Body Weight Changes
Body weight changes have been used in the past to measure acute changes in hydration status . This method’s popularity with sporting clubs can be accredited to the fact that it is a simple, non-expensive and non-invasive tool that can provide a quick estimate of an athlete’s so-called hydration status.
The principle behind measuring an athlete’s body weight before and after exercise to estimate hydration status is relatively simple. It is assumed that 1ml of sweat represents 1g of mass lost . The mass loss can then be used to express the post-training BW as a percentage of pre-training BW through the following equation:
Mass Loss (%) = [BW (post) / BW (pre)] x 100
For example, if an athlete weighed in at 78kg before training and weighed out at 76.8kg after training, the equation would read: [76.8kg / 78kg] x 100 = 98.5%. Meaning a mass loss of 1.5%.
However, it is important to note that there are many factors that could limit the reliability and validity of the results. Using this method assumes that all mass lost during exercise is only due to sweat, but many other factors can contribute towards mass loss . Some of which include:
- Substrate oxidation
- Urination and excrement
From a practical perspective, this method uses minimal equipment, is quick to conduct, cost-effective, and can provide a quick estimation of an athlete’s hydration status immediately after exercise. However, it is important to understand the different variables that can affect the reliability of the results.
Hydration status can be measured by monitoring characteristics such as volume, colour conductance and osmolality . Under normal conditions, the amount of urinary volume excreted can range from 1.5-2.5 L/day, with the colour a pale to light yellow and an osmolality of <500 mOsm/L . When exercise commences, water conservation mechanisms are activated in the kidneys to ensure that both plasma volume and intracellular water are both maintained, this has an effect on the aforementioned urinary characteristics.
Measuring the athlete’s urinary volume quantitatively (i.e. how much the athlete has urinated that day) requires a large amount of compliance and cooperation from the athlete; however, a more qualitative method can be used, such as asking the athlete the frequency of urination during the day . Both approaches require a degree of athlete education to ensure that data collected is accurate.
Measuring urine osmolality involves the collection of urine and using a freezing point osmometer to determine the amount of solutes (e.g. NaCl) per kg of solution . This requires a trained technician and also expensive equipment, though there is an alternative. The use of a Sparta 5 Conductance Metre has been validated in previous research . This method uses a 5-point scale to provide feedback regarding the conductance of a person’s urine, and provides immediate feedback which is simple to use.
A 6-point Likert Scale can be used to estimate hydration status through urinary colour . Copies of the scale can be distributed between athletes. This is a non-invasive, non-expensive and simple to use, however, the athlete must be educated to ensure that they use to scale correctly and must take note of the results.
These methods to estimate hydration status through urinary indices may prove invalid if there are large acute ingestions of fluid after exercise [30, 31]. This may produce diluted urine and mask their true hydration status . Methods must be put in place to ensure that hydration testing is completed as soon as possible after exercise so that the data collected is as accurate and precise as possible.
It is thought that a number of blood-borne indices can be used to test the dehydration status of an athlete. Hypertonic dehydration (e.g. from profuse sweating) can be detected through changes in plasma osmolality and plasma sodium [30, 32), whilst hypotonic or isotonic dehydration can be detected through serial haematocrit or haemoglobin measurements .
This method requires a properly trained professional to ensure that safe and sterile measurements are taken with the appropriate laboratory equipment. Examples of which are outlined below:
- Freezing point osmometer (Plasma osmolality)
- Ion selective electrode (Plasma sodium)
- Centrifuge and capillary tubes (Haematocrit)
- Spectrometer (Haemoglobin)
This method of hydration testing can be costly, invasive and labour intensive . Due to the fact that it requires blood sampling, there will always be a risk of infection, bruising and vein damage.
There have been studies that clearly show that a loss of >3% BW during exercise results in an increase in plasma osmolality [30, 32]. However, there is contradicting research regarding the sensitivity of blood osmolality when <3% BW has been lost . From this study, they concluded that urinary measures may be more accurate during conditions of mild dehydration; which other research supports [29. This could be due to the fact that urine is more concentrated to maintain normal blood chemistry during exercise . Adding further contradiction and complexity, one study even demonstrated that plasma osmolality was highly responsive to a reduction in BW by <1% .
Plasma sodium has been found to increase under conditions of dehydration . This study aimed to investigate the use of plasma sodium as a marker of dehydration during exercise in the heat. It involved 2 hours of cycling (3.7% BW lost) and a further 21 additional hours of fluid retention. Results show an increase in plasma sodium from baseline measurements, and an extra 20-mins cycling (1.5% BW lost) showed a further increase in plasma sodium.
One study compared haematocrit and haemoglobin levels to levels of total body water both before and after an exercise intervention lasting 14 days . It showed that blood indices correlated with total body water throughout the study, suggesting that these may be useful for situations of hypotonic and isotonic dehydration.
Whilst plasma osmolality, blood sodium, haematocrit and haemoglobin may provide accurate information regarding hydration status, the limitations outlined earlier seem to outweigh the benefits at this point in time. Further research and engineering are needed to provide a more practical approach when using these measurements.