How much water do we really need?

Abstract

As is becoming increasingly typical for a British summer, water was everywhere! But this year it was doing much more than just falling from the sky. Water has been appearing in the headlines of the major newspapers, as well as being featured in health and lifestyle magazines, and the question that everyone has been trying to answer has been ‘does anyone know how much water we really need?’ Water is an essential requirement for life – without water, humans can only survive for 2 to 3 days at the most. On average, water makes up 60% of body mass (range 45–75%), which is equivalent to approximately 45 l in a 70kg man and 30 l in a 55kg woman (Olsson & Saltin 1970). Every day we gain and lose water although there is a huge degree of inter-individual variation. This makes setting any blanket recommendations for the entire population very difficult. However, when asked, many health professionals or members of the general public would most likely be familiar with the old adage to drink around 6 to 8 glasses of ‘water’ a day. Indeed, many believe that we need eight glasses of pure water a day just to stay healthy. The use of such recommendations has been called into question in a recent editorial in the Journal of the American Society of Nephrology (Negoianu & Goldfarb 2008). Its authors have concluded that, in the absence of any clear evidence, there is no proof of any health benefit for drinking 6 to 8 glasses of water a day, except perhaps for people in hot climates. This editorial triggered headlines in the Daily Mail (2008a), Daily Mirror (2008), Daily Telegraph (2008), Guardian (2008) and Independent (2008) along with a story on BBC News (2008), proclaiming that there is no evidence that drinking eight glasses of water a day improves skin tone, aids dieting or prevents headaches. Further confusion has been added following news reports of tragic cases where individuals have either died or have had their health seriously compromised by drinking too much water (e.g. Daily Mail 2008b; The Sun 2008). In such instances, the consequences of following ill-advised detoxing regimens or mistakenly over-consuming water during daily life or exercise can be incredibly serious, as the body attempts to cope with the dilution of many important electrolytes.