Hydration

At a glance:

• 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 their water dietary reference value.
• We can get fluid from several 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 older or ill adults 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 (such as 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.

The importance of hydration

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.

Fluid in the body is lost throughout the day via water in urine and faeces as well as smaller amounts through the skin and lungs. The replenishment of these fluids is referred to as hydration. 

What are the biological actions of water?

Although humans can survive for several 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 intake 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 200ml glasses or 8 x 150ml 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 assumes 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:

Table 1: Fluid requirements for adults

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

Fluid intake 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:

_{Source: EFSA (2010) Scientific Opinion on Dietary Reference Values for water}

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 change the concentration of urine accordingly. The sensation of thirst is increased when low levels of dehydration are detected to stimulate fluid intake. The minimum amount of water needed to excrete waste products from the body is decided 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 (450ml/day on average, depending on air temperature, humidity, currents and clothing) and lungs (250-600ml/day on average, depending on physical activity level) and in faeces (100-200ml/day on average, can increase with higher fibre intake).

If fluids are not replaced via drinking or eating 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.

Water is crucial to every bodily action and maintenance of hydration is essential to life. Making sure we drink enough is important. Yet when we discuss diets healthy hydration is often overlooked.

Ayela Spiro, Nutrition Science Manager, British Nutrition Foundation

Implications of water overconsumption

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 symptoms such as fatigue, dizziness, confusion 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.

What are the best 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 (such as coconut water) 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

Pregnant women should limit caffeine intake to 200mg/day. Babies and toddlers should not be given drinks containing caffeine

Helena Gibson-Moore, Nutrition Scientist, British Nutrition Foundation.

Regularly consuming drinks high in energy and sugars (such as 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 help to minimise any dehydrating effects. It is important to take account of safe guidelines for drinking. Learn more about the safe guidelines by reading our FAQs on alcohol.

What are the effects of free sugars in drinks?

‘Free sugar’ is any sugar added to a food or drink. 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. You can read the Carbohydrates and Health report on SACN's website.

Studies show 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 100ml. 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.