The often cited notion that a person needs to drink at least eight glasses of water, the equivalent of two liters, to meet body needs, has wilted under closer scrutiny. A new study shows that the amount of water a person needs to intake daily differs widely and is conditional on several external and internal factors.
Even before the new study, questions have been raised on the appropriateness of using ‘eight glasses’ as a health guideline, among others because very often people did not consider the additional water they consumed daily through beverages and from the food they eat.
For the new study, an international team of more than 90 researchers, headed by scientists at the University of Wisconsin in the United States and the University of Aberdeen in Scotland, measured how much water people actually consume on a daily basis — which is the turnover of water into and out of the body — and looked at the major factors that drive water turnover. It needs to be added that the study does not suggest any new guideline figure, instead it only establishes that the daily water needs of individuals differ widely.
The study measured the water turnover of more than 5,600 people from 26 countries, with ages ranging from 8 days to 96 years old, and found daily averages on a range between 1 liter per day and 6 liters per day.
Previous studies of water turnover relied largely on volunteers to recall and self-report their water and food consumption, or were focused on small groups, such as young volunteers or male soldiers working outdoors in desert conditions, which was not representative of most people.
The new research objectively measured the time it took water to move through the bodies of study participants by following the turnover of ‘labeled water’ — water containing trackable hydrogen and oxygen isotopes. Isotopes are atoms of a single element that have slightly different atomic weights, making them distinguishable from other atoms of the same element in a sample.
By measuring the rate at which a person is eliminating those stable isotopes through their urine over the course of a week, the hydrogen isotope can reveal how much water they are replacing, while the elimination of the oxygen isotope can indicate how many calories they are burning.
The researchers also collected and analyzed data from participants, comparing environmental factors, such as temperature, humidity and altitude of the participants’ hometowns, to measured water turnover, energy expenditure, body mass, sex, age and their athlete status. The researchers also incorporated the United Nations’ Human Development Index, a composite measure of a country that combines life expectancy, schooling and economic factors.
Water turnover volume peaked for men in the study during their 20s, while women held a plateau from 20 through 55 years of age. Newborns, however, turned over the largest proportion daily, replacing about 28 percent of the water in their bodies every day. Physical activity level and athletic status explained the largest proportion of the differences in water turnover, followed by sex, the Human Development Index, and age.
All things equal, men and women differ by about half a liter of water turnover. As a baseline of sorts, the study’s findings expect a male non-athlete (but of otherwise average physical activity) who is 20 years old, weighs 70kg, lives at sea level in a well-developed country in a mean air temperature of 10 degrees C and a relative humidity of 50 percent, would take in and lose about 3.2 liters of water every day. A woman of the same age and activity level, weighing 60 kg and living in the same spot, would go through 2.7 liters (91 ounces).
The researchers also found that doubling the energy a person uses will push their expected daily water turnover up by about a liter. Fifty kilograms more body weight adds 0.7 liters a day, while a 50 percent increase in humidity pushes water use up by 0.3 liters. Athletes use about a liter more than non-athletes.
The researchers found ‘hunter-gatherers, mixed farmers, and subsistence agriculturalists’ all had higher water turnover than people who live in industrialized economies. In all, the lower the person’s home country’s Human Development Index, the more water they go through in a day.
The study reasons that people in low HDI countries are more likely to live in areas with higher average temperatures, more likely to be performing physical labor, and less likely to be inside in a climate-controlled building during the day. In addition, they are less likely to have access to a sip of clean water whenever they need it, which makes their water turnover higher.
Measurements from the new study are expected to improve the ability to predict more specific and accurate future water needs, especially in dire circumstances, such as in places where entire regions have been exposed by a calamity to water shortages. More accurate estimates on the water needs of people caught up in such circumstances, allows the authorities to be better prepared to respond in an emergency.
Advance knowledge of water needs also aids in preparing to meet long-term water requirements and understand the short-term health concerns of affected people.
How much water humans consume is also of increasing importance due to growth in global population and rise in climate change. And also because water turnover is related to other crucial indicators of health, like physical activity and body fat percent, it has the potential to serve as a biomarker for metabolic health.