Water UK

Water requirements in adults

Water is the most important nutrient of life.(1) It is the main constituent of the body and forms 50-60% of body weight.(2) The exact amount varies with age and sex and also depends on body fat content. Men have around 60% of body weight as water and women, 52% of body weight. The lower figure for women is because they have more body fat. One third of body water is outside the cells (extracellular), eg blood, fluid between cells, fluid in lymph vessels etc, and two thirds is inside the cells (intracellular). These extracellular and intracellular compartments are separated by cell membranes that are freely permeable to water movement. This allows water to move into and out of the cells in order to maintain osmotic equilibrium.(3,4)

Water is vital to many functions in the body. These include maintaining cell structure, forming a solvent within which chemical reactions in the body can take place, physically transporting other nutrients and oxygen through the body via the bloodstream, transporting white blood cells to fight infection via the lymphatic system, and enabling the body to get rid of waste products via the excretory systems, such as through the formation of urine.(5)

Body water gains and losses

The main sources of water for the body are the food we eat and the fluid we drink. Water taken in via the mouth is absorbed throughout the gastrointestinal tract, mainly in the jejunum, but also in the colon. The other source of water for the body is metabolic water. This water is the product of normal body metabolism, and arises as a by-product from the oxidation (breakdown) of other food substances such as carbohydrates, protein and fats. These food substances produce different amounts of water when they are broken down in the body, so for example:

•1g of carbohydrate produces 0.60g water
•1g of protein produces 0.41g water
•1g of fat produces 1.07g water(2)

Water is lost from the body as urine, in faeces and by evaporation from the skin and lungs (the latter two make up what is called "insensible water loss"). More water is lost from the skin and lungs in high temperatures, at high altitude and when the air is dry. Even in the absence of visible perspiration, approximately half of water loss occurs through the lungs and skin.(6) Water loss through the skin is usually about 800-1000ml per day. In a hot climate, water loss of 500ml per hour is not unusual, but sweating rates can be as high as 2500ml per hour. Expired air is saturated with water vapour and the average water loss from this source is about 300ml per day - this figure may increase if the air is very dry, or during hyperventilation.(2) Urine output can range from 1-2 litres per day, but can be more when large volumes of fluid are consumed. Varying urine output is the main method by which the body regulates net water balance in response to a wide range of fluid intakes and losses.(1)

Table 1: Example of water balance in the body in a temperate climate (5)

Daily water requirements

The normal daily turnover of total body water is approximately 5-10 per cent in adults.(1) The amount of water required by the body can vary markedly, depending, for example on dietary factors, physical activity level, environmental conditions, metabolism and health status. The kidneys also require a minimum amount of water to excrete soluble waste products. Studies indicate that daily water turnover is 3.3 litres for sedentary men and 4.5 litres for active men. For more active adults, particularly those living in a warm environment, daily water need can increase to about 6 litres.(1) It is therefore difficult to estimate a general water requirement, because of the wide variability within and between individuals.

A review into the origins of advice regarding daily water requirements found that a recommendation from the US National Research Council may have been the source of advice to "drink at least eight glasses of water a day".(7) In 1945, the Food and Nutrition Board of the National Research Council had advised: "A suitable allowance of water for adults is 2.5 liters daily in most instances. An ordinary standard for diverse persons is 1 milliliter for each calorie of food. Most of this quantity is contained in prepared foods." It could be that the last sentence may not have been heeded, and the recommendation was therefore interpreted as eight glasses of water to be drunk each day. (1 glass/cup - approx. 8 ounces = 240ml; therefore 8 glasses = approx. 1920ml).

The National Research Council updated its advice in 1989, recommending that for practical purposes the water requirement for adults could be estimated as "1ml per kcal energy expenditure for adults living under average conditions of energy expenditure and environmental exposure" - thus a person eating 2900kcal would require 2900ml of water.(6) It is interesting to note that on this occasion, the Council did not comment on the source of the water required. This may have perpetuated the misconception that all of this water requirement must be in the form of drink.(6) Whereas, in accordance with the table above, if an adult gains in the region of 1 litre of their 2500ml water requirement from food, and metabolic water contributes a further 250ml, this leaves only 1250ml to be consumed in the form of drink.

In spite of this, some authorities(8,9) still advise that we should drink "six to eight glasses" or "2500-3000ml" of water per day. However, this guidance may have been influenced by the additional recommendation made by the Food and Nutrition Board of the National Research Council in 1989, that as "there is so seldom a risk of water intoxication … the specified requirement for water is often increased to 1.5ml/kcal to cover variations in activity level, sweating, and solute load."(6)

In 2004 the National Academies, Institute of Medicine, Food and Nutrition Board issued revised recommendations on the Dietary Reference Intakes for Electrolytes and Water.(10) The Panel on Dietary Reference Intakes undertook an extensive review of the literature in addition to conducting surveys as part of their review of this subject. They commented that “although a low intake of total water (which includes a combination of drinking water, water in beverages, and water that is part of food) has been associated with some chronic diseases, the evidence is insufficient to establish water intake recommendations as a means to reduce the risk of chronic diseases. Instead, an Adequate Intake for total water is set to prevent deleterious, primarily acute, effects of dehydration, which include metabolic and functional abnormalities.” Based upon their review of water balance studies for inactive adults in temperate climates, the panel recommended that the minimal water requirement should be approximately 1 to 3.1 litres per day to replace respiratory, urinary, faecal and insensible fluid losses. Since individual water requirements can vary greatly, even on a day-to-day basis, because of differences in physical activity, climate and diet, they recommended the following Adequate Intake for water:

Table 2: Recommended Adequate Intake for water, United States (10)

*The panel noted that higher intakes of total water would be required for individuals who were physically active or exposed to hot environments.

This level corresponds to about 1.31 ml/kcal for men and 1.2 ml/kcal for women. It should be noted, however, that these recommended intake levels are based on studies of the US population. Recommended intakes based on other populations may be different, at least in part because of cultural differences in drinking patterns. For example from the 1900s until the 1990s, the German population was advised by scientists to keep their total fluid intake low. Over the last 15 years there has been a trend to increase water intake by adults and children. Nevertheless, in Germany studies of 24-hour urine samples showed that more than 3 per cent of the subjects were under-hydrated, compared with the US population in which even the subjects with the lowest water intake were probably still adequately hydrated over the 24 hour period. This difference in the general state of hydration between the US and German populations is reflected in the corresponding recommendations. Those based on the German population, called “Guiding Values” for German children and adults, range from 1.01 to 1.08 ml/kcal.(11)

Outlined below are some other examples of advice from various authorities regarding the recommended daily intake of water.

The UK Food Standards Agency recommends: "In climates such as the UK, we should drink approximately 1.2 litres (6 to 8 glasses) of fluid every day to stop us getting dehydrated. In hotter climates the body needs more than this.”(12)

The advice from the British Dietetic Association is: "In a moderate climate like ours, most of us need around 6-8 glasses of fluid a day; for example, water, milk, fruit juice, tea or coffee, to keep the balance right. However, if we have sweated a lot, because it's hot or we've been exercising, our requirements increase; a good guide is to have an extra one litre of water for every hour of strenuous exercise."(8)

The World Health Organization states that "the 'absolute minimum' quantity of water to sustain hydration remains elusive, as this is dependent on climate, activity level and diet". WHO notes that some hydration needs are met through fluid obtained from food, however they disregard this contribution in their recommendation of daily water requirements, because, on a global basis, "the proportion of fluid obtained from food may vary significantly in response to diet and culture from negligible to all hydration needs". They note that allocating the full hydration component to drinking water alone may over-estimate the quantity of water required, but "this is believed to be no more significant than the variation likely to occur due to activity levels and temperature." The WHO recommendations for daily requirements of drinking water are given in the table below.(9)

Table 3: WHO recommendations for daily requirements of drinking water

Last updated: January 2007

1. Sawka MN, Cheuvront SN and Carter III R. Human water needs. Nutr Rev 2005;63:S30-S39
2. Principles of Human Nutrition. Ed M Eastwood. Chapter 8: Water, electrolytes, minerals and trace elements. London: Chapman & Hall 1997
3. Electrolytes, body fluids and acid base balance. Ed R Eccles. London: Edward Arnold 1993
4. Nephrology and hypertension. Ed CC Tisher, CS Wilcox, 4th ed. Philadelphia: Lippincott Williams and Wilkins 1999
5. Food Science, Nutrition and Health. Ed BA Fox, AG Cameron. 6th Edn. London: Edward Arnold 1995
6. Food and Nutrition Board. Recommended Dietary Allowances. 10th ed. Washington, DC: National Academy Press 1989
7. Valtin H. "Drink at least eight glasses of water a day." Really? Is there scientific evidence for "8x8"? Am J Physiol Regul Integr Comp Physiol 2002;283:R993-R1004
8. The British Dietetic Association. Food Facts. Fluid: Why you need it and how to get enough. BDA November 2003
9.Howard G, Gartram J. Domestic water quantity, service level and health. World Health Organization: Geneva 2003
(www.who.int/water_sanitation_health/diseases/wsh0302/en/print.html)10. Dietary Reference Intakes for Water, Potassium, Sodium, Chloride and Sulfate (2004) Institute of Medicine of the National Academies. Washington DC: The National Academies Press (http://books.nap.edu/catalog/10925.html)
11. Manz F and Wentz A. Hydration status in the United States and Germany. Nutr Rev 2005;63:S55-S62
12. Food Standards Agency (accessed Jan 2007): www.eatwell.gov.uk/healthydiet/nutritionessentials/drinks/drinkingenough/