There are many different methods for testing hydration status. That's because hydration (and dehydration) is actually a fairly complex process. We've already compared urine color versus saliva; in this post, we'll compare urine color versus body weight for measuring hydration.
When is body weight best for hydration monitoring?
Body weight measurements work very well for assessing changes in hydration. In fact, body weight change is often used as a gold standard comparator in research articles interested in evaluating other hydration monitoring techniques.
Why does it work so well?
The simple fact that if you control for everything properly between weigh-ins, the only thing that will have materially changed is that you lost fluids. Therefore, if you have a good understanding of an individuals initial hydration condition and ensure good controls, you will know exactly how much fluid they lost to guide rehydration strategies and assess the relative degree of dehydration that has occurred.
Note: Not all weight lost is fluid from extracellular stores. For example, as glycogen is used for fuel and calories are burned, some weight is lost. And approximately 1-3g of complexed water are lost as glycogen is metabolized, leading to additional water loss. And particularly in endurance events, fat oxidation leads to additional weight loss.So, when properly controlled, body weight change can provide a good approximation of total fluid loss with guidelines to follow.
Often, a 2% loss in body weight is used as the start of performance decrements in sport (see ACSM Fluid Replacement Guidelines or National Athletic Trainers' Association Position Statement: Fluid Replacement for the Physically Active).
And though some studies have identified this may begin at even lower changes, the 2% figure has become the standard metric to aim for.
Beyond 2%, performance decreases further, until safety concerns begin to emerge. Beyond 10% does occur in some athletes during competitions, but this should be followed by closer monitoring.
Body weight measurements are also a fantastic solution for assessing hydration because its both simple and noninvasive. There are two primary ways to use body weight for hydration assessment:
Relative hydration change: By measuring weight before and after a time period, for example, training or competition, you can assess the degree of fluid loss and associated dehydration.
Absolute hydration status: By measuring weight on a regular basis under well-controlled conditions with a "calibrated" starting point, you can obtain an understanding of absolute hydration with a decent degree of accuracy.
As mentioned earlier, there is one significant caveat to ensure body weight is being used appropriately for hydration monitoring. You must control for all other possible weight-related changes that occur between weigh-ins.
Again, if nothing else occurs between weigh-ins except sweating and breathing, the change in weight is, for all practical purposes, entirely due to fluid losses.
But as soon as many "real-world" variables enter the picture (e.g. water breaks, bathroom breaks, clothing) the accuracy of weigh-ins will decline if not taken into account. Now, for some cases, this loss in accuracy is within the margin of error that is acceptable .
What to pay attention to when using weigh-ins for hydration testing
Here is a non-exhaustive list of a few things to pay attention to when using weigh-ins for hydration monitoring. Do you best to control for these items as much as possible to ensure more accurate results.
Food and fluid intake: If the goal is to understand fluid loss and dehydration, then any food or fluid ingested between weigh-ins should be considered. This becomes increasingly variable when looking to obtain absolute hydration status measurements because the variability in nutrition consumption and timing is large.
Bathroom breaks: Urination and defecation both result in weigh loss and should be accounted for. This, like food and fluid intake, should be considered more heavily when interested in obtaining absolute hydration status measurements.
Clothing: Any weigh-ins should be performed in the same state regarding clothing, because clothing has weight. Ideally, this is performed in the nude or close to nude to avoid potential variation from clothing. This is especially important after training or competition, as clothing can hold moisture from sweating and significantly confound the final measurement results.
Body composition change: This becomes a factor when you are looking to obtain absolute hydration status measurements. A common approach is to take the weigh-in measurement at the same time every day and monitor the value as a function of a "calibrated" starting hydration point. The calibration step is necessary to understand what weight is a good starting point, and this can usually be done by performing a parallel hydration test like a urine test. But over time, changes to fat and lean muscle mass may gradually change and result in changes to the hydration status calibration point. Thus, re-calibration should be performed from time to time to ensure the hydrated "set-point" remains relevant.
One final issue with body weight measurements that occurs in some environments, like football team locker rooms, is the time. Although weigh-ins can be simple and fast, they still require time for the scale to equilibrate and for a performance staff member to record the value for assessment. In a high-volume locker room where lots of players need tests all at once and trying to ensure fluid intake, bathroom breaks, and clothing are accounted for, without several scales and staff members, it can take a while to go through each player one-by-one.
In summary, body weight measurements offer an excellent choice for hydration status measurements due to their simplicity and non-invasiveness, but they are not without their shortcomings. It is important to know and understand these shortcomings so that you either control for them to get more accurate results, or if nothing else, be aware of them to give you an idea of where increased variability may require you to read the measurements with a grain of salt.
Urine color for hydration testing
Here at Intake Health's blog, we've covered a lot of the depth of urine hydration science and many of it's positives and negatives. Here are some of the highlights:
Urine color correlates significantly with body weight change. Both markers are valuable for assessing hydration status among athletes and non-athletes alike.
Urine color assessment is noninvasive, and simple real-time checks can be performed by the players themselves if no data-collection is required to help self-manage hydration.
Food and fluid intake can confound results during acute changes. Like body weight measurements, food and fluid intake have the potential to alter acute changes in urine color. This is true if significant fluids are ingested (e.g. someone is rehydrating) or someone is taking diuretics. Though, over time, the body equilibrates from absorbed fluids and nutrients.
Certain supplements can confound urine color results. Particularly riboflavin (vitamin B2) and certain supplements like beet juice, are known to alter urine color and can make it more difficult to assess hydration status.
Calibration "set point" does not change. The relative urine color scale stays relatively consistent between genders, age, and body composition, making consistency easier to monitor.
Sample collection is timing consuming and messy. When more quantitative assessments by performance staff are required, sample collection and evaluation can be time consuming.
Conclusion
Body weight change is a tried and true method for measuring hydration change in well-controlled environments. This method has been used for a long time, and it is reliable when you ensure contributions to variability are accounted for. Relative changes, such as differences in weight before and after training or competition, are some of the most reliable uses of body weight change measurements because confounders become more condensed and limited. Absolute hydration status monitoring using body weight, on the other hand, is a challenge, as long term changes to calibration set-points and wider variation in confounders makes consistency a challenge.
Urine color is another tried and true method used for measuring hydration. Similarly non-invasive, this simple check provides insight into one's hydration status but must also consider confounders such as recent ingestion of large volumes of water or diuretics. However, urine color as a scale stays consistent and makes for a more useful biomarker for chronic hydration status trends as well as acute changes.
And with tools like InFlow, instant analysis and feedback without the need for sample collection eliminate some of the prior negatives associated with urine color testing.
The best approaches to full hydration management should ideally include a variety of testing methods to obtain higher accuracy and account for the variety of internal processes associated with dehydration. Both body weight and urine color provide valuable means for assessment and both can be used for simple, frequent hydration status monitoring over time.