Six to eight glasses of water ‘still best’
Many newspapers have reported today that “government advice to drink six to eight glasses of water a day is nothing but "thoroughly debunked nonsense" (The Daily Telegraph).
The news stories are based on an opinion piece in the British Medical Journal, in which Dr Margaret McCartney, a GP from Glasgow, discusses the evidence behind the claims that we all need to drink more water to maintain our health. Specifically, the author questions messages that are being publicised by “Hydration for Health”, an initiative sponsored by Danone, the producer of Volvic, Evian and Badoit bottled waters.
Dr McCartney also questions the advice from NHS Choices that people should try to drink about six to eight glasses of water or other fluids a day to prevent dehydration.
This article, which has not been externally peer-reviewed, contains some good points regarding the lack of evidence that there are health benefits in drinking increased amounts of water, or that people are not drinking enough water and should aim to drink more.
However, McCartney’s argument that government advice is ‘thoroughly debunked nonsense’ is flawed. The two studies on which she bases her argument looked at volumes of water much greater than the current UK advice. Both are American studies that refer to research into the ‘8x8’ rule, which promotes drinking at least eight 8-oz glasses of water a day (1.9 litres). This is much more than the ‘about six to eight glasses’ (1.2 litres) of fluid a day advocated by the UK government.
Keeping hydrated is important. It is recommended that six to eight glasses of water or other fluid are consumed everyday to replace normal water loss, rather than to obtain any broader health benefits.
Does this article 'debunk' the advice to drink about six to eight glasses?
No. The two studies on which Dr McCartney bases her argument looked at drinking volumes of water greater than the current UK advice.
Dr McCartney cites a 2002 study by Heinz Valtin to support her argument. In that study, Valtin questioned the US advice that people should aim to drink “at least eight 8-oz glasses of water a day”. Importantly, Valtin specifically examines the evidence behind the 8x8 advice, and not whether six to eight glasses a day is required. In fact, Valtin states “there is a huge difference between “somewhere around 6 to 8 glasses” and “at least eight glasses””.
The second study that McCartney cites is an editorial which says that the “classic recommendation is known as "8 x 8": eight glasses of 8 oz of liquid per day—not including caffeinated and alcoholic beverages”.
Both are American studies which refer to research into the ‘8x8’ rule, i.e. that US citizens should aim to drink at least eight 8-oz glasses of water a day (1.9 litres). This is more water than the six to eight glasses (1.2 litres) a day advocated in the UK.
How much water does NHS Choices advise people to drink?
NHS Choices’ page on Water and drinks says:
Your body needs water or other fluids to work properly and to avoid dehydration. That’s why it's important to drink enough fluids. In climates such as the UK's, we should drink about 1.2 litres (six to eight glasses) of fluid every day to stop us getting dehydrated. In hotter climates, the body needs more than this. We also get some fluid from the food we eat.
Our Health A-Z article on dehydration has more detailed advice for specific groups.