The ideal amount of water to keep the body hydrated is not universal.

posted on 11/25/2022 06:00


(credit: FRED TANNEAU)

Eight glasses of water a day. Who has never heard that this is the ideal amount to stay hydrated? The problem is that, behind the recommendation, there is no scientific support. This is what sustains a study published in the journal Science, carried out with data from more than 5,600 people. According to North American, Asian and European researchers, the volume needed to perform body functions is individual and depends on each person’s metabolism.

The results were collected in 26 countries (Brazil was left out), and the study covers data from babies aged just 8 days to elderly people aged 96 years. The average daily water intake varied greatly: from 1 liter to 6 liters, and in some cases, scientists recorded up to 10 liters of consumption per day. “There is no average. Many things are related to differences in the intake and elimination of water from the body”, says one of the authors of the study, Dale Schoeller, from the University of Wisconsin-Madison, in the United States.

The professor of nutritional sciences has been studying the relationship between water and metabolism for decades. “This work is the best we’ve done so far to measure how much water people actually consume on a daily basis and the key factors that drive water turnover in and out of the body,” says Schoeller. According to him, previous studies were largely designed based on self-report of participants or observation of a very small number of volunteers, not being representative of the majority of people.

Now, the researchers objectively measured the time it took for the water to move through the participants’ bodies. Volunteers drank a set amount of so-called “labelled water,” which contains traceable isotopes of hydrogen and oxygen. Isotopes are atoms of a single element that have slightly different atomic weights, so scientists can tell them apart in a sample.

“If you measure the rate at which a person is eliminating these stable isotopes through their urine over the course of a week, the hydrogen isotope can tell you how much water they are replacing. are burning,” explains Schoeller. The researcher explains that his laboratory at the North American university was the first to use labeled water in the study of human metabolism.


water exchange

In total, more than 90 researchers participated in the study, led by Yosuke Yamada, who did postdoctoral work in Schoeller’s laboratory and today heads the National Institute of Biomedical Innovation, Health and Nutrition in Japan. John Speakman, professor of zoology at the University of Aberdeen in Scotland, co-led the study. The researchers collected and analyzed participant data by comparing environmental factors — such as temperature, humidity and altitude of the cities where the volunteers lived — with measured water volume, energy expenditure, body mass, gender, age and degree of physical activity.

In men, body water turnover peaked at age 20, while women maintained a plateau from 20 to 55 years. Newborns were the ones with the highest water exchange rate, replacing 28% of the water in their bodies daily. According to the study, the participants’ level of physical activity and physical fitness were the items that most influenced differences in renewal, followed by gender, country’s human development index (which measures socioeconomic status) and age.

On average, men and women with similar profiles differ by about half a liter of body water. A 20-year-old non-athlete who practices moderate physical activity, weighs 70 kg, lives at sea level in a well-developed country, in an average air temperature of 10ºC and a relative humidity of 50% would absorb 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 place would consume 2.7 liters, exemplify the authors.

Doubling the energy used by the body increases the daily volume of water by about 1 liter. Fifty extra pounds of body weight add 0.7 liters per day. A 50% increase in humidity increases consumption by 0.3 liters. Also, athletes use about 1 liter more than non-athletes.

Social aspects

In all, the lower the Human Development Index (HDI) of the country of origin, the more water a person spends in a day. “These people in low HDI countries are more likely to live in areas with higher average temperatures, are more likely to do manual labor and less likely to be inside a climate-controlled building during the day”, justifies Schoeller. “That, plus less access to a sip of clean water whenever you need it, increases water turnover.”

According to the co-author, the measurements made in the study could improve the ability to predict future water needs, especially in circumstances of calamity due to the scarcity of the liquid. “The better we understand how much people need it, the more prepared we will be to respond to an emergency. Determining how much water humans consume is of increasing importance because of population growth and increasing climate change,” Yamada said in a statement. “As water renewal is related to other important health indicators, such as physical activity and body fat percentage, it has potential as a biomarker for metabolic health”, he adds.

According to Tamara Hew-Butler, a sports scientist at Wayne State University in the United States, overhydration can be just as dangerous as dehydration. Author of several studies on water and body metabolism, she agrees that eight glasses a day is not a scientific parameter. “It’s not clear where this came from. Perhaps they are misinterpretations of health guidelines”, she ponders. “If you’re thirsty, you need to drink water. But if you’re not, you don’t. The brain has sensors that identify the need for water replacement. Saying that everyone needs eight glasses a day to be healthy is like saying that everyone world needs to eat a 2,000-calorie diet,” he says.

According to Hew-Butler, the liquid lost daily needs to be replaced, but not necessarily with glasses of water. “Almost everything you drink or eat contains water, which makes up for that daily loss. Coffee, tea, soup, fruit, vegetables — it all counts,” he says.

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