How much water do you need to drink per day?

Adequate Intake for Water

The Institute of Medicine (IOM) in the U.S. has established the Adequate Intakes (AI) for water (Chart 1). For example, the AI for adult men is 3.7 liters of water per day, which should cover the water needs of most healthy, sedentary adult men living in temperate climates. You may need to drink less or more than AI, which mainly depends on how much you sweat. A very sedentary adult who sweats very little may need as little as 1 liter ^[1], but the average water needs for adult men in temperate climates are probably about 2 liters per day (from beverages and foods combined). A physical worker in a hot climate may need as much as 15 liters of water per day, though.

The most reliable way to check if you have enough water in your body is to weigh yourself. One day take care to drink enough and the next morning weigh yourself after emptying your bladder and bowel and before eating or drinking anything ─ this is then your actual body weight. Anytime you think you may be dehydrated, weigh yourself in the morning in the same circumstances: any decrease of your weight is probably from dehydration and it also tells you have much water you need to take to regain your usual weight.

NOTE: When you eat a lot of foods with high water content, such as fruits, salads, oatmeal, boiled potatoes, rice or soups, you need less water from beverages.

Chart 1. Adequate Intakes (AI) for Water
Age	Water (liters/day)*
Infants 0-6 months	0.7 L (from breast milk)
Infants 7-12 months	0.8 L (from breast milk and other foods)
1-3 years	1.3 L
3-8 years	1.7 L
9-13 years, boys	boys: 2.4 L ; girls: 2.1 L
14-18 years, boys	boys: 3.3 L ; girls: 2.3 L
19 y. or older men	men: 3.7 L ; women: 2.7 L
Pregnancy	3.0 liters
Lactation	3.8 liters

Chart 1 source: ^[2]  * Water intake from beverages and foods combined

Healthy, breastfed infants do not need any extra water, even not in a hot climate ^[3,4]. Human breast milk contains 88% water ^[5]. Infants 0-6 months drink about 800 milliliters breast milk per day and infants 7-12 months old about 600 milliliters per day ^[7]. After weaning an infant needs 100 mL of water per kg body weigh per day, for example, a 15 lbs (7 kg) infants needs about 700 mL water per day ^[6].

Healthy, formula-fed infants do not need any extra water before six month of age ^[8], but for older infants a pediatrician should tell. Well water containing more than 10 mg nitrates per liter should not be used because it can cause nitrate poisoning ^[9]. Some doctors say water to prepare formula for infants under 3 months of age should be boiled and cooled before using ^[11].

Toddlers 1-3 years old need about 1.3 liter water per day from fluids and beverages combined ^[10].

If you use fluoridated tap or well water, note that infants should not get more than 0.7 g fluoride per day, otherwise they can develop dental fluorosis (stained teeth) ^[12].

Infant and toddlers with diarrhea should get an oral rehydration solution (ORS): 1 teaspoon (5 mL) every minute as long they accept it and until they get back the lost weight and their urine becomes clear. If the infant vomits, wait for 15 minutes or so and continue with ORS. If the infant refuses to drink, take him or her to the hospital immediately. Fluids NOT appropriate for the treatment of diarrhea in small children under age of 3: plain water, tea, coffee, soda, energy drinks, fruit juices, ginger ale, gelatin desserts, chicken broth, sport drinks and alcoholic beverages ^[13].

Adequate Intakes for older children, adolescents, adults and pregnant and breastfeeding women are listed in Chart 1. Old people, in general, do not need to drink more or less than young ones. However, old people tend to eat less and often do not feel thirsty even when they are dehydrated.

Heavy physical workers in a hot climate might need more than 15 liters of water per day ^[14,15,16].

How much water do athletes need?

The amount of water you need to drink during a sport event is about the same as the amount you lose with sweat. The average sweat rate in athletes may range from 0.5 to more than 2.5 liters per hour ^[17].

To find out your personal sweat rate, check your naked weight, then exercise for one hour without drinking, eating or urinating in the meantime, then towel yourself and check your naked weight again. The difference in weights is your sweat rate, which tells you how much water you need to drink in one hour during the competition to prevent dehydration or hyperhydration. Be aware that sweat rate rises with the exercise intensity, clothing, ambient temperature, stress level and the state of your heat acclimatization. After exercising in a hot climate for 5-10 days, your body will acclimatize and start to excrete more sweat and less sodium ^[17]. The level of physical fitness only modestly affects sweat rate, though ^[18].

A general recommendation for sport is to drink about every 20 minutes, but not more than you are thirsty and not more than 800 milliliters (27 fl. oz) per hour, since larger amounts of water may not be absorbed ^[19]. During intense cycling, as low as 500 mL of water per hour may be absorbed ^[20].

Dehydration that results in as little as 2% loss of body weight (3 lbs in a 154 lbs person, or 1.4 kg in a 70 kg person) may decrease athletic performance ^[17]. An increase of body weight after drinking and no eating during exercise speaks for overhydration.

What to drink during sport events? Plain water is appropriate in any exercise lasting for less than 4 hours or in a prolonged but less intense (fitness-level) exercise ^[21]. Sport drinks are intended for intense exercise lasting more than 4 hours (marathon, cycling) or exercise in hot weather ^[21].

Beverages, NOT appropriate during exercise: Fruit juices, soft drinks and energy drinks usually contain 10% or more sugars, which may slow down the emptying of the fluids from the stomach or cause bloating or diarrhea. Carbonated drinks may also cause bloating.

One day before the sport event, drink water according to thirst to keep yourself well hydrated. You can eat salty foods as usually to have enough sodium in the body. There is no need for excessive salt or water intake, because the goal is to be normally hydrated and not overhydrated before the race ^[23]. Overhydrating with drinking plenty of water and taking glycerol (which temporarily helps to retain water in the body) does not seem to have any clear benefit and will likely make you urinate more during the competition ^[23].

Two hours before the sport event you can drink two cups or about 500 mL of water or sport drink. 20 minutes before the sport event drink about one cup (237 mL) of water or sport drink. Hyperhydration does not increase physical performance and it is not clear if it increases resistance to heat exhaustion, so it is generally not recommended ^[23,25].

During the sport activity, drink water or sport drinks every 20 minutes or so. To avoid water intoxication, drink only to thirst and not more than 400 mL (per hour for small persons with low sweat rate in a cool weather) to 800 mL per hour (for big persons with high sweat rates in hot weather) ^[23,26]. The amount of fluid needed depends on the amount of sweat lost; the goal is to prevent more than 2% loss of body weight.

After the sport event, it is recommended to drink 1.5 liters (50 oz) of water for every kilogram (2.2 lbs) of the body weight lost ^[17].

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Water Intake Recommendations During Disease

Heart, Kidney Or Liver Disease

Your doctor may instruct you to limit your water intake if you have one of the following conditions:

• Chronic heart disease
• Chronic kidney disease
• Chronic liver disease
• Swelling of the legs or general body swelling
• Brain swelling (cerebral edema)
• Lung swelling
• Low blood sodium levels (hyponatremia)
• Water intoxication caused by excessive water drinking
• Before or after certain surgical procedures
• During chemotherapy or certain other therapy

DO NOT start to drink significantly less or more than usually without discussing with your doctor.

Orthostatic Hypotension

In one study, in individuals with orthostatic hypotension (a drop of blood pressure upon standing), drinking two cups (about 1/2 liter) of water increased blood pressure and reduced symptoms at 15 and 35 minutes after drinking ^[27,28]. In healthy individuals prone to get dizzy after standing, water drinking before standing up may also slightly reduce symptoms ^[29].

Cystic Fibrosis

Individuals with cystic fibrosis may excrete up to about 140 mmol of sodium (8 g of salt) with one liter of sweat ^[30]. They are less thirsty after an exercise than healthy individuals, since they lose a lot of sodium through sweating resulting in lower blood sodium levels, which decrease thirst. Adding at least 50 mmol/l sodium chloride to drink may stimulate thirst ^[30]. For athletes with cystic fibrosis, sport drinks containing more than 115 mg sodium per 100 mL are recommended ^[43].

Water Requirements at High Altitudes

Unacclimated persons arriving at altitudes between 11,500 and 17,500 feet (3,500 and 5,334 meters) may lose 1-2 liters of the body water within the first three days and may remain partially dehydrated for about 14 days ^[32]. The water loss may be caused by increased urination stimulated by cold (cold-induced diuresis), faster breathing due to low oxygen levels, increased water evaporation from the skin due to lower ambient water vapor pressure and warm clothing ^[31,34,35].

On the other hand, physical activity (climbing) soon after arriving at high altitude causes marked water and sodium retention ^[33].

Staying at altitudes over 18,000 feet (5,500 m) for several weeks results in the accumulation of water and sodium in the body ^[32]. In one study, in soldiers staying at 22,000 feet (6,700 m) for at least 10 weeks, body water increased for about 20% and body sodium for 18% ^[34]. About 20% of these soldiers experienced generalized body swelling and shortness of breath, a condition later termed as subacute mountain sickness.

Benefits of Drinking a Lot of Water

High water intake has been long considered to be preventative against kidney stones, especially in individuals with history of kidney stones, but more research is warranted ^[36,37].

Some people try to lose weight by eating nothing and drinking large amounts of water to suppress hunger. This method is dangerous since it can result in water intoxication. Water by itself does not help to reduce weight; it is low calorie intake that does.

Drinking excessive amounts of water is NOT a medically approved method of losing weight or “removing toxins from the body.”

The Body Water Stores

Your body cannot store water for “later use.” Any water in excess of your body needs will be excreted with urine in few hours.

You also cannot obtain enough water exclusively from the body fat stores. For example, the breakdown of 1 pound of the body fat yields about 500 mL of metabolic water, but to burn 1 pound of fat (3,500 Calories) you would need to be quite active for the whole day during which you would likely lose more than 500 mL of water through sweating and urination ^[38].

How long can you survive without water?

You may survive for three or more days without water or food, but only when you were well hydrated on the beginning and you do not lose a lot of water by sweating, fast breathing, diarrhea or excessive urination. A 14 days old boy, one of the “miracle babies of Mexico” was rescued the 10th day after being trapped in a nursery home ruined by the 1985 Mexico City earthquake ^[39]. According to newspaper reports, several other infants, children, young and old women have survived for as much as 7 or 8 days under the ruins of earthquakes without any water or food ^[40,41,42].

On the other hand, a small child left in a hot car or anyone lost in the desert under the direct sun can die within few hours due to dehydration.

In general, bigger, well hydrated persons survive longer without water than small, dehydrated ones.