Fluoride, Water, and Kids' Brains: It's Complicated

This transcript has been edited for clarity. 

Welcome to Impact Factor, your weekly dose of commentary on a new medical study. I'm Dr F. Perry Wilson of the Yale School of Medicine.

I recently looked back at my folder full of these medical study commentaries, this weekly video series we call Impact Factor, and realized that I've been doing this for a long time. More than 400 articles, believe it or not. 

I've learned a lot in that time — about medicine, of course — but also about how people react to certain topics. If you've been with me this whole time, or even for just a chunk of it, you'll know that I tend to take a measured approach to most topics. No one study is ever truly definitive, after all. But regardless of how even-keeled I may be, there are some topics that I just know in advance are going to be a bit divisive: studies about gun control; studies about vitamin D; and, of course, studies about fluoride.

Shall we shake this hornet's nest? 

The fluoridation of the US water system began in 1945 with the goal of reducing cavities in the population. The CDC named water fluoridation one of the 10 great public health achievements of the 20th century, along with such inarguable achievements as the recognition of tobacco as a health hazard.

But fluoridation has never been without its detractors. One problem is that the spectrum of beliefs about the potential harm of fluoridation is huge. On one end, you have science-based concerns such as the recognition that excessive fluoride intake can cause fluorosis and stain tooth enamel. I'll note that the EPA regulates fluoride levels — there is a fair amount of naturally occurring fluoride in water tables around the world — to prevent this. And, of course, on the other end of the spectrum, you have beliefs that are essentially conspiracy theories: "They" add fluoride to the water supply to control us.

The challenge for me is that when one "side" of a scientific debate includes the crazy theories, it can be hard to discuss that whole spectrum, since there are those who will see evidence of any adverse fluoride effect as confirmation that the conspiracy theory is true. 

I can't help this. So I'll just say this up front: I am about to tell you about a study that shows some potential risk from fluoride exposure. I will tell you up front that there are some significant caveats to the study that call the results into question. And I will tell you up front that no one is controlling your mind, or my mind, with fluoride; they do it with social media.

Let's dive into these shark-infested, fluoridated waters.

We're talking about this study, "Maternal Urinary Fluoride and Child Neurobehavior at Age 36 Months," which appears in JAMA Network Open.

It's a study of 229 mother-child pairs from the Los Angeles area. The moms had their urinary fluoride level measured once before 30 weeks of gestation. A neurobehavioral battery called the Preschool Child Behavior Checklist was administered to the children at age 36 months. 

The main thing you'll hear about this study — in headlines, Facebook posts, and manifestos locked in drawers somewhere — is the primary result: A 0.68 mg/L increase in urinary fluoride in the mothers, about 25 percentile points, was associated with a doubling of the risk for neurobehavioral problems in their kids when they were 3 years old.

Yikes.

But this is not a randomized trial. Researchers didn't randomly assign some women to have high fluoride intake and some women to have low fluoride intake. They knew that other factors that might lead to neurobehavioral problems could also lead to higher fluoride intake. They represent these factors in what's known as a directed acyclic graph, as seen here, and account for them statistically using a regression equation.

Not represented here are neighborhood characteristics. Los Angeles does not have uniformly fluoridated water, as you can see here, and neurobehavioral problems in kids are strongly linked to stressors in their environments. Fluoride level could be an innocent bystander.

I'm really just describing the classic issue of correlation vs causation here, the bane of all observational research and — let's be honest — a bit of a crutch that allows us to disregard the results of studies we don't like, provided the study wasn't a randomized trial. 

But I have a deeper issue with this study than the old "failure to adjust for relevant confounders" thing, as important as that is.

The exposure of interest in this study is maternal urinary fluoride, as measured in a spot sample. It's not often that I get to go deep on nephrology in this space, but let's think about that for a second. Let's assume for a moment that fluoride is toxic to the developing fetal brain, the main concern raised by the results of the study. How would that work? Presumably, mom would be ingesting fluoride from various sources (like the water supply), and that fluoride would get into her blood, and from her blood across the placenta to the baby's blood, and into the baby's brain.

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.

F. Perry Wilson, MD, MSCE, is an associate professor of medicine and public health and director of Yale's Clinical and Translational Research Accelerator. His science communication work can be found in the Huffington Post, on NPR, and here on Medscape. He tweets @fperrywilsonand his book, How Medicine Works and When It Doesn't, is available now.