How much water should you drink? A scientific assessment.

Water: Mysterious and marvellous

Water needs to be consumed according to need, not rules, so knowing when and how often to drink it involves staying once step ahead of thirst.

Author: Professor David Cameron Smith

(Originally published in CHOICE Health Reader, June 2013, page 3.)

Water is essential for life. The right to clean water, along with food, are basic human rights endorsed by the General Assembly of the United Nations. Many people, however, continue to live in areas where the scarcity or pollution of water makes daily life a struggle.

Water makes up between 30-70% of total body mass, depending upon levels of body fat. Of the parts of the body that are not comprised of fat, including the muscle, brain and internal organs, water is on average 73.2% of the total weight. Within each of these tissues the level of water is carefully managed to ensure optimal cellular function.

Managing water levels in cells is an ongoing process where electrolytes (salts) are continually pumped in or out of cells to control the osmotic pressure. Water moves from areas of low electrolytes (and hence high water concentration) to areas of high electrolytes (and lower water concentration).

Managing water in the body is even more complex because it is continually lost through a variety of ways. Everyday water is lost in respiration (expired breath), urine, faeces and sweat. Balancing this loss is water that is consumed in liquids or as part of foods, like some fruits and vegetables.

The desire for fluids is driven by changes in blood volume and the concentrations of electrolytes. Both mechanisms are not activated by subtle changes in hydrated status, so thirst is often felt well after considerable amounts of water are lost from the body.

Consumption guidelines

The wide variation in physical activity, climate and body size make prescribing required water each day extremely difficult. Given the widespread (and sometimes inaccurate) reporting that inadequate hydration impairs mental and sporting performance, it has become commonplace to accept the dogma that humans need ‘at least eight glasses of water a day’. There is no scientific basis to this statement.

Fluid intake, preferably water, needs to be adaptable and increase when water loss is likely to be greater. Continuous swigging of water from an ever-present water bottle has no advantages, particularly in the absence of exertion in an ambient environment. In fact, there is some evidence that athletes perform slightly better by running themselves ‘a little dry’.

Daily fluid needs remain difficult to determine and fluid intake should be dynamic and not rigid. Water has few health risks, although there is a small risk of hyponatraemia (when the amount of water in the body dilutes the electrolytes enough to affect heart rhythms). Only dangerous in extremes and found mainly in endurance athletes, hyponatraemia is unlikely in the general population. The other side effect of excessive drinking is frequent urination. Exactly what constitutes enough water depends on your day, so drink responsibly and according to your body’s needs.

For reference: Rush EC et al. Water: neglected, unappreciated and under researched. European Journal of Clinical Nutrition 2013; Jan 30. doi 10.1038/ejcn.2013.11. [Epub ahead of print].