How Much Water Do We Really Need?

The Earth consists of about 70% water, our bodies are more than 65% water and our brains are approximately 75% water but how much do we need in our diets?

Health care professionals often tell us we need 8 glasses of water a day; however the origins of this guideline remain unclear. In 1945 the Food and Nutrition Board of the National Research Council stated: “A suitable allowance of water for adults is 2.5L/day. An ordinary standard for diverse persons is 1mm for each calorie of food. Most of this quantity is contained in prepared food”. It seems plausible that the last sentence was not heeded and the recommendation was misunderstood and then interpreted as eight glasses of water to be drank every day.

Heinz (2002) recommended that people drink when they feel thirsty and reported that caffeinated drinks can be valuable in adding to our hydration status.  In February 2004, the Institute of Medicine (IOM) issued new recommendations agreeing with Heinz, disclaiming the eight-glasses-a-day recommendation. They concluded that healthy adults can use thirst as a determinant of their water needs. These recommendations are based on the average person – excluding those with medical conditions, those enduring prolonged physical activity or living in extreme conditions. The IOM advised that ~80% of overall water intake comes from all kinds of beverages and the remaining ~20% from food (Institute of Medicine 2005).

Inadequate hydration is likely to have many negative effects on the body, primarily causing a reduction in physical and cognitive ability. A reduction in fluid levels of 4% causes a shocking decrease in performance of 20-30% which results in difficulties in concentration, headaches, irritability and sleepiness (WHO 2005). Dehydration often causes an increase in body temperature and respiratory rates, affecting cardiovascular function. Water losses of 10% or more are likely be life-threatening (Scientific Committee 2007), commonly causing fatal consequences such as stroke and inadequate cardiac output which eventually may lead to organ failure (Bouchama and Knochel, 2002).

Caffeine can have a diuretic effect, thus resulting in decreased hydration (Riesenhuber et al. 2006).  Regular and moderate consumption does not result in dehydration (Maughan & Griffin 2003) but intake of >600mg/day (an average of 6 cups of coffee a day) has to be compensated for in water intakes. Alcohol also has a diuretic effect and people with usual intakes of >50g/day (an average of 4/5 drinks per day) need to take measures to compensate for this water lowering effect (Stookey 1999). The average glass of white wine has 9.3g of alcohol.

A 2010 EFSA scientific opinion report (European Food Safety Authority) supports the 8 glasses of water a day mantra we hear from many nutritionists worldwide. According to EFSA, adolescents over 14 years, older people and adults have an AI (Adequate Intake) of 2.0L/day for females and 2.5L/day for males. EFSA concludes that an adequate water intake is crucial for health and life (Panel & NDA 2010). WHO (World Health Organisation) also agrees that plenty of water is important to remain healthy, recommending that adult men and women under average conditions should have 2.9L/day and 2.2L/day (Howard et al. 2003).

Although EFSA supports 8 glasses a day they state that the AI for water intake should include water from drinking water, beverages of all kind, and from food moisture, which disclaims the myth that the “8 glasses a day” requirement needs to come from drinking water alone. Contrary to the previous belief that eight glasses of water should come from drinking water alone, EFSA concludes that it is normal for 70% to 80% to be provided by all kind of beverages.

The main conclusion drawn from this research is the importance of having approximately 6-8 250ml glasses of fluid throughout the day. As highlighted by both the figures and comments in the table below, global, recognised health organisations agree with this recommendation.

OrganisationTotal AmountFluid AmountPublic Guideline/Comments made
IOM Institute of MedicineW: 91oz/11cups or 2.7L/day


M:125oz/15coups or 3.7 L/day

These are from TOTAL fluid intake but 80% should be obtained from beverages so:


W:1.6L/day or 9 cups
M:2.1L/day or 12.5 cups

The remaining 20% should be derived from food. Prolonged physical activity and heat exposure will increase water losses and therefore may raise daily fluid needs.
Australia- National Health and Medical Research CouncilW: 2.8L/day






Solid foods contribute approx 20% of total water intake (NHS 1995). The remainder comes from water or other fluids (NHMRC 2003). Dehydration of as little as 2% loss of body weight results in impaired physiological responses and performance.
EFSA – European Food Safety AuthorityW:
AI of 2.0L/day (P95 3.1L )


M: 2.5L/day (P95 4.0L)

70- 80% from beverages


W:1.6 L/day

This is for adolescents over 14 years, elderly and adults.The AI for water intake includes drinking water, beverages of all kind, and water from food moisture.
EFSA concludes it is normal that the contribution of food to total dietary water intake is 20% to 30%, while 70% to 80% are provided by beverages.
FSAI- Food Safety Authority of Ireland8-10 cups/day – FSAI states that most of this comes from food.FSAI states that this can come from  drinks such as tea, coffee, milk and waterFSAI advices to drink enough before you feel thirsty, as thirst can be a sign that you are already becoming dehydrated.
WHO- World Health OrganizationW:
AI of



WHO states that these are the recommendations for sedentary, temperate people.This recommendation is based on a 70kg adult male and a 58kg adult woman. For physically active people/increased temperature  WHO recommends 4.5L/day for both men and women due to added losses through perspiration (Howard et al. 2003).


  • WHO (World Health Organization), 2005. Nutrients in drinking water. ISBN 92-4-159398-9.
  • Howard G, Bartram J. Domestic Water Quantity, Service, Level and Health. World Health Organization, 2003. Ref Type: Report
  • EFSA Panel, E. & Nda, A., 2010. Scientific Opinion on Dietary Reference Values for water 1. , 8(3), pp.1-48.
  • Institute of Medicine, 2005. Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate, Washington, D.C.: The National Academies Press. Available at: [Accessed February 2, 2012].
  • Heinz, V. et al., 2011. Drink at least eight glasses of water a day.’’ Really? Is there scientific evidence for 8 × 8’’? American Journal Of Physiology, (August 2002).
  • Food and Nutrition Board, National Academy of Sciences. Recommended Dietary Allowances, revised 1945. National research Council, Reprint and Circular Series, No. 122, 1945 (Aug), p. 3-18.

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