Assessing hydration status

There are different reasons for assessing hydration status, and the method of choice will depend on the population of interest, the level of precision required, on the facilities and expertise available, and on budgetary constraints. This article will look at various ways of measuring hydration.

Know your terminology!

Hydration – the condition of being hydrated
Euhydration – being in water balance
Hyperhydration – positive water balance or water excess
Hypohydration – negative water balance or water deficit
Dehydration – process of losing body water
Rehydration – process of gaining body water

Why measure hydration?

Normal hydration, often called euhydration, is important for health and wellbeing. Even small losses of body water can have a negative effect on muscle strength, endurance and maximal oxygen uptake.1,2 Hydration may be measured:

  • In the laboratory to test the effects of different sports drinks, diets or exercise sessions
  • In a clinical environment to ensure the wellbeing of patients
  • By the military to ensure that personnel are able to fulfill active duties while remaining healthy
  • At major sporting events to ensure that athletes can achieve their potential
  • At home to check if someone in the family, particularly an elderly person or child, is drinking enough.

So, what methods should be used in which situations?

Where is the water in our bodies?

Water makes up 50-65% of the total mass of our bodies. It is present inside cells, in muscle tissue, in the gut, in major organs and even in adipose (fat tissue) which is 10-20% water.3,4

body water Assessing hydration status

Methods for estimating hydration

It is difficult to measure hydration as it is a dynamic process, i.e. constantly changing, and is different for each individual. Another issue is that many tests of hydration are really looking at ‘change’ rather than absolute hydration level.

  • Body mass
  • Blood
  • Urine
  • Saliva

Acute changes in body mass over a short period of time are commonly assumed to be due to body water loss or gain. As 1ml of water has a mass of 1g,5 changes in body mass can be used to quantify water gain or loss. A study in well-hydrated soldiers undertaking physical activity in hot environments6 found a daily variability of 1% while other studies have found average figures of 0.3%7 and 0.1%8, although in the latter study, individual variation was -800 g to +800 g in subjects with a body mass of 53-68 kg.

Selecting the right methods

There is no single reliable, consistent method for measuring hydration so either a low precision has to be accepted (e.g. in the case of making a general assessment about one’s own hydration or that of a friend/relative) or several methods have to be used in combination.

Home settings – urine colour and volume would be acceptable ways to estimate hydration status, as well as keeping a diary of how much fluid has been consumed. Urine colour charts are available to compare against the colour of sampled urine.

Clinical settings – health care staff normally collect regular data on fluid intakes and urine volume and will have access to blood measures of osmolality, electrolytes and haematocrit when required. In addition, nurses are trained to spot more subjective changes in hydration, e.g. dry mouth, skin turgor and thirst. It is worth noting, however, that all of these measures can be influenced by disease state, certain drugs and clinical interventions.

Research settings – here, a higher level of precision is required so several hydration markers should be used in combination. Urine volume, urinary markers of hydration and blood markers of hydration all tend to be used in research studies. Care has to be taken to standardize subjects before conducting an intervention to ensure that everyone begins the process in a state of euhydration.

Conclusion

While it is important to monitor hydration levels in order to promote health and wellbeing, the practicalities of doing so are not simple due to the dynamic state of hydration and differences in how individuals respond to fluid gains and losses. Often more than one measure of hydration is needed to give a true indication, although visual methods, such as urine output and colour, would suffice in the home setting.

The EHI thanks Dr Susan Shirreffs for providing the content used as a basis for the text in this section.


References

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2. Goulet EDB, Lamontagne-Lacasse M, Gigou P-Y, Kenefick RW, Ely BR, Cheuvront S. Pre-exercise hydration effects on jumping ability and muscle strength, endurance and anaerobic capacity: a meta-analysis. Med Sci Sports Exerc. 2010, 42, S254.
3. Institute of Medicine. Water. In: Dietary reference intakes for water, sodium, chloride, potassium and sulfate. National Academy Press: Washington, D.C. pp. 73–185, 2005.
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8. Dore C, Weiner JS, Wheeler EF, El-Neil H. Water balance and body weight: studies in a tropical climate. Ann Hum Biol. 1975, 2, 25-33.
9. Gomez & Park. Key facts in anaesthesia and intensive care. Greenich Medical Media Ltd. 2003.
10. Diem K. Documenta Geigy Scientific Tables. pp 538-539. Manchester: Geigy Pharmaceutical Company Limited, 1962.
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12. Ely BR, Cheuvront SN, Kenefick RW, Sawka MN. Limitations of salivary osmolality as a marker of hydration status. Med Sci Sports Exerc. 2011, 43, 1080-4.
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