When it comes to non-invasive hydration testing methods, two common approaches involve urine testing and saliva testing. In this article, we’ll discuss the pros and cons of each, and at which times one test may be more appropriate than the other.
A Quick Background on the Science
Before we get into the pros and cons of urine color and saliva for hydration monitoring, we’ll quickly recap some of the physiology behind these two biomarkers.
Urine Color
As employees in hot environments and athletes perform work, they lose fluids. And when it’s both hot and humid, they lose a lot of fluids, quickly. If these fluids aren’t replaced, your body will gradually dehydrate, and you will enter a state of hypohydration (under-hydrated).
You lose much of your total body water in these conditions through sweat as well as exhalation. Although sweat is salty, it is less salty than your blood. That means, as you sweat, you lose more water than you do salt and other electrolytes. This, in turn, makes your blood more dense.
As your blood gets more dense, your body compensates by taking water from nearby cells and moving it into the blood to dilute it. This helps keep your blood volume in a very tight window so your heart does not have to work harder than it already is.
However, your cells only have so much water they can spare, so your body senses this stress and triggers two primary mechanisms to help out.
Thirst. Your body triggers your thirst sensation to bring more water into your system. This works great, but as you can already see, your body has been under a state of dehydration to trigger this sense, and therefore thirst is a lagging indicator. It also doesn’t help that thirst sensation varies between individuals, and also gets quenched before you fully replace fluids.
Urine concentration. In addition to thirst, the other mechanism that gets triggered is the release of antidiuretic hormones that work to help your body hold onto what fluids it still has. It does this by concentrating your urine so you don’t pee out as much water. This is the reason your urine color gets darker as you become more and more dehydrated.
Saliva
Salivary flow rate and saliva osmolality have been evaluated for use as biomarkers of hydration status. Saliva is mostly water, and similar to urine, during conditions of dehydration, physiological mechanisms inhibit the production of saliva which are thought to derive from changes to local sodium concentrations (that correlate to blood plasma osmolality, as mentioned above) and possibly antidiuretic hormone release as well.
Under conditions of acute exercise in the heat, salivary flow rate and saliva osmolality correlate well to blood and urine biomarkers of dehydration. In these conditions, saliva can provide a good marker of dehydration.
Pros and Cons of Urine Color Vs. Saliva for Hydration Testing
Non-invasiveness
Both urine color and saliva are both non-invasive hydration testing procedures. That is, they do not require access to blood and are therefore both very welcome when it comes to routine hydration testing among athletes and outdoor laborers.
Winner: Tie
Accuracy during acute dehydration
Adapted from Munoz, et al.
During periods of acute hypertonic-hypovolemia, urine indices such as osmolality, specific gravity, and color, as well as salivary osmolality, correlate well to blood plasma osmolality. Therefore, while individuals are undergoing a process of dehydration, both indices can provide accurate assessments of dehydration.
Winner: Tie
Accuracy during ad-libitum drinking
Adapted from Ely, et al.
In many real-world conditions outside of scientific research, athletes and outdoor laborers are allowed, and typically encouraged, to drink fluids to maintain proper hydration. Recent food and fluid ingestion can influence salivary biomarkers and render them inaccurate for hydration testing if performed recently to a test. In fact, even a simple water mouth rinse will significantly shift saliva-based hydration tests, requiring 15 minutes of recovery time before a test becomes valid again (see image above).
Therefore, food and fluid intake must be controlled and accounted for during saliva-based hydration testing for usable results, which can hinder practicality in many situations.
Winner: Urine Color
Anytime access
Despite the need for well controlled timing of food and fluid intake, saliva samples can be taken essentially at will. In other words, saliva is always present and if testing is required to be performed at very precise times, saliva will be available for testing. Urine, as we all are aware, is only produced at intervals as the bladder fills. Thus, urine testing has less flexibility when fixed test timing is required.
Winner: Saliva
Accuracy during chronic dehydration
Adapted from Perrier, et al.
Salivary biomarkers of dehydration have been shown to correlate poorly with high and low water-drinkers, while urine and blood indices correspondingly change to reflect fluid intake. For example, the study image above shows how “high” and “low” water drinkers are distinct between each other for urine indices but there is no change with respect to saliva indices.
In addition, saliva osmolality has a high interindividual variability and has the lowest area-under-the-curve (AUC) during ROC analysis when assessing its diagnostic ability to identify dehydration when compared to other common blood and urine biomarkers.
Therefore, hydration testing before or after phases of acute dehydration should consider tests other than saliva to better assess hydration status. In these conditions, blood, urine, and body weight change when properly controlled allow usable hydration test results when monitoring over time.
Winner: Urine Color
Ease of testing
Hydration testing has traditionally required both individual and staff cooperation. For example, athletic performance coaches or health and safety staff might wrangle athletes and employees, respectively, to all get tested. In this scenario, licking a saliva test can be a simpler and faster process than urinating into cups and performing refractometer or dipstick tests.
However, automated systems like InFlow allow for real-time, hands-free urine hydration testing that does not require collection cups, does not require staff time, and provides instant results during the act of urination.
Winner: Urine Color
Urine color is generally preferred over saliva for hydration testing
Overall, both urine color and saliva can provide good results for hydration testing. Both can trend well with blood biomarkers of hydration status in certain conditions. And saliva, in contrast to urine, is always available.
Overall Winner: Urine Color
However, when it comes to accuracy in real-world conditions where individuals are allowed to eat and drink at will, robustness for both acute and chronic hydration monitoring, and ease of use (with InFlow), urine color testing wins over saliva as a means of hydration testing.