How Much Water We Should Drink?
How much water we need to drink has been controversial to say the least. It has also been full of unscientific guessing. Recent research has examined the evidence, with some interesting conclusions.
Is 6-8 glasses enough?
Very few people will argue that we don’t need to drink at least 6-8 glasses of water a day. But is 6-8 glasses enough?
Water is necessary for practically all of our body’s metabolic functions. The body can survive for months without food. Water is a more urgent need.
There are many symptoms of dehydration. Many of these are sub-clinical – they aren’t readily observed. Many symptoms of dehydration are also misdiagnosed.
Research supporting need for adequate water
A 2002 study involving 20,297 people found those who drank at least five glasses of water a day were 41% less likely to develop fatal coronary artery disease.
A 1999 Harvard study of 47,909 health professionals showed that six cups of water per day reduced the risk of bladder cancer by over 50%.
A 2009 study of 58 children showed that those who drank more water performed better on cognitive tests. And there’s more.
Water consumption research
Research from Australia has reviewed the various human studies done over the past 80 years on water consumption requirements. The findings of the study has found that, even with a large variance of study protocols and parameters, the need for considerable water intake is necessary to health.
A 2012 study published in the European Journal of Clinical Nutrition provides more dimension to water consumption. The study comes from Dr. A.P Vavanti, Nutrition and Dietetics expert from the Princess Alexandra Hospital in Queensland. Dr. Vavanti and his team reviewed 89 conventional medical studies and papers from peer-reviewed medical research journals that measured and discussed water requirements for humans over the past 80 years.
Dr. Vavanti’s research found that there have been a variety of different techniques employed to measure water requirements, and these resulted in various formulae and calculations to ascertain how much water we really need to drink.
These come from a variety of different protocols, including water-body weight calculations, water-kilocalorie calculations to water-body surface area calculations.
Body surface calculations
Body surface area calculations for water requirements began in the 1930s because water needs in animals had been extensively studied using the body surface areas of the different animals. Well over 300 animal studies led to a consensus of water requirements in animals using surface area calculations.
This led to humans being referenced using similar calculations, using a proportion of milliliters of water requirement per meter squared (ml/m2) of body surface area. Multiple studies using these calculations arrived at a general consensus of 1500-1600 ml/m2 of water requirement for adults.
As an average adult has between 1.5 and 2 m2 of body surface area, this converts to from about 80 ounces a day for women to about 108 ounces a day for men.
Water need based on calories
Research has also calculated water requirements by calories burned, using the scientific kilocalorie. This research also began in the 1930s and has continued through the 90s. Initial studies concluded that about one milliliter of water per kcal was the minimal requirement – with the ranges being 1.5ml/kcal to 66 ml/100 kcal. This generalized 1 ml/kcal of minimal water requirement for adults has been supported by the National Research Council.
Children require more water per kilocalorie than adults according to the research, with the consensus being about 1.5 ml/kcal.
Large water consumption study
Meanwhile a German water consumption study of more than 2,000 adults documented average water consumption at 1.01 ml/kcal for men and 1.06 ml/kcal for women.
However, research that included the loss of water due to sweat and stool – not calculated in some of the early calculations, arrived at a 1989 calculation of 1.5 ml/kcal.
This standardization produces a usable conversion from calorie requirements to water needs. Many consider 2,000 common calories (a common calorie is roughly equivalent to a kcal) a day to be healthy, which would convert to a water need of about 3,000 ml of water per day (101.4 ounces). Active adults, however, can easily burn 2,500 or more calories a day, boosting their water requirements considerably.
These calculations produce some consistency with the surface area calculations.
Water requirement rather than drinking water
Note that these water need calculations have determined complete water requirement, not drinking water. Foods also contain water, and several studies have calculated the average water content of a typical diet, ranging from 700 to 1,000 milliliters of water (24 to 34 ounces) from foods. The consensus is that about 20 percent of our water consumption comes from the foods we eat. This leaves about 80 percent to fluids.
Fluids include both caffeinated and decaffeinated drinks, but preferably, pure drinking water.
Water need per body weight has been the most prominently used calculation among the various studies over the past century, especially among the most recent studies. Many of these also included population studies, concluding that a range of 35-45 milliliters per kilogram of body weight was an appropriate water requirement.
Bottom line water consumption
This means that a 150-pound adult would have a water consumption requirement of about 2,380 to 3,060 milliliters of water per day – equating to 80 to 104 ounces.
This comes close to the conclusions of The National Academies of Sciences, Engineering, and Medicine. Their examination of the evidence determined that fluid consumption should be (not including water in foods):
- An adult woman should drink about 91 ounces of fluids per day = 11.375 cups.
- An adult man should drink about 125 ounces of fluids per day = 15.625 cups.
Because the average adult man typically weighs more than 150 lbs, we find the 80-104 ounce estimate to be in the middle of the adult men to adult women range.
Fluid loss calculation
Dr. Jethro Kloss studied water use and fluid loss in the 1930’s.
He found that every day the average person loses about 550 cubic centimeters of water through the skin, 440 cc through the lungs, 1550 through the urine, and another 150 cc through the stool.
This adds up to 2650 cc per day in fluid loss, equivalent to a little over two and a half quarts (about 85 fluid ounces). He wasn’t too far off.
6-8 glasses is not enough
Regardless of which study we connect with, the evidence agrees that the famous six to eight glasses of water a day recommended by so many over recent years comes up a bit short.
If we assume a glass of water is eight ounces, then six glasses a day would only equate to 48, and eight glasses would equal 64 ounces. Both come up short of the conclusions of scientific research.
Vivanti AP. Origins for the estimations of water requirements in adults. Eur J Clin Nutr. 2012 Dec;66(12):1282-9.
Dietary Reference Intakes: Water, Potassium, Sodium, Chloride, and Sulfate. The National Academies of Sciences, Engineering, and Medicine. Feb. 2004.
Hydration: Why it’s so important. American Academy of Family Physicians. Acc. , 2017.
Mentes JC. Hydration management protocol. J Gerontol Nurs 2000; 26: 6–15.
Panel on Dietary Reference Intakes for Electrolytes and Water and Standing Committee on the Scientific Evaluation of Dietary Reference Intakes. Dietary Reference Intakes for Water, Potassium, Sodium, Chloride and Sulfate. National Academies Press: Washington DC, 2004.
Grandjean AC, Reimers KJ, Bannick KE, Haven MC. The effect of caffeinated, non-caffeinated, caloric and non-caloric beverages on hydration. J Am Coll Nutr. 2000 Oct;19(5):591-600.
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Adams C. Pure Water: The Science of Water, Waves, Water Pollution, Water Treatment, Water Therapy and Water Ecology. Logical Books, 2010.