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The science of nutrition and healthy eating
The science of nutrition and healthy eating

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1 What is hydration?

This week is all about fluids. So far, you have considered food as macro and micro nutrients, absorption and how these nutrients are used. Although a person can survive for several weeks without food, without fluids, they can only survive for a few days.

An image of a tap with running water filling up a glass.
Figure 1 Fluids are important for survival.

The term ‘hydration’ will be used which the English Oxford Dictionary defines as: ‘the process of causing something to absorb water’.

This ‘something’ is the cells in our body (intracellular) and blood and the spaces between cells (extracellular). There is a lot of water in the human body. On average, it is 60% water in adult men and 51% in adult women, 75% in infants and about 55% in elderly people. There are variations with age and gender: males have more water than females. This is because women have a larger percentage fat content and fat contains less water (about 20%) than muscle.

According to the Office for National Statistics (2010) an average male in England weighs 83.6kg and an average woman in England weighs 70.2kg therefore a person weighing 70 kg has 42 litres of water. Most of this fluid is in the cells (about 65%). Water can move easily in and out of cells, so it can go wherever it is needed.

Different parts of the body have different concentrations of fluid. This is related to the function of the fluid in that part of the body. Did you know that an adult brain is approximately 73% water? What about other parts of the body?

Activity 1 water in different parts of the body

Timing: Allow approximately 15 minutes.

Move the relevant percentages next to the relevant organ for average water contents in adults.

Using the following two lists, match each numbered item with the correct letter.

  1. 83%

  2. 79%

  3. 73%

  4. 73%

  5. 71%

  6. 64%

  7. 31%

  • a.brain

  • b.heart

  • c.kidneys


  • e.bone

  • f.liver

  • g.lungs

The correct answers are:
  • 1 = g
  • 2 = c
  • 3 = b
  • 4 = a
  • 5 = f
  • 6 = d
  • 7 = e


Order is highest to lowest: lungs 83%, kidneys 79%, heart 73%, brain 73%, liver 71%, skin 64%, bone 31%. (Source: National Hydration Council, 2018)