Water makes up approximately 50-60% of the adult human body. Roughly 65% of this water is within our trillions of cells, and the remaining 35% is outside our cells in blood plasma, synovial fluid (cushioning the joints), urine, vitreous fluid (shock-absorber in the eye), tears, and cerebrospinal fluid, which delivers nutrients to the brain and protects the brain and spinal cord from injury. Water is essential for digestion, breathing, temperature control, and metabolism.
Around 20% of our water intake comes via the food we eat, and a very small amount is generated through the metabolic process. The remainder comes from drinking.
According to the European Food Safety Authority (EFSA), adequate quantities of water for children are as follows:
The younger the child, the more fluid they require in relation to their body weight.
At four years old, a child should be drinking 1.2 litres per day – approximately 7.8% of their body weight. For an eight-year-old, 1.2 litres of fluid equates to 4.8% of the child’s body weight. At 13 years old, a child needs around 1.5 litres per day – approximately 3.3% of their body weight.
In anyone, of any age, the thirst reflex is triggered only when a body is already dehydrated.
For young children, though, the risk of becoming dehydrated is greater because they don’t always recognise the early signs of thirst. Therefore, it’s important for children to drink small amounts of water frequently, as dehydration can cause headaches, constipation, irritability, fatigue, reduced concentration, and reduced cognitive function.
Fluid is vital for effective respiration. In fact, our lungs are 85% water, and the average adult loses up to a litre of water, each day, just by breathing. Because children expend a huge amount of energy rushing about and vocalising, a lot of fluid is expelled from the lungs.
One aspect of dehydration in children that often gets overlooked is daytime and night-time incontinence.
Dehydration stimulates reabsorption of water from the kidneys, leading to a lower volume of concentrated urine, which can irritate the bladder and cause daytime wetting.
When a child habitually drinks very little, the bladder’s capacity to hold urine is reduced, so if a child makes a habit of drinking nothing at school, but makes up fluid intake after school, the bladder could struggle with the sheer volume of water. This situation can exacerbate an existing bedwetting problem.
When a child starts to drink more, and to drink regularly, bladder capacity will increase, and the child will pass more urine at one time but will be able to hold on for longer.
In October 2000, Education and Resources for Improving Childhood Continence (ERIC) launched a national campaign to improve the quality of provision and access to fresh drinking water for all pupils in UK primary and secondary schools.
The campaign, Water is cool in school, set out to educate school staff and parents about the importance of hydration in children. It asserted that access to drinking water is a fundamental human right and that adequate hydration is vital for good health and wellbeing.
ERIC campaigned for every school to provide fresh, mains-fed drinking water that was chilled in summer, free of charge, and readily available from hydration points around the school, including dining areas. The importance of equal opportunities and making water accessible to all was stressed.
A 2007 study of the association between children's access to drinking water in primary schools and their fluid intake showed that free access to drinking water in class is associated with better hydration. The study concluded that primary schools should promote the drinking of water in class.
In 2020, the head teacher of a London primary school talked about his school’s water-only policy. “Staff taught children about the importance of hydration and how not drinking enough water can make it difficult to concentrate and play. I asked children to bring in their own bottles which they can refill at school. During the day staff reminded children to drink water and refill their bottles.” He added, “Schools must consider taps in the classrooms and allow the children to fill up their water bottles.”