Water
What it does
Water makes up roughly half to two-thirds of an adult body and supplies most of the fluid inside cells, between cells, and in the bloodstream. It helps maintain blood volume, carries dissolved nutrients and waste, lubricates joints, cushions sensitive tissues, and removes heat when sweat evaporates from the skin.
Water is lost continuously through urine, breathing, stool, and the skin. Exercise, heat, fever, vomiting, and diarrhea can increase those losses quickly.
Why water has an Adequate Intake instead of an RDA
Water has no Recommended Dietary Allowance (RDA). It has an Adequate Intake (AI).
An RDA is calculated from an estimated average requirement and the variation around it. For water, researchers have not established a reliable average requirement because needs vary widely with activity, temperature, humidity, body size, diet, illness, and other factors. Healthy adults can maintain normal hydration across a wide range of water intakes.
The National Academies therefore based the water AI on the median total water intake of people who appeared adequately hydrated in national survey data.
There is also no Tolerable Upper Intake Level for water. Healthy kidneys can excrete substantial excess water under ordinary conditions, so a fixed daily toxicity threshold has not been established. Drinking excessive amounts over a short period can still be dangerous because it can dilute the sodium concentration in the blood.
How much water do adults generally need?
The National Academies’ Adequate Intakes refer to total water from all sources, not just plain drinking water.
| Group | Total water per day |
|---|---|
| Men 19+ | 3.7 L / about 125 fl oz |
| Women 19+ | 2.7 L / about 91 fl oz |
| Pregnancy | 3.0 L / about 101 fl oz |
| Breastfeeding | 3.8 L / about 128 fl oz |
In the U.S. data used to establish the adult AI, beverages supplied about 81% of total water and food supplied about 19%. For younger adults, that worked out to roughly 3.0 liters per day from beverages for men and 2.2 liters for women.
These are reference intakes, not instructions to drink 3.7 or 2.7 liters of plain water.
The familiar “eight glasses a day” rule is also not an official reference value. Eight 8-ounce glasses equal about 1.9 liters of beverage. That number cannot be compared directly with the 2.7- and 3.7-liter AIs, because the AIs include water from food and every beverage.
Where water comes from
Plain water is only one source. Coffee, tea, milk, sparkling water, soups, and other drinks all contribute to total water intake. Food can supply a meaningful amount as well, particularly when the diet contains plenty of fruit, vegetables, cooked grains, yogurt, soups, and similar foods.
Representative foods vary widely in water content:
| Food | Approximate water content |
|---|---|
| Cucumber | ~95% |
| Watermelon | ~91% |
| Milk | ~88% |
| Apples and oranges | ~86% |
| Plain yogurt | ~85% |
| Cooked rice | ~68% |
| Bread | ~36% |
Coffee and tea still count toward hydration. Caffeine can temporarily increase urine production, but evidence does not show a cumulative body-water deficit from normal caffeinated beverage intake.
Thirst usually works
For healthy adults under ordinary conditions, thirst and normal drinking behavior generally keep hydration within a normal range.
As the concentration of body fluids rises, the brain increases thirst and releases vasopressin, which tells the kidneys to conserve water. Blood-volume losses provide another signal when losses become larger.
Thirst can become less reliable with age. Older adults may feel less thirsty as body water falls, especially when illness, cognitive impairment, mobility problems, or limited access to drinks are also factors.
Urine color is a rough clue. First-morning urine is normally more concentrated. Riboflavin from food, a B-complex, or a multivitamin can turn urine bright yellow without indicating dehydration.
Who may need to pay closer attention
Fluid intake can require more attention when losses rise, thirst is less reliable, or access to drinks is limited:
- older adults, particularly with reduced mobility or cognitive impairment
- people working or exercising in hot or humid conditions
- people wearing heavy or protective clothing for long periods
- people doing prolonged or high-intensity exercise
- people with fever, vomiting, or diarrhea
- pregnant or breastfeeding women
- people taking diuretics, lithium, or other medications that affect fluid balance
- people whose work, travel, or schedule makes regular access to fluids difficult
General advice to drink more does not apply to everyone. Kidney disease, heart failure, some liver and endocrine conditions, and certain medications can require individualized fluid limits or replacement plans.
Exercise, heat, and sweat
Exercise can increase water losses substantially, especially in heat.
Sweat rates around 0.5 to 2.0 liters per hour are common during exercise, with substantially higher rates measured in some people and conditions. Body size, exercise intensity, heat, humidity, clothing, and heat acclimatization all affect the result.
Sweat composition varies too. Sodium losses differ dramatically between individuals, so two people completing the same workout can lose different amounts of both water and sodium.
A practical way to estimate sweat rate is to weigh yourself immediately before and after a representative session. For a session without bathroom breaks:
Approximate sweat loss = body mass lost + fluid consumed during exercise
A loss of about 1 kilogram of body mass is roughly equivalent to 1 liter of fluid. Divide the estimated loss by the number of hours exercised to get an approximate hourly sweat rate. If urine is produced during the measurement period, account for that as well.
Repeat the measurement under conditions you actually train in. A cool indoor workout and a summer run may produce very different sweat rates.
For shorter sessions in moderate conditions, water and ordinary meals are often enough. Longer sessions, high heat, or heavy sweating can make sodium replacement more relevant. Longer exercise can also create a separate need for carbohydrate, depending on intensity and duration.
Exercise hydration source: National Athletic Trainers’ Association Position Statement: Fluid Replacement for the Physically Active.
Drinking too much
Exercise-associated hyponatremia occurs when blood sodium becomes abnormally low during or shortly after physical activity. The main preventable driver is consuming water faster than the body is losing and excreting it, often combined with continued vasopressin activity during exercise.
Sodium in a drink does not make unlimited fluid intake safe. Studies of endurance events have found overhydration to be more important in the development of exercise-associated hyponatremia than low sodium-supplement intake.
Weight gain during a long event is a useful warning sign that fluid intake has exceeded losses.
Early symptoms can include headache, nausea, bloating, vomiting, and confusion. Some of those symptoms overlap with dehydration, which is why symptoms alone can point in the wrong direction. Drinking more water is not a safe default for someone who develops these symptoms during a prolonged event after consuming large amounts of fluid.
Severe confusion, seizures, loss of consciousness, or other neurological symptoms during or after prolonged exercise require urgent medical care.
For many recreational situations, drinking in response to thirst is a reasonable way to avoid overdrinking. People with high sweat losses, long events, or performance goals may benefit from measuring sweat rate and planning fluid and sodium intake around their own losses rather than a generic schedule.
Exercise-associated hyponatremia source: Hew-Butler T, et al., Statement of the Third International Exercise-Associated Hyponatremia Consensus Development Conference, Carlsbad, California, 2015. Clinical Journal of Sport Medicine, 2015;25(4):303-320.
What to check in your routine
People with a prescribed fluid restriction or hydration plan should follow that guidance rather than general hydration advice.
For most healthy adults, normal access to drinks and meals is enough to replace ordinary daily losses. Heat, illness, pregnancy, breastfeeding, and heavy sweating can raise water needs.
For repeated long or hot training sessions, a personal sweat-rate estimate is more useful than a generic ounces-per-hour target.
The information on this page is educational and does not constitute medical advice. Talk to a qualified healthcare provider before making major changes to fluid intake if you have kidney, heart, liver, or endocrine disease or take medication that affects fluid or electrolyte balance.