By Professor Loc Do, Professor of Dental Public Health, The University of Queensland
Fluoride is a common natural element found in water, soil, rocks and food. For the past several decades, fluoride has also been a cornerstone of dentistry and public health, owing to its ability to protect against tooth decay.
Water fluoridation is a population-based program where a precise, small amount of fluoride is added to public drinking water systems. Water fluoridation began in New Zealand in the 1950s.
Today, around 60 per cent of New Zealand’s population has access to fluoridated tap water. But a recently published review found higher fluoride exposure is linked to lower intelligence quotient (IQ) in children. So how can we interpret the results?
Much of the data analysed in this review is poor quality. Overall, the findings don’t give us reason to be concerned about the fluoride levels in our water supplies.
Not a new controversy
Tooth decay (also known as caries or cavities) can have negative effects on dental health, overall health and quality of life. Fluoride strengthens our teeth, making them more resistant to decay. There is scientific consensus that water fluoridation is a safe, effective and equitable way to improve oral health.
Nonetheless, water fluoridation has historically been somewhat controversial.
A potential link between fluoride and IQ (and cognitive function more broadly) has been a contentious topic for more than a decade. This started with reports from studies in China and India.
But it’s important to note that these studies were limited by poor methodology, and water in these countries had high levels of natural fluoride when the studies were conducted – many times higher than the levels recommended for water fluoridation programs. Also, the studies did not control for other contaminants in the water supply.
Recent reviews focusing on the level of fluoride used in water fluoridation have concluded that fluoride is not linked to lower IQ. Despite this, some have continued to raise concerns. The United States National Toxicology Program conducted a review of the potential link; however, this review did not pass the quality assessment by the US National Academies of Sciences, Engineering and Medicine due to significant limitations in the conduct of the review.
The authors followed through with their study and published it as an independent publication in the JAMA Paediatrics journal. This is the study that has been generating media attention in recent days.
What the study did
This study was a systematic review and meta-analysis, where the researchers evaluated 74 studies from different parts of the world. A total of 52 studies were rated as having a high risk of bias, and 64 were cross-sectional studies, which often can’t provide evidence of causal relationship.
Most of the studies were conducted in developing countries, such as China (45), India (12), Iran (four), Mexico (four) and Pakistan (two). Only a few studies were conducted in developed countries with established public water systems, where regular monitoring and treatment of drinking water ensures it’s free from contaminants.
The vast majority of studies were conducted in populations with high to very high levels of natural fluoride and without water fluoridation programs, where fluoride levels are controlled within recommended levels.
The study concluded there was an inverse association between fluoride levels and IQ in children. This means those children who had a higher intake of fluoride had lower IQ scores than their counterparts.
Limitations to consider
While this review combined many studies, there are several limitations that cast serious doubt over its conclusion. Scientists immediately raised concerns about the quality of the review, including in a linked editorial published in JAMA Paediatrics.
The low quality of the majority of included studies is a major concern, rendering the quality of the review equally low. Importantly, most studies were not relevant to the recommended levels of fluoride in water fluoridation programs.
Several included studies from countries with controlled public water systems (Canada, New Zealand and Taiwan) showed no negative effects. Other recent studies from comparable populations (such as Spain and Denmark) also have not shown any negative effect of fluoride on IQ, but they were not included in the meta-analysis.
For context, the review found there was no significant association with IQ when fluoride was measured at less than 1.5 milligrams per litre in water. In New Zealand, the recommended levels of fluoride in public water supplies range from 0.7 to 1.0 milligrams per litre.
Also, the primary outcome (the IQ score) is difficult to collect. Most included studies varied widely on the methods used to collect IQ data, and did not specify their focus on ensuring reliable and consistent IQ data. Though this is a challenge in most research on this topic, the significant variations between studies in this review raise further doubts about the combined results.
No cause for alarm
One New Zealand study was included in the review. This was from the Dunedin Multidisciplinary Health and Development Study, a longitudinal investigation of the health and behaviour of a complete birth cohort of people born in 1972 and 1973, in Dunedin, New Zealand. A strong feature of this study was that the IQ assessments were done repeatedly at ages seven to 13, and at 38 years. The study did not find any association between living in fluoridated areas and IQ.
I’ve been involved in population-based longitudinal studies in Australia investigating a link between fluoride and child behavioural development and executive functioning, and between fluoride and IQ. The IQ data in the second study was collected by qualified, trained psychologists – and calibrated against a senior psychologist – to ensure quality and consistency. Both studies have provided strong evidence that fluoride exposure in Australia does not negatively impact child development.
This new review is not a reason to be concerned about fluoride levels in New Zealand and other developed countries with water fluoridation programs. Fluoride remains important in maintaining the public’s dental health, particularly that of more vulnerable groups.
That said, high and uncontrolled levels of fluoride in water supplies in less developed countries warrant attention. There are programs underway in a range of countries to reduce natural fluoride to the recommended level.
Professor Loc Do has edited and expanded this article based on his original story that first appeared on The Conversation.