A new high-quality study of the effect of fluoride on the memory and learning behaviour of rats has produced definitive results. Anti-fluoride campaigners had great hopes this study would bring an end to community water fluoridation (CWF) – but their hopes have been dashed.
The study showed no effect of fluoride on the memory, learning and motor skills of rats thus reinforcing the consensus that CWF is safe.
Animal experiments are commonly used to investigate possible health effects of chemicals like fluoride. This enables strict research protocols without the ethical problems faced by human studies. Consequently, there have been a large number of investigations of the effect of fluoride on animals. Some of these have suggested harmful effects. The US anti-fluoride activist organisation, the Fluoride Action Network (FAN) lists 45 studies “where mice or rats treated with fluoride were found to suffer impairments in their learning and/or memory abilities” (see FLUORIDE AFFECTS LEARNING & MEMORY IN ANIMALS).
FAN claims these and similar studies as irrefutable evidence that CWF is harmful – particularly in their major campaign claiming CWF lowers IQ and should be stopped. However, a more scientific assessment is far less dogmatic.
The US National Toxicity Program (NTP) examined published research of potential neurological effects from fluoride exposures in experimental rodent animals in a systematic review published in 2016 (see Systematic literature review on the effects of fluoride on learning and memory in animal studies). They found many of the studies had limitations due to confounding in the learning and memory assessments and there was a lack of discrimination between motor and learning skills. Very few of the studies were made at drinking water concentrations relevant to CWF and the evidence for adverse effects was “low to moderate,” and weakest for animals during their developmental phase.
The NTP concluded further research was needed and undertook laboratory studies with rodents to fill the research gaps it had identified. Those studies are now complete and have been published in a research paper:
McPherson, C. A., Zhang, G., Gilliam, R., Brar, S. S., Wilson, R., Brix, A., … Harry, G. J. (2018). An Evaluation of Neurotoxicity Following Fluoride Exposure from Gestational Through Adult Ages in Long-Evans Hooded Rats. Neurotoxicity Research. Neurotoxicity Research.
The laboratory experiment
The authors used four treatments for the rats:
- G1: Fed standard rodent chow;
- G2: Fed low-fluoride chow;
- G3: Fed low-fluoride chow + drinking water with 10 ppm F;
- G4 Fed low-fluoride chow + drinking water with 20 ppm F.
Effects of drinking water F were determined by comparing results for G3 and G4 with G2.
The drinking water fluoride concentrations still seem high (compared with the recommended level of 0.75 ppm for CWF) but are lower than used in most earlier studies (often around 100 ppm). However, the basis for these choices was the use of the US secondary drinking water standard (2 ppm) and US UPA maximum contaminant level (4 ppm) and “the conventional wisdom that a 5-fold increase in dose is required to achieve comparable human serum levels.” However, this “wisdom” is debated as blood serum levels fluctuate.
These drinking water concentrations are still far higher than the recommended optimum level for CWF (0.75 ppm) so the results should be seen as more related to the defined upper limits than to CWF itself.
A range of behavioural assessments was made. These included:
“motor, sensory, or learning and memory performance on running wheel, open-field activity, light/dark place preference, elevated plus maze, pre-pulse startle inhibition, passive avoidance, hot-plate latency, Morris water maze acquisition, probe test, reversal learning, and Y-maze.”
The purpose of using such a wide range was to overcome deficiencies of the measurements made in earlier studies and to fill in gaps. Animals at the developmental stage were included as most earlier studies had been made with adult rats.
“No significant differences observed”
One of the most commonly used phrases in this paper as the results are presented and discussed is that there were “no significant differences observed across groups.”
The authors note in their abstract that they “observed no exposure-related differences” in any of the behavioural tests listed above.
This result is important. The study is authoritative. The chosen experimental protocols resulted from an extensive systematic review of the earlier work which identified gaps and deficiencies. A very wide range of behavioural tests was used. And the experimental plans were discussed very widely before the experiments began.
We can conclude, therefore, that rodent experiments are unlikely to show behavioural effects related to fluoride exposure at the concentrations which, the authors argue, are relevant to the recommended maximum drinking water standard (2 ppm) and maximum contaminant level (4 ppm) for humans. The argument that this result is relevant to humans is strengthened by the possibility that ““the conventional wisdom that a 5-fold increase in dose is required” to make results relevant for humans may be inflated.
The argument is further strengthened for humans as the recommended drinking water fluoride concentrations for humans is even lower than the maximum drinking water standard and the maximum contaminant level.
The researchers also analysed thyroid hormones and examined collected tissues. They reported:
“No exposure-related pathology was observed in the heart, liver, kidney, testes, seminal vesicles, or epididymides.”
No evidence of neuronal death or glial activation was observed in the hippocampus at 20 ppm F.”
In fact, the only statistically significant effects they found were a “mild inflammation in the prostate gland” and “evidence of mild fluorosis in adults” at 20 ppm F (treatment G4). Remember this level corresponds to the maximum contaminant level for humans and dental fluorosis has also been reported for humans at that concentration.
The anti-fluoride spin
Several years ago I discussed the planned NTP work and the reaction of anti-fluoride campaigners to it in my article Fluoride and IQ – another study coming up.
These campaigners seemed ecstatic about the planned NTP work, although I did comment:
“You wouldn’t think the anti-fluoride crowd would welcome such a careful analysis of the poor-quality articles they promote”
However, Fluoride Free NZ revealed the spin they placed on the NTP document describing the systematic review and the planned work in their press release at the time (see Fluoride-Brain Studies Set to Expose Fluoridation Damage):
“Results could mean the end to fluoridation world-wide, and definitely should put a halt to any plans to start fluoridation in places not currently fluoridated.
Because it is now well established that fluoride affects the brain, the NTP plans to conduct new animal studies to determine the lowest dose at which this damage occurs. They also plan to do a systematic review of all the existing scientific literature. To date, there have been 314 studies that have investigated fluoride’s effects on the brain and nervous system. These include 181 animal studies, 112 human studies, and 21 cell studies.”
I commented on this:
“The confirmation bias and dogmatic agenda stick out like a sore thumb – don’t expect these people to accurately report this study’s findings.”
Well, it seems that these campaigners are still stuck in dumb shock of the denial phase as they have yet to make any comment on these research results. When they do get around to overcoming their speechlessness they are going to be hard put to reconcile this denial with their earlier hopes for the research findings.
There is no way this study can be used to argue for “the end to fluoridation worldwide” or that there “definitely should” be “a halt to any plans to start fluoridation in places not currently fluoridated.