Drinking recommended two litres of water per day likely too high, research reveals

  • Largest study to date into water turnover, total water intake and water loss, which is closely related to water requirement
  • Research produces equation for predicting water turnover which can be used to anticipate the effects of future changes such as climate and population demography

The common suggestion that humans should drink two litres of water (or eight 8-ounce glasses) per day is probably too high for most people in most situations, new research from the University of Roehampton London has shown.

The biggest study of its kind comprised over 5,600 participants aged 8-96 from 23 different countries and analysed participants’ water turnover, the total intake and water loss movement of the body which is closely related to our water requirement. While health experts including the European Food Safety Authority (EFSA) recommend an adequate total daily water intake of 2 litres, the study found this is likely too high for most people in most situations and a one size fits all policy for water intake is not supported by the research data.

The product of a large multinational collaboration, the research revealed that water turnover is higher in hot and humid environments and at high altitudes, as well as among athletes, pregnant and breast-feeding women and individuals with high levels of physical activity. The biggest factor however was energy expenditure. The highest water turnover was observed in males between the ages of 20 and 35, which is the group with the highest energy expenditure, whose water turnover averaged 4.2 L/day. Thereafter, it decreased with increasing age, averaging only 2.5 L/day in males in their 90s. Among women, the average water turnover at age 20 to 40 was 3.3 L/day, and also declined to around 2.5 L/d by the age of 90.

The main outcome of the paper was a general equation for predicting water turnover which can be used to anticipate the effects of future changes such as climate and population demography. This will help countries anticipate their future water needs. The data revealed that developing countries had higher water turnover than developed countries, as developed countries are more likely to use air conditioning and heating to buffer individuals from increased water demands.

It should be noted that water turnover is not equal to the requirement for drinking water, with surface water exchange, water produced from metabolism and water from food reducing humans’ amount of required water intake per day.  The research also sheds light on the underestimated role of food, which accounts for more than half of the standard 3.6L/day water intake within our bodies. It found that a typical male person in the US or Europe should have around 1.5 to 1.8 L/day coming from water. For women, this should be 1.3 to 1.4 L/day. Older people will generally require less than this, while hot climates, being pregnant or breast-feeding and greater physical activity will require an increase this water demand.

Professor Lewis Halsey from the University of Roehampton London said: “Water is essential for human survival and measuring our exact water requirement intake for our bodies continues to remains a challenge. This research sheds light as to how factors including climate, age, physical activity, pregnancy and water intake of food can determine how much water intake we actually need. It is an important finding that can help aid us to create global public health policies regarding the provision of drinking water and water rich food as concerns surrounding climate change, clean drinking water and global water security continue to grow around the world.”

Mary Henderson, PhD student at the University of Roehampton London said: “The majority of previous research into water requirements for humans has depended on subjective questionnaires applied to relatively small numbers of people. By collaborating with scientists from across the world, the study provides the most comprehensive look into people’s water intake requirements to date and is a big step forward in predicting future water needs.”

Dr. Yosuke Yamada, Section Head of National Institute of Biomedical Innovation, Health and Nutrition, who was a leader of the work and co-first author on the paper said: “The equations we have generated to predict water turnover will be of great benefit in modelling global water requirements. This work could not have been accomplished without the international collaboration of over 90 researchers.” 

The research paper, “Variation in human water turnover associated with environmental and lifestyle factors” has been published in Science and can be read in full here.