How much water should I drink a day?

• Normal hydration is usually maintained by our thirst mechanism driving normal drinking habits, such as drinking with meals and at other times during the day

• The amount of water a person needs in a day varies and depends on how physically active they are, the temperature and other factors

• The recommendation to drink eight glasses of water per day isn’t based on scientific evidence

• The US Food and Nutrition Board has published Adequate Intake (AI) values for daily water consumption by age group, based on data from the Third National Health and Nutrition Examination Survey (NHANES III, 1988-1994)

• Plain water isn’t our only source of water, we also absorb water from other beverages and foods including fruits and vegetables

Water is essential for life and accounts for about 50-60 percent of body weight in adults. Our bodies need water for a number of important tasks, including to flush out waste, perform major bodily functions, regulate body temperature and more.

We get water from drinking and eating, but at the same time it is lost through the respiratory system, skin and the renal and gastrointestinal tracts. To ensure we don’t get dehydrated we need our water intake to cover what we’ve lost.

You’ve probably heard the old saying that you should drink eight glasses of water a day. The recommendation to drink eight ounces of water eight times a day is often referred to as 8x8. But where did this recommendation come from and is it still the right advice?

Where did the recommendation to drink 8 glasses of water a day come from?

It’s unclear exactly where the recommendation to drink eight glasses of water a day came from.

It’s been suggested that it’s based on the Recommended Dietary Allowances published by the US Food and Nutrition Board in 1945, which stated the following.

It is also thought it may be based on comments made by nutritionist Fredrick J. Stare. In 1974, Dr. Stare wrote the following at the end of the book “Nutrition for Good Health”.

Interestingly, both of the suggested sources report that water isn’t just obtained from drinks and beverages, but also the foods we eat - a message that appears to have been lost when it became popular to promote drinking 8x8.

Is drinking 8 glasses of water a day still recommended?

Eight glasses a day is no longer the target. How much you need depends on your specific circumstances, such as your level of physical activity and environmental factors, including the temperature and altitude. Certain medical conditions and medications can also impact how much water you need each day.

Instead of recommending people drink a specific number of glasses of water a day, an Adequate Intake (AI) for total water has been established by experts from the panel on Dietary Reference Intakes for Electrolytes and Water of the US Food and Nutrition Board, Institute of Medicine, The National Academies, in collaboration with Health Canada. Total water includes water from drinking water, as well as the water obtained from other beverages and food.

A wide range of water intake can maintain normal hydration levels, so the AI for total water has been based on the median total water intake from the Third National Health and Nutrition Examination Survey (NHANES III, 1988-1994).

The AI is not a recommendation as to how much everyone should ideally drink a day, but an amount of water that is expected to prevent the immediate negative effects of dehydration, such as functional and metabolic abnormalities, in somewhat sedentary individuals in a temperate climate.

The panel highlighted that there isn’t enough data available to recommend an average daily intake of water that’d meet the needs of nearly all of the healthy population (Recommended Dietary Allowance, RDA), or even half of the healthy population (Estimated Average Requirement, EAR).

How much water should I drink a day?

The amount of water a person needs in a day varies and depends on how physically active they are, the temperature, their age, health status and other factors. Normal drinking behavior and letting thirst guide us generally results in adequate water intake.

For those wanting a guide as to how much they should be drinking, AI values for total water have been established by experts from the panel on Dietary Reference Intakes for Electrolytes and Water of the US Food and Nutrition Board.

It’s important to remember that the AI value is simply an amount of water that is expected to prevent dehydration and it’s the median total water intake of people who took part in a national survey that involved them recording what they ate and drank over a period of time. People may drink more or less than this amount and still maintain a normal level of hydration. Our body’s homeostatic responses, which work to keep everything in balance, are able to deal with under- and overhydration to some degree for short periods of time.

Separate AI values for infants, children and pregnant and breastfeeding people are reported because requirements for these groups are different from the average adult. Different AI values have also been established for men and women.

The AI values also indicate how much water people usually obtain from drinking water and other beverages and how much they obtain from food. Typically 20-30 percent of water comes from the foods we eat.

Do caffeine and alcohol count towards my water intake for the day?

Caffeinated beverages (such as tea and coffee) and alcohol are known to have a diuretic effect, which makes you pee more. Because of this it was thought that they could make you lose more water than they provide. However, the diuretic effects of caffeinated beverages and alcohol are thought to be transient and the current evidence suggests that they are unlikely to cause you to lose more water than they provide and so they can contribute towards your daily total water intake.

How do I know if I'm not drinking enough water?

If you’re not drinking enough water or other fluids you can become dehydrated and this can result in an imbalance in the electrolytes in your body, including sodium, potassium and chloride.

Signs of dehydration

Mild-to-moderate dehydration can be corrected by rehydrating - drinking more fluids including rehydration solutions available at the pharmacy (especially, for babies and young children), water, diluted fruit juice or sports drinks with electrolytes. Severe dehydration requires urgent medical attention.

Dehydration can be caused by being too busy to eat or drink, not feeling like eating or drinking because you’re unwell, or not having access to water. Excessive sweating, a prolonged fever, severe vomiting and/or diarrhea, and uncontrolled diabetes or certain medications can cause dehydration.

Those most at risk of dehydration include:

• Older adults
• Infants and young children
• People with certain illnesses including diabetes, cystic fibrosis or kidney disease
• People who take medications that make them sweat or urinate more
• People who work or exercise outdoors in hot weather

Is it possible to drink too much water?

It’s possible to drink too much water, which can lead to water intoxication or poisoning, particularly if a lot of water is consumed in a short space of time.

Drinking too much water can cause the level of sodium in your blood to become too low (hyponatremia) and this can be life threatening. Having a high total water intake most days isn’t thought to be a problem because most healthy people can self-regulate the amount of water they take in from beverages and foods. Healthy adults also have considerable ability to get rid of excess water and maintain their water balance.

When too much water is consumed too quickly, however, it can greatly exceed the ability of the kidney’s to remove the excess - they can usually get rid of 0.7-1.0 liters an hour - and this can lead to acute water toxicity.

Signs that you’re drinking too much water include:

• Consistently colorless or clear urine
• Urinating more than 7-10 times a day
• Drinking a lot even when you don’t feel thirsty
• Nausea or vomiting
• Headache
• Fatigue or loss of energy
• Muscle weakness, twitching or cramps
• Confusion or disorientation
• Bad temper or restlessness
• Seizures or coma
• Death

Bottom line

The amount of water you need to drink each day will vary depending on a number of factors, such as the temperature and your levels of physical activity. Your water intake doesn’t just come from drinking water and other beverages, roughly 20 percent comes from the foods you eat. AI values provide some guidance as to how much water you should drink, but normal hydration is usually maintained by our thirst mechanism driving normal drinking habits.