Skeptoid PodcastSkeptoid on Facebook   Skeptoid on Twitter   Skeptoid on Stitcher   iTunes   Google Play

Members Portal



Get a Free Book



Does Water Fluoridation Cause Diabetes, Obesity, Or Depression?

by Stephen Propatier

February 28, 2015

Share Tweet Reddit

Donate For some reason water fluoridation seems to generate fear and dread. Brian Dunning covered this in Skeptoid episode #58. Generally anything can be toxic if given in high enough doses. Conversely anything is safe at low enough doses. Fluoride, like all things, can be toxic at high enough dosage. Fluoride at recommended dosages can be beneficial when added to drinking water. It helps reduce the incidence of dental caries by strengthening tooth enamel during early growth of permanent teeth.

Recently some research has made the news media cycle. In that research, a correlation is drawn between water fluoridation and low thyroid. Low thyroid, or hypothyroidism, is a medical condition. Mostly it is endogenous medical problem like diabetes or high cholesterol, meaning outside factors influence but don't cause the condition. Rarely hypothyroidism can be induced by low iodine intake. Modern iodized salt has mostly eliminated this problem in the western world. Hypothyroidism causes symptoms similar to depression, and can cause weight gain, fatigue and affect other endocrine systems. Although the symptoms are similar they are not the same diseases. Weight gain, diabetes, and depression are really completely different from hypothyroidism. It is patently incorrect to say that fluoride can affect the thyroid and can therefore give you diabetes. It is also misleading and medically untrue to say that if fluoride affects the thyroid it gives you depression or obesity. If it affects the thyroid it may give you symptoms similar to depression, and make you gain weight. Nonetheless, such claims have been extrapolated by media outlets reporting on the aforementioned new research. The real question is, does fluoride affect your thyroid? Let's take a look.

Some water supplies have too much or too little fluoride. Water fluoridation either adds or removes fluoride depending on how much is in the water supply. In order to prevent fluorosis or permanent discoloration of teeth, fluoride is also removed from water in areas that have naturally high levels.

There has long been vague yet persistent resistance to fluoridation in areas that have it or in area that want to include it. Opponents believe that it is dangerous in some way despite mountains of evidence supporting safety and efficacy if given at the recommended levels. A recent article in the Journal of Epidemiology & Community Health, called "Are fluoride levels in drinking water associated with hypothyroidism prevalence in England? A large observational study of GP practice data and fluoride levels in drinking water," added fuel to this scaremongering.

The primary researcher on the paper, Dr. Stephen Peckham, was quoted in an article on HealthDay News giving this recommendation:
"Consideration needs to be given to reducing fluoride exposure," the researchers wrote. They believe that public efforts to strengthen dental health should move away from fluoridated water and instead "switch to topical fluoride-based and non-fluoride-based interventions."
Dr. Peckham and the University of Kent, where he is a faculty member, have been promoting the study, which has disseminated into the news cycle. The narrative has become distilled into headlines such as "Fluoride in water could cause depression, weight gain, scientists say."

Total and complete bunk. Still, it is being credibly reported as scientific evidence. The researchers have recommended that fluoride should be removed from drinking water. Although this is a provocative position that generates media attention it says little about the quality of the research. The public statements from the researcher do say a great deal about the researchers' mindset.

If a good scientist suddenly comes across a piece of data that is contrary to a large body of research they don't suddenly make sweeping recommendations. A careful scientist questions what they are looking at and questions if they have made any mistakes. Good scientists generally suggest that further study is necessary and make measured conservative statements about their findings. Reasonably so, since they may have made some error or have found a rare exception, a statistical outlier. In this case the primary researcher is proposing a sweeping health change based on one lone correlational study. Good scientists know that correlational studies are unreliable. They demonstrate correlation, not causation. They prudently hedge their bets even if they firmly believe that they have performed a rigorous study. That is the mark of a good scientist: cautious optimism in the face of substantial oppositional research.

Dr. Peckham seems to feel that he has a definitive conclusion, and has made broad and sweeping recommendations about water fluoridation in the UK. That behavior is consistent with an bias preconception confirmed by the research and is not a good sign that we have a reliable bit of research here. Let's take a look at the actual research and train our skeptical eye on it.

The study looked at 2012 levels of fluoride in drinking water in the United Kingdom and the national prevalence of underactive thyroid (hypothyroidism) diagnosed by family doctors across the country. They then compared an area that had fluoridated water supply with another that did not and found an association between rates of hypothyroidism and fluoridated water.

They did this by reviewing charts and correlating data geographically. Frankly, I wonder how this study even got published. Structurally they had no blinding and no randomization. They chose the records based on a non-random-size criteria. They made no accommodation for selection variables. They restricted their study to women, ostensibly because women have higher rates of hypothyroidism. Which raises the medical question: why not restrict the study to men? If there was, in fact, an effect, you would expect a large mean deviation in the male population. This is not even really an observational study. It is, in fact, a geographic study. From an epidemiological sense that makes it confounded by too many variables. Like many other disproven, poorly structured geographic correlations, there are just too many factors involved, similar to studies showing high voltage wires causing cancer, or high rates of cancer surrounding nuclear plants. It is just too broad; there are millions of things that correlate when you break up by geography rather than population.

More significantly, the basic assumption that low fluoride affects the thyroid gland is based on poor, outdated information. It is plausible that fluoride affects the thyroid since fluoride and iodine are chemically similar. Small amounts of iodine stimulate the thyroid, but large doses inhibit it. You could draw the conclusion that fluoride could block the thyroid like large doses of iodine. There is research to show this. Drawing the connection between that research and this study is tenuous at best. First of all, the thyroid gland prefers iodine over fluoride, meaning you would have to be iodine deficient to have any effect from fluoride. The iodine would block the use of fluoride by the thyroid. We know that in the UK the general population studied does not suffer from iodine deficiency. Plus in the research from the 1950s mentioned in this research the doses were massive, 100 times the dosage in drinking water. If you consumed this much fluoride from regular drinking water you would die from hyponatremia due to water intoxication before you could ingest enough fluoride to reach half the dose that affected the thyroid.

So the research has flaws in its construction and its supposed effects. What is the author's response to criticisms of the findings? According to The Conversation, when he was asked about limits and issues of the analysis given their broad recommendations,
study author Stephen Peckham, professor of health policy at the Centre for Health Services Studies at the University of Kent said since the study was conducted at the GP level, the data was fine-grained enough to not require statistical analyses that could take some of these confounders into account.
If I was peer reviewing this paper, I would be shocked that the paper failed to take these confounding factors into account, since statistical tools for doing this are in common use. Hypothyroidism has many confounding factors, which may be related and which the study was not picking up on.

The Conversation also points out that the authors write of the necessary iodine deficiency component,
In the UK, while iodine intake levels have been considered adequate since the middle of the 20th century, concern has been expressed about iodine deficiency in pregnant women and teenage girls.
Again: it's inexplicable why such flaws were not controlled for and are simply dismissed with facile assertions.

The researchers' responses to criticism are admittedly disappointing but consistent with his overall narrative. It sheds suspicious light on his probable bias in both choosing and researching this subject.

Overall there are no findings that support making sweeping changes to water fluoridation practices. There is absolutely no chance that this is solid evidence linking fluoridation to hypothyroidism. I am certainly not surprised that scientifically illiterate news outlets are credulously reporting such a study. Realistically the research doesn't stand up to scrutiny. It doesn't even convince me that further research is necessary, except to disprove all the cranks that will hop on this study and the subsequent news reports. Again: frankly I am surprised that it got published and made it past peer review, as is, with those claims.

Whenever you have some bit of data or research that is in opposition to long standing mountains of data you should be suspicious. And when you have a controversial and pseudoscience-laden subject involved, look at it very closely. When the data underlying that narrative is an unreplicated, structurally poor, correlational study, you have good reason to be skeptical.

Take a minute and support Skeptoid. The money doesn't go to me, but instead goes to keep Skeptoid running as a resource of science and skepticism. Remember: All donations and gifts to Skeptoid Media, Inc. are tax deductible under section 501(c)(3) of the Internal Revenue Code (sections 170, 2055, 2106, 2522).

by Stephen Propatier

Share Tweet Reddit

@Skeptoid Media, a 501(c)(3) nonprofit