We’ve all heard the advice to drink at least 8 glasses of water per day. It’s enough to make you feel like your back teeth are floating! But is it the right amount?

The Importance Of Water

You know that going more than a few days without enough to drink can be fatal. Do you know why?

We have trillions of cells in our bodies in each of which millions of chemical reactions are taking place. Most of these reactions are facilitated by the actions of hydrogen (H) and hydroxide (OH) ions. Water (H2O = H+OH) is a prime source of these ions. If there isn’t enough water to provide them, the chemical reactions don’t occur and the cells begin to die.

Thinking about this on a higher level, water is essential for most of our body fluids, including blood, tears, cerebral spinal fluid and the fluid in our joints. If we are not sufficiently hydrated, we also don’t make enough of these protective fluids.

How Much Is Not Enough?

With respect to fluid intake, our goal is to avoid dehydration. So, how do you know if you’re dehydrated? Here is a list of some signs and symptoms†:

Mild to moderate dehydration:

• Dry, sticky mouth
• Sleepiness or tiredness — children are likely to be less active than usual
• Thirst
• Decreased urine output — fewer than six wet diapers a day for infants and eight hours or more without urination for older children and teens
• Few or no tears when crying
• Muscle weakness
• Headache
• Dizziness or lightheadedness

Severe dehydration (treat as a medical emergency):

• Extreme thirst
• Extreme fussiness or sleepiness in infants and children; irritability and confusion in adults
• Very dry mouth, skin and mucous membranes
• Lack of sweating
• Little or no urination — any urine that is produced will be dark yellow or amber
• Sunken eyes
• Shriveled and dry skin that lacks elasticity and doesn’t “bounce back” when pinched into a fold
• In infants, sunken fontanels — the soft spots on the top of a baby’s head
• Low blood pressure
• Rapid heartbeat
• Fever
• In the most serious cases, delirium or unconsciousness

†adapted from https://www.mayoclinic.org/diseases-conditions/dehydration/symptoms-causes/syc-20354086

Gauging How Much You Need

Does it really make sense that each of us needs 8 glasses of water per day, given we come in different ages, sizes, and body compositions and engage in different levels of activity?

Perhaps not. I think this idea derives from an estimate of daily physiological water loss, thought to be about 2 litres for an “average” person.

Other ways of calculating water intake are based on body weight. For example, the idea that half your weight in pounds equals the number of ounces you should drink. This means a person who weighs 130 pounds should drink 65 ounces – or about 8 glasses that are 8 ounces each (which ties into the 8 glasses a day rule of thumb).

None of these methods is perfect for ensuring good hydration. Age and stage of life have a big impact on the amount of water needed as well as perceptions of thirst.

For example, children and the elderly are less likely to experience thirst even when they are dehydrated, and pregnant women may need to drink more as their weights (and blood volumes) increase during their pregnancies.

Also, the thirst and urine colour methods evaluate water intake after the fact. That’s not too helpful if your goal is to prevent dehydration.

Our suggestion:

Combine methods to estimate how much water you need. Start by using the weight calculation to determine an estimate of what you should drink on an average day. Increase your intake if you’ve been perspiring or drinking “dehydrating” fluids that contain caffeine or alcohol. Make a quick check of the colour of your urine before you flush and adjust your intake accordingly. Lastly, drink some water if your mouth feels dry.