Is 24 hour urine potassium a valid measure of potassium intake?

Because it is hard to understand how much someone is eating based on self-reports, there is interest in the research community to identify and use biomarkers.  Biomarkers is a fancy way to say that we measure something (e.g. urine or blood) and use these results to make inferences about what people are eating.

Nutrition researchers consider 24 hour urine potassium a valid marker of dietary potassium intake. But how did they test that? Is the relationship 1:1? Are there considerations that we should be aware of when interpreting these results? Let’s dig in.

How did researchers establish 24-hour urine as a biomarker?

Tasevska et al authored one of the most important studies that established 24-hour urine potassium as a biomarker.

This was an incredible study! They took 13 healthy adults, provided them with all of their food for 30 days and collected every urine and poop during the study period! WOW! That’s a lot of work.

Study strengths included:

1. Each participant had their own unique diet created, based on what they normally eat.  This is a super strength of this study because it likely made a huge difference with compliance.  Personalized menu’s means that the participants were eating foods that they liked. This would have made it easier for the participants to follow the diet.
2. Researchers prepared and analyzed extra food from each day in a lab to determine the true potassium content.  This is also a super strength because sometimes nutrient databases are incorrect.  This confirms that we know EXACTLY how much potassium everyone was eating.
3. Researchers confirmed the completeness of urine and stool samples with recovery markers. This means that they accounted for all potassium excreted through either urine or stool.

Why did they find?

The urine excreted an average of 77% of the consumed potassium. The range was 63-88%. The authors considered day-to-day urine potassium excretion highly variable.. Though, it is important to note that the daily potassium intake was variable too. 

However, when the researchers looked at the correlation between urine potassium excretion and actual potassium intake for most participants the correlation was moderate (not high!). This implies that day-to-day variations in potassium excretion do not solely arise from day-to-day variations in intake.

The stool contained 17.5% (95% CI 14.8-20.3%) of the potassium.  This means that researchers were able to account for 94.4% of all potassium consumed. By consuming different diets, researchers were able to explore factors associated with increased potassium excretion in the stool. They reported associations between higher potassium in the stool and:

1. Higher fibre diets
2. Higher stool weight
3. Faster intestinal transit time

Recognizing that not 100% of the potassium is found in the urine, it is worth noting that it has been proposed that 24-hour urine potassium should be converted by a factor 0.8 to convert urine potassium excretion to dietary potassium intake.

Take Aways

Researchers consider 24-hour urine potassium intake a valid biomarker for true potassium intake..  BUT, consider that not 100% of potassium intake is found in the urine.  And also consider that there will be differences between people (for some it was low at 63% and for some it was high at 88%).  There is also a high degree of day to day variability and this day to day variability isn’t well correlated with day to day variability in potassium intake.

So, while we can use 24 hour urine potassium in observational studies to explore health outcomes, we should interpret these studies with caution because:

1. Not all ingested potassium is found in the urine
2. There is between person variability (some people excrete more and some excrete less)
3. There is within-person variability (some days we excrete more and some days we excrete less)