A recommendation to drink eight glasses of water a day is one of the most recognized dietary recommendations and it even appears on the recommendations of government websites. However, a new and stringent study on water consumption, reveals that the amount of water that one needs to consume varies widely from person to person. Some only need 1.5 to 1.8 liters a day, less than the two liters normally recommended, according to the study.
"The current recommendation is not scientifically supported at all," said Yusuke Yamada of the National Institute for Biomedical Innovation, Health and Nutrition in Japan and one of the first authors of the paper. "In fact, most scientists are not sure where this recommendation came from."
One problem is that previous estimates of water requirements tended to ignore the water content in food, which can contribute a significant portion of our overall consumption. "If you only eat bread, sausage and eggs, you won't get much water from the food, but if you eat meat, vegetables, fish, pasta and rice you can get about 50% of your water needs from food," Yamada said.
How was the study conducted?
The study was published in the journal Science, which estimated the water consumption of 5,604 people aged eight days to 96 from 23 countries. The study also examined people who drank a glass of water in which some of the hydrogen atoms were replaced by deuterium, or heavy hydrogen, which is a stable (non-radioactive) isotope of hydrogen that is naturally present in the human body and is harmless.
The rate at which the additional deuterium is eliminated reveals how quickly the water in the body evaporates, and the study found that the index varied greatly depending on the person's age, gender, activity levels and environment. Doubling the energy a person uses pushes their expected cycle up to about a liter, or four cups. Fifty pounds more than body weight adds 0.7 liters a day. A 50% increase in humidity increases water consumption by 0.3 liters. When it comes to sports, athletes use about a liter more. An average 20-year-old man of normal weight living in a temperature climate would pump and lose about 3.2 liters every day. For women, this number is closer to 2.7.
Energy expenditure is the largest factor in water turnover, with the highest values observed in men aged 20 to 35, with an average of 4.2 liters per day. This has decreased with age, averaging 2.5 liters a day for men in their 90s. Women between the ages of 20 and 40 had an average cycle of 3.3 liters, which dropped to 2.5 liters by the age of 90. Newborn babies made the largest proportion, replacing about 28% of the water in their bodies every day.
"This study shows that the common suggestion that we should all drink eight glasses of water - or around two liters a day - is probably too high for most people in most situations and a 'one-size-fits-all policy' for water consumption is not supported by this data," said Prof. John Spickman of the University of Aberdeen, co-author of the study. "I think it's a recommendation that many people just ignore and follow what their bodies are telling them."
Although drinking more water than the body requires is unlikely to be harmful to health, clean drinking water is not free to produce, the authors note. "There is a price to drink more than we should, even if it is not a health cost," said Spikeman. "If 40 million adults in the UK were following the guidelines and they drank half a liter of cleaner water than they needed every day, that's 20 million liters of water wasted every day."