No, the recommendation to drink 8 glasses of water per day is not based on evidence. The idea came from a 1945 US Food and Nutrition Board which recommended 2.5 L of daily water intake from both food and liquid sources. This was derived from the notion that 1 mL of water per calorie of food consumed seemed adequate enough (1). Over the years this was likely construed to the infamous 8 glasses of water per day standard - 8 x 8 oz being 64 total oz, or about 2 L, which is close to that 1 mL per calorie per day average. Again, this is based on zero scientific evidence.

Your body tells you when you need to drink water. The hypothalamus is a structure in the brain that regulates thirst, amongst other things. It does so by monitoring the blood’s osmolality (e.g. sodium concentration) and by receiving hormonal signals from the kidneys; a cascade set off by both blood pressure and osmolality, ultimately telling the brain that more water is needed to maintain adequate blood volume and concentration (2). Mechanoreceptors in specific blood vessels project neurons into the brainstem, and when blood pressure and volume drops this also leads to thirst regulation (2). To put it simply, your body knows via a series of mechanisms when it needs more water, and it’ll bring this into conscious awareness through the sensation of thirst.

Water requirements vary between individuals. Given the complexity of this, we can usually simplify things by relying on our thirst reflex. Keep in mind that the sensation of thirst may be diminished in the elderly and in those with damage to certain areas of the brain; thereby making a conscious effort to drink water necessary in some individuals. Water intake may also need to be modified based on environmental factors, activity level, body composition, breastfeeding status, and in certain health conditions (3). It’s always best to be prepared by carrying water with you and to speak to your doctor about your individual health concerns.

Although water is a good option it isn’t the only way to stay hydrated. A good portion of our water intake comes from food and non-plain water beverages. Yes, this also includes coffee, teas, and even alcoholic drinks (4). Foods with higher water content, including fresh fruit and vegetables, stews, soups, and sauces, will help maintain hydration throughout the day. Water is typically recommended because of its calorie-free profile, not because of any intrinsic health benefits related to drinking plain H20.

Bottom line: don’t stress about meeting a specific water quota each day. Drink when you’re thirsty, stop drinking when you’re no longer thirsty. It’s like your very own built-in water reminder app, but way more sophisticated. Plus, quenching your thirst with a tall glass of cold water is one of life’s simple pleasures, let’s not ruin it with a gallon water challenge.


Author: Emily Howard, ND, BSc


References

1. Food and Nutrition Board. Recommended Dietary Allowances. National Academy Press; 1945.

2. Leib DE, Zimmerman CA, Knight ZA. Thirst. Curr Biol. 2016;26(24):R1260-R1265. doi:10.1016/j.cub.2016.11.019

3. Myo Clinic Staff. Water: How much should you drink every day. Mayo Clinic. October 14, 2020. Accessed October 4, 2022. https://www.mayoclinic.org/healthy-lifestyle/nutrition-and-healthy-eating/in-depth/water/art-20044256#:~:text=You've%20probably%20heard%20the,a%20day%20might%20be%20enough.

4. Institute of Medicine. Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate. The National Academies Press; 2005.