October 28, 2019


Water makes up over 70% of the Earth's surface. Most of it is salty. Humans are made up of between 45% and 78% water at any given time in our lives with higher volumes at younger years. Muscles are 70% water and fat stores are 10-40% water. (Reibi et. al. 2013)

Water is vital to our survival. We can survive for many weeks without food, however, without water, death will creep up in days. (Popkin et. al. 2010)

We obtain 70 to 80% of our water from fluid intake and the remaining 20 to 30% from food based water. There are all kinds of recommendations out there on how much water to drink daily. Is it 6 to 8 cups of pure water a day? 4 to 6? In truth, the answer to this question is very complex as we are all unique in our day to day activities and our physiology as well.


After reviewing the meaningful scientific literature, here is what I found: According to one good study, a typical sedentary adult needs to take in roughly 2.5 liters of water a day to maintain euhydration. (Reibi et. al. 2013) The best study and most comprehensive analysis for this complex issue states that 1.8 liters per 24 hours is ideal for the average adult. (Armstrong et. al. 2018) The problem with this or any generalized number, as the authors point out, is that we are all unique and this is a general rule which, like the bell curve, will leave the outliers getting too much or too little water for their needs at this intake.


The theory is that in order to maintain this euhydration status, a person must intake as much water as they release through everyday activity. Exercise, illness, ambient heat and many other variables can cause a person to need more water to stay in balance.

"National Academy of Medicine (NAM) publication, which presented dietary reference intakes for water, included a lengthy review of water balance studies and water needs (i.e., using the stable isotope of water D2O) of children and adults. However, this report concluded that: (a) individual water requirements can vary greatly on a day-to-day basis because of differences in physical activity, climates, and dietary contents; and (b) there is no single daily water requirement for a given person." (Armstrong et. al. 2018)

What are the known dehydration levels and symptoms?:
"Dehydration is the adverse consequence of inadequate water intake. The symptoms of acute dehydration vary with the degree of water deficit. For example, fluid loss at 1% of body weight impairs thermoregulation and, thirst occurs at this level of dehydration. Thirst increases at 2%, with dry mouth appearing at approximately 3%. Vague discomfort and loss of appetite appear at 2%. The threshold for impaired exercise thermoregulation is 1% dehydration, and at 4% decrements of 20-30% is seen in work capacity. Difficulty concentrating, headache, and sleepiness are observed at 5%. Tingling and numbness of extremities can be seen at 6%, and collapse can occur at around 7% dehydration. A 10% loss of body water through dehydration is life-threatening." (Grandjean A. WHO Book Chapter)

When we think of these levels of dehydration, we usually think of an infectious disease causing a loss of fluids and a reduced intake, like cholera or noroviral diarrheal illness. These diseases are a major cause of death worldwide through water loss. At home, heat stroke in the intense summers during sporting events or children left in cars are another wakeup call for us as Americans. These issues are easily prevented through adequate hydration, safety precautions and rest.


Recent literature suggests that even mild dehydration - a body water loss of 1-2% - can impair cognitive performance. There are many other side effects of mild dehydration chronically including: the disruption of how water acts as a transporter of nutrients, regulates body temperature, lubricates joints and internal organs, provides structure to cells and tissues, and can help preserve cardiovascular function. Water consumption may also facilitate weight management. Water deficits can impact physical performance. (Reibi et. al. 2013)

Not knowing exactly what is the ideal amount because there is as of yet no true one size fits all answer, how do we stay adequately hydrated in any scenario? Clearly drinking some adequate level of water is almost always the simple answer.The volume is the variable. In moderate to severely dehydrated individuals, using an oral rehydration solution can be useful again with a volume variable.

The World Health Organization publishes a recipe for rehydration as follows:


*Six (6) level teaspoons of Sugar
*Half (1/2) level teaspoon of Salt
*One Litre of clean drinking or boiled water and then cooled - 5 cupfuls (each cup about 200 ml.)
Preparation Method:
*Stir the mixture till the salt and sugar dissolve.

The bottom line: There is no straightforward answer to how much. I could sit here and say that you need 1.8 liters per day or 2.5 liters per day as both study authors recommend, however, this is not how physiology works in my mind. My rule of thumb for hydration when you are not sick with kidney or heart disease is to have clear urine. If your urine is yellow, then drink more water. If you feel thirst, then you are already behind in your intake and need a cup or two immediately. I think that drinking a glass of water immediately upon awakening in the am makes great logical sense because of the long pause in intake overnight. Eating foods throughout the day like watermelon, cucumbers, strawberries, cantaloupe, celery and tomatoes are a lovely and tasty way to add water.

Athletes should drink often during games and practice especially on hot days. At least one 12 ounce rehydration solution with as much free water as possible is warranted on hot days and heavy (2 hour plus) workouts.

Breastfeeding and pregnant mothers need to drink adequate volumes to maintain their and their infant's hydration status. I recommend carrying water around with you all day as a reminder.

What a hot summer it was!

My take home point today: Think water!

Dr. M


Reibi ACsMs Health Fit Journal Article
Armstrong Nutrients Article
Popkin Nutrition Reviews Article
Grandjean WHO PDF Article
CDC Water Fact Sheet