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

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6 Effects of dehydration

Two plums with two prunes in front of them.
Figure 5 The difference between hydration and dehydration.

Medical dictionaries define dehydration as the excessive loss of body water due to restricting fluid intake, sweating, diarrhoea, vomiting and certain medications. Dehydration is classified according to water weight loss as mild (1–2%), moderate (5%) and severe (10%). Dehydration is defined as a 1% or greater loss of body mass when there is no weight loss due to a negative energy balance. For example, you may be trying to lose weight and following an eating pattern that means you are eating less energy from food than you are using in your activity levels.

Controlled fluid restriction experiments have shown that it can only take 13 hours for 1% dehydration, 24 hours for 2% dehydration, and 3% after 37 hours. Ethically, it was not safe to continue the experiment but it did demonstrate how quickly dehydration can happen.

The symptoms of dehydration depend on the degree of dehydration.

  • Mild to moderate dehydration has symptoms such as constipation, dark urine, headache, increased thirst and dry mouth, muscle tiredness and general tiredness.
  • Fluid losses of 2% or more can reduce mental (cognitive) performance.
  • Regular inadequate fluid intake can contribute to chronic kidney (renal) disease.
  • Older people are at increased risk of dehydration which can lead to confusion and even hospitalisation.

Diarrhoea and vomiting can also cause dehydration. Therefore, fluid and electrolytes should be replaced.

Activity 6 Are you dehydrated?

Timing: Allow approximately 20 minutes.

Are you aware of the Bristol Stool Chart? Search online for it and see what type you tend to have.

Now answer the following questions.

  • Have you ever experienced symptoms of dehydration?
  • Why do you think you became dehydrated?
  • Is it possible to have too much water?

Click ‘Save’ when you are satisfied with what you have written.

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