys"
- National Kidney Foundation: "Hyponatremia"
- Office on Women's Health: "Breastfeeding and Everyday Life"
- Office on Women's Health: "Staying Healthy and Safe"
- Annals of Pediatric Endocrinology & Metabolism: "Hyponatremia Caused by Excessive Intake of Water as a Form of Child Abuse"
- Journal of Clinical Medicine: "Effects of Hyponatremia on the Brain"