How Much Water Do You Need Per Day?
News stories are abounding about the fact that health professionals really don’t know what they are talking about when they make recommendations regarding how much water a person needs per day. It is true that hydration needs are individual based on age, sex, activity and overall health conditions. However, Registered Dietitians have had calculations for years to determine the general amounts of fluid that is needed for a variety of individuals in a variety of situations.
In 2004, the Institute of Medicine (IOM) issued a report presenting reference intake information about water needs for Americans and Canadians. The report defined for the first time adequate intakes (AI) for total water from drinking water, beverages and food for healthy people. An AI is the recommended average daily intake level based on observed or experimentally-determined approximations or estimates of nutrient intake by a group or groups of apparently healthy people.The AI for water in this case is based on US survey data and do not apply to at-risk individuals who may need more or less hydration due to medical conditions or circumstances.
The IOM states that the average individual meets their hydration needs by quenching “thirst”. However, their studies show that healthy adults need about 91 ounces for women (from fluids and food) and men need an average of 125 ounces. About 80% of hydration needs are met from drinking beverages, so for a woman 80% of 91 ounces = 9.1 glasses per day of 8 oz. glasses. Men need about 13 cups per day between the ages of 18-30. Higher intakes of total water may be needed by those who are physically active or who live in hotter climates. And, these recommendations are for adults, not children. The IOM panel did not set an upper limit for fluids. Fluid restriction is a common medical treatment for those who have conditions such as heart disease and needs to be evaluated on an individual medical basis by a health professional.
The usual equation for hydration for children is 0-10 kg = 100 ml/kg; 11-20 kg 1000 ml + 50 ml for every kg > 10; >20 kg use 1500 cc + 20 cc/kg each kg > 20 kg. For adults, we use an approximation of 30 cc per kg with 30 cc = 1 oz. This formula approximates a lower need for fluid than the IOM recommendations but is based on body weight as opposed to US survey data. Using the 30 cc formula and a 110 (50 kg) pound woman her hydration need would equal 1,500 cc or 6.25 ( 8 oz.) glasses of fluid per day. A 180 pound man would need 10.22 cups, so clearly there is a difference of opinion here.
Whatever the case, it looks like this nutritional issue needs to be re-visited. Are our formulas for hydration needs in err or do we really need as much fluid as the IOM suggests? I think there is a definite difference between making general public health guidelines based on US survey data and making an individualized assessment of a person’s hydration needs. Health professionals are needed to consider the age, sex, wt, temperature, ventilation status, activity level, metabolic rate and other variables in considering an individual’s hydration status, which may not match the public health guidelines because of inclusion of these variables in the assessment of hydration need.
I guess the conclusion is to drink when you are thirsty. Seek a medical professional’s help when ill and more studies are needed!
About the Author
Kathy Shattler received her Master of Science degree from Michigan State University in E. Lansing Michigan in Human Nutrition. Her twenty-two years of practice includes holding positions as a Lecturer, Chief Clinical Dietitian and Program Manager. Kathy is the Founder of Nutri-Care Consulting and is currently the Nutrition Director of www.CEU4U.COM, an online continuing education management company for Registered Dietitians and Dietetic Technicians.
SandyD responds:
Posted: April 19th, 2008 at 1:22 pm →
I am 147kg (down from 155kg and still losing) and aim for a maximum of 4.5 litres of water a day. This aim freaks people out, but to me it is just basic maths. If the recommendation for the average person (65kg) is 2.2 litres, then the needs of a heavier person should increase proportionately, since we lose more fluid through sweat and elimination. My weight loss has occurred only since I drastically increased my water consumption to 3.5 to 4.5 litres a day, and with little other effort than that. I now feel so much more energetic as well. Of course as my weight decreases, I will decrease my water consumption accordingly.