Is Urinary Fluoride a Good Measure of Blood Fluoride?
It’s not great. Empirically, we have data that tell us that levels of urine fluoride are not all that similar to levels of serum fluoride. In 2014, a study investigated the correlation between urine and serum fluoride in a cohort of 60 schoolchildren and found a correlation coefficient of around 0.5.
Why isn’t urine fluoride a great proxy for serum fluoride? The most obvious reason is the urine concentration. Human urine concentration can range from about 50 mmol to 1200 mmol (a 24-fold difference) depending on hydration status. Over the course of 24 hours, for example, the amount of fluoride you put out in your urine may be fairly stable in relation to intake, but for a spot urine sample it would be wildly variable. The authors know this, of course, and so they divide the measured urine fluoride by the specific gravity of the urine to give a sort of “dilution adjusted” value. That’s what is actually used in this study. But specific gravity is, itself, an imperfect measure of how dilute the urine is.
This is something that comes up a lot in urinary biomarker research and it’s not that hard to get around. The best thing would be to just measure blood levels of fluoride. The second best option is 24-hour fluoride excretion. After that, the next best thing would be to adjust the spot concentration by other markers of urinary dilution — creatinine or osmolality — as sensitivity analyses. Any of these approaches would lend credence to the results of the study.
Urinary fluoride excretion is pH dependent. The more acidic the urine, the less fluoride is excreted. Many things — including, importantly, diet — affect urine pH. And it is not a stretch to think that diet may also affect the developing fetus. Neither urine pH nor dietary habits were accounted for in this study.
So, here we are. We have an observational study suggesting a harm that may be associated with fluoride. There may be a causal link here, in which case we need further studies to weigh the harm against the more well-established public health benefit. Or, this is all correlation — an illusion created by the limitations of observational data, and the unique challenges of estimating intake from a single urine sample. In other words, this study has something for everyone, fluoride boosters and skeptics alike. Let the arguments begin. But, if possible, leave me out of it.
Dr. Wilson is associate professor of medicine and public health and director of the Clinical and Translational Research Accelerator at Yale University, New Haven, Conn. He has disclosed no relevant financial relationships.
A version of this article appeared on Medscape.com.