Hydration

About the role of fluid in the body and healthy hydration

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The science of hydration

Key points

  • Water is the most abundant constituent of the human body and regular fluid intake is essential for the body to function optimally.
  • Fluid requirements vary between people and according to age, time of year, climatic conditions, diet and levels of physical activity. Current UK recommendations are to drink 6-8 glasses of fluid per day. More specific recommendations about the quantity of fluid needed at different ages have been provided by the European Food Safety Authority (EFSA) in the water dietary reference value.
  • We can get fluid from a number of sources such as water and other drinks (teas, coffee, fruit juice, squash and milk) as well as the foods we eat.
  • Dehydration can impair cognitive and physical performance in mild cases, and in extreme cases can be fatal. It can also be dangerous to drink excessive amounts of water as, in rare cases, water intoxication can lead to hyponatraemia (low levels of sodium in the blood).
  • The body has sensitive mechanisms to maintain hydration status and to prompt drinking but young children and the elderly or unwell may need support to maintain adequate hydration.
  • Fluid intake is important to maintain hydration when physically active and is advised before, during and after activity. If active at higher levels for long periods (e.g. intense activity lasting more than 1 hour), drinks with added carbohydrate and electrolytes may be advisable to rehydrate more quickly and replace electrolytes lost in sweat.

Summary

Water is essential for life. It is the major component of body fluids and has many other biological roles including helping to regulate body temperature through sweating. Fluid balance in the body is carefully controlled by varying the concentration of urine and stimulating thirst.

The body loses water in urine and faeces as well as smaller amounts through the skin and lungs. If fluids are not consumed to replace water losses then the body becomes dehydrated. Mild dehydration (1-2% body weight lost) can result in headaches, tiredness, lack of concentration and reduced athletic performance.

Fluid requirements vary between individuals depending on factors such as age, level of activity, and climate. In the UK, guidance is to drink 6-8 glasses of fluid per day (roughly 1.2 litres).

We can get fluids from foods and drinks; not only water but also drinks like squash, fruit juice, soft drinks, milk) and tea and coffee. As alcohol is a diuretic, alcoholic drinks should not be considered as contributing to fluid intake.

Physical activity can increase fluid requirements and more fluids may be needed to maintain hydration.  Sports drinks with added carbohydrates and sodium are usually not needed when doing moderate activities but when doing intensive exercise for longer than an hour then commercial or home-made sports drinks may help to rehydrate more quickly, provide energy and replace sodium losses.

Biological actions of water

Although humans can survive for a number of weeks without food, they cannot normally go without fluids for more than a few days. On average, water makes up around 60% of body weight in males, 50-55% for females; though this varies depending on age and body composition (a higher proportion of body fat means that the body water content as a proportion is lower). Water is essential for the function of all the cells in the body. It is the major component of body fluids (such as lymph, saliva, bile) and has many other biological roles:

  • acting as a lubricant for joints and eyes
  • providing the medium in which most reactions in the body occur
  • acting as a cushion for the nervous system
  • helping to get rid of waste and transport nutrients (in urine and plasma respectively)
  • helping to regulate body temperature.

Fluid requirements for adults

UK Government advice on fluids is to consume 6-8 glasses per day (around 1.2 litres in total, which equates to 6 x 200 ml glasses or 8 x 150 ml glasses).

In 2010 the European Food Safety Authority (EFSA) published a Scientific Opinion on dietary reference values (DRVs) for water. These DRVs differ somewhat from UK recommendations (although it should be noted they include fluid from both foods and drinks – EFSA assume that 20% of fluid intake comes from food and 80% from drinks). Assuming a temperate climate and moderate physical activity levels, adequate intake values for those 14 years and up from EFSA’s report are as follows:

Population group

Adequate intake

Adult and adolescent females aged 14 years and over

2.0 litres/day

Adult and adolescent males aged 14 years and over

2.5 litres/day

Pregnant women

As above for women of same age + 300 ml

Lactating (breastfeeding) women

As above for women of same age + 700 ml

Those doing strenuous physical activity, or living in hotter or more humid climates may need more than this.

Fluid requirements for children

In general, older children will need more fluids than younger children due to their greater body weight. There are no UK-specific recommendations for children’s fluid intakes, apart from the general recommendation to have 6-8 glasses of fluid a day. In Europe figures were set by EFSA in 2010  as part of their DRV report on water and are as follows:

Population group

Adequate intake

Infants aged 0-6 months

100-190 ml/kg as milk

Infants aged 6-12 months

800-1000 ml/day

Infants aged 1-2 years

1100-1200 ml/day

Infants aged 2-3 years

1300 ml/day

Children aged 4-8 years

1600 ml/day

Girls aged 9-13 years

1900 ml/day

Boys aged 9-13 years

2.1 litres/day

 You can view our hydration guides for children here.

Fluid balance in the body

Fluid balance in the body is tightly controlled. The kidneys monitor the volume and composition of body fluids and can modify the concentration of urine accordingly.  The sensation of thirst is increased when low levels of dehydration are detected in order to stimulate fluid intake. The minimum amount of water needed in order to excrete waste products from the body is determined by the macronutrient and salt content of the diet.

As well as urine output, which totals 1-2 litres/day on average (but can be more if large amounts of fluid are consumed); water is also lost in smaller amounts via evaporation through both the skin (450 ml/day on average, depending on air temperature, humidity, currents and clothing) and lungs (250-600 ml/day on average, depending on physical activity level) and in faeces (100-200 ml/day on average, can increase with higher fibre intake).

If fluids are not replaced via drinking or fluid-rich foods then dehydration will set in over time. Dehydration can be potentially serious. A loss of 15-20% of body weight as water is fatal, and a loss of as little as 2% (equivalent to an overall loss of about 1.4L water in a 70kg man) can result in impaired cognitive and physiological responses and athletic performance. Symptoms of dehydration include headache and fatigue.

Babies, children and older adults can be particularly susceptible to dehydration as they may not recognise the symptoms of dehydration (headache, fatigue) or respond to thirst and need support to access and drink enough fluids.

People who are physically active for long periods, those who sweat excessively, those living in warm or humid climates will generally have higher fluid requirements and care should be taken to avoid dehydration and keep adequate fluid balance.

The body has no way of storing water and so any excess fluid consumed will be excreted in urine. Extreme overconsumption of water can, under rare circumstances, overwhelm the body’s ability to excrete the excess water and lead to hyponatraemia (low sodium levels in the blood), which can be life-threatening in severe cases. 

Hyponatraemia affects the balance of intra and extracellular fluids causing symptom such as lethargy, dizziness, disorientation and seizures. The severity of hyponatraemia depends on the volume and rate at which fluids are consumed and may be affected by other factors such as sodium losses from sweat during endurance exercise.

Sources of fluid in the diet

Dietary water can come from drinks as well as from the water contained in foods, for example in fruit and vegetables, soups and stews. It is estimated that foods contribute to about 20% of total fluid intakes.  A small amount of water is also produced via metabolism in the body.

Regarding drinks; squash, fruit juice, soft drinks, milk, dairy-free milk alternatives, plant-based drinks (e.g., coconut water, soy drinks) and tea and coffee can all count towards fluid intakes. Whilst caffeine acts as a mild diuretic (it increases urine output to a small extent), drinks that contain caffeine do contribute to fluid intakes under normal circumstances and studies have found that consuming moderate amounts of caffeine does not seem to affect hydration status (although pregnant women should limit caffeine intake to 200 mg/d and babies and toddlers should not be given drinks containing caffeine).

Regularly consuming drinks high in energy and sugars (e.g. sugary drinks, fruit juices) may result in excess energy consumption, and can contribute to weight gain and tooth decay, as well as increasing risk of type 2 diabetes, so choosing low energy and low sugars versions where possible is recommended.

Alcohol, like caffeine, also has a diuretic effect due to its action on the hormone arginine vasopressin. The level of this effect depends on the type of alcoholic drink consumed. Drinking water alongside alcoholic drinks will minimise any dehydrating effects. It is important to take account of safe guidelines for drinking.

You can view our hydration guide for adults and teenagers here.

Beverage intakes in UK adults (average grams per person per day including non-consumers)

 

Adults (19-64 years)

Older adults (65+ years)

Milk (whole, semi-skimmed, 1% and skimmed)

133g

165 g

Tea, coffee and water

1205g

1183g

Fruit juice

40g

40g

Soft drinks (not low calorie)

130g

40g

Soft drinks (low calorie)

145g

46g

Alcoholic drinks (spirits and liqueurs, wine, beer, lager, cider and perry).

216g

142g

Source: National Diet and Nutrition Survey (years 15-9 2012/13 - 2016/17)

Free sugars in drinks

The effects of drinks containing free sugars were reviewed in the report Carbohydrates and health published by the Scientific Advisory Committee on Nutrition (SACN) in 2015. On review of the evidence, SACN concluded that consumption of drinks containing sugars are associated with increased risk of type 2 diabetes and dental caries in observational studies.

Studies indicate that, compared with non-caloric drinks, the consumption of drinks containing sugars is associated with increased energy consumption in adults and with weight gain and BMI increases in children. SACN therefore recommended that the consumption of drinks containing sugars should be minimised as well as recommending a reduction in free sugars intakes from across the diet.

Policy actions to help reduce intakes have included a levy on drinks with added sugars that contain more than 5g total sugars per 100 ml. Advice on fruit and vegetable juices and smoothies, which also contain ‘free sugars’, is to keep portion sizes to no more than 150ml per day.

Pure fruit and vegetable juices and smoothies still count towards the 5 A DAY target, as they can provide a variety of vitamins and minerals, but only as a maximum of 1 portion. Sugars-containing drinks currently contribute 10% to the daily energy intakes of adults (aged 19-64 years) and a higher amount (17%) to energy intakes of adolescents aged 11-18 years. The mean consumption of sugar-sweetened soft drinks (g/day) decreased between 2008 and 2019 for all age groups in the UK by between 1% (adults 19-64 years) and 7% (children 1.5-3 years), although this change was not significant for adults aged 19-64 years and 75+ years. Over this 11-year period, the proportion of children consuming sugar-sweetened soft drinks also fell by 32 percentage points for those aged 1.5-3 years, 44 percentage points for 4-10 year-olds, 25 percentage points for 11-18 year-olds and by 20 percentage points for adults aged 19-64 years.

Fluid requirements during physical activity

Fluid requirements increase during physical activity as more fluid is lost through sweating. Dehydration can occur in people doing intense physical activity, particularly those exercising in hot climates and/or at altitude. The amount of fluid lost when exercising will vary according to the intensity of exercise, the environmental conditions and the sweat rate of the individual (which can reach 2.4L/hour under extreme conditions).

Having an adequate fluid intake before, during and after periods of intense activity is important as loss of as little as 2% of body weight as water (equivalent to a net loss of 1.4L in a 70 kg man) may impair performance.

The International Olympic Committee recommends that 1.2-1.5 litres of fluid are consumed for each kilogram of body weight lost during a sports or exercise session to restore losses and that sodium should be included in fluids consumed after physical activity which lasts for more than 1-2 hours. This is because sweat contains sodium at an average concentration of around 1 g/L (though this is highly variable). Some individuals have a high sweat rate (e.g. 3 L/hour) or particularly salty sweat and therefore sodium losses can be significant (e.g. 3-4 grams in total) after vigorous or prolonged exercise.  Thus, replacing lost sodium is important and can be achieved simply by eating a meal or snack containing salt, or by using sports drinks which contain sodium. Potassium is also lost in sweat, though in much smaller amounts, and replacing this after exercise may enhance water retention, though replacing sodium after exercise is more important than other electrolytes which are less significant.

For strenuous exercise that lasts longer than 1 hour it may be helpful to include drinks that contain a source of carbohydrate that is rapidly converted to glucose in order to provide energy. Most commercial sports drinks contain 4-8% carbohydrates and also contain some sodium in order to replace any sodium lost in sweat. Home-made versions can also be prepared using a source of sugars (e.g. sugared squash or fruit juice) and salt.

 

 

Key references

EFSA Panel on Dietetic Products, Nutrition, and Allergies (NDA) (2010) Scientific Opinion on Dietary reference values for water. EFSA Journal 8(3):1459.

NHS Choices (2017) Dehydration  https://www.nhs.uk/conditions/dehydration/

NHS Choices (2018) Water, drinks and your health. https://www.nhs.uk/live-well/eat-well/water-drinks-nutrition/

Nutrition for athletes. A practical guide to eating for health and performance: based on an international consensus conference held at the IOC in Lausanne in October 2010 / prep. By the nutrition working group of the International Olympic Committee; updated in April 2012 by Ron Maughan and Louise Burke. https://library.olympics.com/Default/doc/SYRACUSE/74010/nutrition-for-athletes-a-practical-guide-to-eating-for-health-and-performance-based-on-an-internatio?_lg=en-GB

Kenefick, R. W., Cheuvront, S. N. et al. (2012) Human Water and Electrolyte Balance. In: Present knowledge in nutrition. Erdman Jr, J. W., MacDonald, I. A. and Zeisel, S. H. John Wiley & Sons: 493-505.

National Diet and Nutrition Survey Reports https://www.gov.uk/government/collections/national-diet-and-nutrition-survey

Hargraves, M., Lockyer, S. and Chambers, L. (2018) Are energy drinks suitable for children? , Nutrition Bulletin, 43(2), 112-118. Available online at: https://onlinelibrary.wiley.com/doi/abs/10.1111/nbu.12316

Useful resources

These resources provide key information about healthy hydration at different life stages.

Healthy hydration for children 1-4 years

A resource showing healthy hydration options for children 1-4 years.

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Healthy hydration for children aged 5-11

A resource showing healthy hydration options for children aged 5-11.

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Healthy hydration for teenagers and adults

A resource showing healthy hydration options for teenagers and adults.

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Healthy hydration for older adults with poor appetites

A resource showing healthy hydration options for older adults with poor appetites.

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Consumer
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pdf

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