How much water do we need a day?

So the actually required water intake is about...

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Water is necessary for survival, but one in three people globally lack access to clean water for drinking. Requirements for water consumption are mostly based on daily water use by the body or water turnover (WT). However, it is challenging to evaluate water needs objectively. Most earlier research relied on subjective surveys given to relatively few individuals.

A new study by Prof. John Speakman from the Shenzhen Institute of Advanced Technology (SIAT) of the Chinese Academy of Sciences (CAS) and the University of Aberdeen investigated the determinants of human water turnover in 5604 people aged eight days to 96 years from 23 countries. They used an isotope-labeling technique to follow water intake and loss in individuals.

Their study shows that the recommended water intake of eight 8-oz glasses of water per day (around 2 L/day) is too high for our actual needs in many situations. The water turnover was higher in hot and humid environments and at high altitudes among athletes, pregnant and breastfeeding women, and individuals with high physical activity levels.

However, energy use has the highest impact on water turnover. Since this group had the highest energy expenditure, the highest values were seen in males between the ages of 20 and 35. Their daily water turnover was 4.2 L on average. As people aged, this amount fell, with guys in their 90s averaging only 2.5 L/day. At ages 20 to 40, women’s average water turnover was 3.3 L/day; by 90, it had decreased to about 2.5 L/d.

Additionally, water turnover was higher in developing nations. This is likely due to the fact that developed nations’ air conditioning and heating protect people from environmental extremes that increase water demand.

Prof. Speakman said, “It should be noted that water turnover is not equal to the requirement for drinking water. Even if a male in his twenties has a water turnover of, on average, 4.2 L/day, he does not need to drink 4.2 L of water each day. About 15% of this value reflects surface water exchange and water produced from metabolism. So the required water intake is about 3.6 L/day.”

“Since most food also contains water, a substantial amount is provided just by eating. Because the water content of foods varies so much, though, working out the exact drinking water requirement is difficult.”

“For a typical man in his twenties in the US or Europe, probably more than half of the 3.6 L of water needed each day comes from food, which means that the amount that should be consumed by drinking is around 1.5–1.8 L/day. For a woman in her twenties, it is probably about 1.3–1.4 L/day.”

“Older people will generally require less than this while living in a hot climate, greater physical activity and being pregnant or breast-feeding will increase this figure.”

Prof. ZHANG Xueying from SIAT, co-first author of the study, said“Figuring out how much water humans require is significant due to explosive population growth and growing climate change. Water turnover is related to many health parameters like physical activity, body fat percent, etc., making it a new potential biomarker for metabolic health.”

Journal Reference:

  1. Yosuke Yamada, Xueying Zhang et al. Variation in human water turnover associated with environmental and lifestyle factors. Science. DOI: 10.1126/science.abm8668