Science: Fluoride: wat het doet voor tanden, en de doseringsvraag in het water

Gepoold over veel gerandomiseerde studies voorkomt poetsen met fluoridetandpasta een aanzienlijk deel van tandbederf vergeleken met een fluoridevrije pasta, en het effect neemt toe met de concentratie: standaardpasta's van ongeveer 1000 tot 1500 ppm zijn duidelijk effectief, terwijl pasta's onder ongeveer 1000 ppm weinig of geen voordeel tonen ten opzichte van placebo. Het voordeel is lokaal aan het tandoppervlak en vereist niet dat de pasta wordt ingeslikt. De studies zijn voornamelijk bij kinderen en adolescenten, waar nieuw tandbederf het gemakkelijkst te meten is.

Fluoridering van drinkwater vermindert tandbederf bij kinderen, maar de omvang van het voordeel, gemeten in studies uitgevoerd sinds 1975, nadat fluoridetandpasta wijdverspreid raakte, is aanzienlijk kleiner dan in de studies uit het midden van de eeuw waarop het beleid was gebaseerd, in de orde van grootte van een fractie van één aangetaste tand per kind. Veel van het hedendaagse bewijs wordt beoordeeld als laag zeker, en moderne studies zijn moeilijk uit te voeren omdat vrijwel iedereen nu enige blootstelling aan fluoride heeft, wat het contrast tussen gefluorideerde en niet-gefluorideerde gebieden verkleint.

Across industrialised countries, childhood tooth decay fell steeply from the 1970s onward by similar magnitudes in nations that fluoridate their water and nations that never have, tracking the spread of fluoride toothpaste, not water fluoridation (WHO country data). This is an ecological comparison across countries that differ in diet, dental care, sugar intake and reporting, so it cannot isolate a single cause; it undercuts the necessity of water fluoridation, not proving it contributes nothing.

Two Canadian natural experiments followed tooth decay after Calgary ended community water fluoridation in 2011 while Edmonton kept it. In repeated population samples of Grade 2 children, primary-tooth decay rose in both cities from 2004/05 to 2013/14, but the increase was larger in Calgary about 2.5 to 3 years after cessation. A separate Alberta analysis of 2,659 children under 12 treated under general anesthesia for decay found cessation associated with a higher rate of caries-related treatments, with the largest effect in children under 5. Measured in: Grade 2 schoolchildren in Calgary and Edmonton, and children under 12 treated under general anaesthesia across Alberta, Canada. What could explain it instead: Diet, dental-care access, immigration and socioeconomic shifts differ between Calgary and Edmonton and changed over the study period, and total fluoride exposure was not measured for each child.. These are before-and-after community comparisons, not randomized trials, so other differences between the two cities and across the decade contribute; the direction is consistent across both studies.

Fluoride protects teeth mainly by acting at the tooth surface: a low, steady concentration in saliva and plaque slows the acid dissolution of enamel and speeds its rebuilding, and it drives formation of fluorapatite, a mineral more resistant to acid than the native hydroxyapatite. This is a local, post-eruptive action, not one that depends on fluoride being swallowed and built into forming teeth. The understanding that the benefit is chiefly topical is itself the reason the swallowed route is questioned: if the teeth are protected from the surface, systemic intake adds exposure to the rest of the body without adding much to the teeth.

Fluoride reaches people from several sources at once, water, toothpaste, professional treatments, food and beverages, and it is total intake that determines where a person sits between the caries benefit and the risk of dental fluorosis. Guideline reviews put the window that separates useful from excessive intake as fairly narrow in young children, which is why swallowed toothpaste and multiple systemic sources matter. This is a synthesis of intake and guideline data, not a trial, and recommended intakes are set to capture the caries benefit while limiting fluorosis, using assumptions that vary between authorities.

Most US water fluoridation uses fluorosilicic acid or its salts, recovered as a byproduct of phosphate-fertilizer manufacturing, not the pharmaceutical sodium fluoride used in toothpaste. Whether the industrial source meaningfully changes the toxicology at treatment concentrations is debated; the provenance itself is documented and not in dispute.

Pooling dozens of studies, higher fluoride exposure is associated with lower children's IQ, with an inverse dose-response: each 1 mg/L increase in urinary fluoride tracked with roughly a 1.6-point lower IQ. The federal review that accompanied this work concluded with moderate confidence that fluoride above 1.5 mg/L in water, about twice the 0.7 mg/L US fluoridation target, is associated with lower IQ in children. Most of the direct evidence comes from regions with naturally high fluoride, well above US water levels; the reviews judged the data below 1.5 mg/L too sparse to draw a conclusion, so this finding is about higher exposures, not a settled verdict on US tap water.

In a Mexican birth cohort, higher maternal urinary fluoride in pregnancy was associated with lower cognitive scores in the children: each roughly 0.5 mg/L increase in maternal urinary fluoride tracked with about 2 to 3 points (GCI 3.15, IQ 2.50) lower on the general cognitive index at age 4 and on IQ at ages 6 to 12. Measured in: About 299 mother-child pairs at age 4 and 211 at ages 6 to 12 in Mexico City (the ELEMENT cohort). What could explain it instead: Residual confounding by co-exposure to other developmental neurotoxicants common in the same population, such as lead and arsenic, and by social and educational factors that shape childhood test scores.. This is a single observational cohort. It shows an association rather than cause, and the exposure was measured through the mother, not assigned.

In a Canadian pregnancy cohort spanning fluoridated and non-fluoridated cities, higher maternal fluoride exposure was associated with lower child IQ. A 1 mg/L higher maternal urinary fluoride tracked with about a 4.5-point lower IQ in boys, while higher estimated fluoride intake tracked with lower IQ across both sexes. Measured in: 512 mother-child pairs across six Canadian cities (the MIREC cohort). What could explain it instead: Residual confounding by socioeconomic and home-environment factors, and reliance on a single spot urine measure and self-reported intake to estimate exposure.. Observational, so it cannot establish cause, and the sex-specific pattern, seen in boys but not girls, could be a real difference or a chance finding that later work will have to settle.

At the low exposures typical of US and New Zealand fluoridation (about 0.7 mg/L), the picture is unsettled. A long-running New Zealand birth cohort found no IQ difference between people raised in fluoridated and non-fluoridated areas after adjustment, while the pooled higher-exposure evidence thins out to too few studies to judge at this level. Measured in: About 992 people followed from birth to adulthood in Dunedin, New Zealand (the Dunedin cohort). What could explain it instead: Both groups in the New Zealand cohort used fluoride toothpaste and, in some cases, fluoride supplements, which narrows the real difference in total fluoride between them and can mask a small effect.. The low-dose question is contested, not closed. It is the crux of the current debate, and it is not established in either direction at 0.7 mg/L.