Based on a review of previous studies a group of researchers have shortlisted a number of environmental risk factors for dementia in a paper published in the journal BMC Geriatrics.
Dr Doug Brown, Director of Research and Development at the Alzheimer’s Society, said:
“Dementia is incredibly complex and we are yet to fully piece together how genes, lifestyle and environmental factors interact to determine a person’s risk of developing the condition.
This review looks at a vast number of environmental toxins and finds only weak evidence to link each one to an increased risk of dementia. Research connecting air pollution, pesticides and vitamin D deficiency to dementia is stronger – but these factors still need to be followed up in more robust studies before any health advice can be considered.
There is no good evidence to suggest any environmental toxin directly causes dementia. Studies that report an association with dementia must be carefully interpreted because other factors such as regional differences in diagnosis rates could be swaying the findings.”
Prof. Kevin McConway, Emeritus Professor of Applied Statistics at The Open University, said:
“This is a well-conducted piece of work, but it’s really important to understand what the researchers set out to do and what they found.
“This is a review of previous studies, so in a sense it tells us nothing new. Reviews like this can sometimes clarify things by putting together a strong piece of evidence from several weaker pieces. But on the whole, this study doesn’t do that. Most of the factors that it reviewed were covered by only one or two previous studies.
“All the studies covered in the review were observational, and a few such studies on their own really can’t tell us what causes what. For instance, some studies found that dementia was more common in people with low levels of Vitamin D. But they don’t tell us whether the low vitamin level caused the dementia. Perhaps the people with low levels had something else in common, and maybe that something else caused the dementia. In that case, increasing someone’s Vitamin D level would have no effect on their chances of dementia. We just can’t tell from this kind of research.
“In any case, the main virtue of this review is to provide better ways forward in trying to pin down the causes of dementia. Overall, this new study concludes that the evidence on specific environmental risk factors for dementia is “generally not strong”. But the researchers did find many possible risk factors where the evidence of a link to dementia is weak (or effectively non-existent). Rightly in my view, they suggest that future work should concentrate on the relatively small number of possible risk factors where there is at least moderate evidence of correlation with dementia, even though we can’t yet tell whether they play a role in causing it.
“Better studies of these factors, using methods that can tell us a bit more about what causes what, might well get us further down the road of understanding the causes of these awful diseases, and might lead to strategies to help prevent dementia. But that could be quite a long way into the future.”
Dr Rosa Sancho, Head of Research at Alzheimer’s Research UK, said:
“Dementia is one of the most feared conditions and public interest in what might influence dementia risk is understandably high. This new analysis of existing studies has identified a shortlist of environmental factors that could be associated with dementia risk, but strong evidence behind these factors is still lacking. Many of the factors identified in this study have only been observed in a single study, or multiple studies have presented a mixed picture, making it hard to draw firm conclusions.
“It is important to remember that association does not necessarily indicate causality, and that diseases like Alzheimer’s have a complex mix of risk factors including age and genetics. Alzheimer’s Research UK has been working with experts to identify key challenges in risk reduction research that could be tackled to boost the number of studies in this important area. We are working with researchers across the globe to develop new ways to support the most promising research into dementia risk reduction.
“This study indicated an association between vitamin D deficiency and increased dementia risk. While the reasons for this relationship are unclear, current government advice recommends that people consider a daily supplement of vitamin D, particularly if they are unlikely to gain enough through their diet and daily exposure to sunlight. Anyone concerned that they may be deficient in vitamin D should speak to their GP.”
Prof. Robert Howard, Professor of Old Age Psychiatry and Psychopathology at UCL, said:
“I am a psychiatrist working with people who have dementia and there is nothing in the data presented in this study that would make me change my lifestyle.
“There is robust evidence that head trauma and poor cardiovascular health increase dementia risk. But most of the environmental factors identified in this review probably represent no realistic increase or only a vanishingly tiny increased risk for dementia.
“If you want to avoid dementia, look after your heart and try to avoid getting knocked unconscious.”
Dr Tara Spires-Jones, Interim Director of the Centre for Cognitive and Neural Systems at the University of Edinburgh, said:
“The study by Killin and colleagues looked at the published evidence about environmental risk factors associated with dementia. They found that there is moderately good evidence that air pollution, some metals, pesticides, and vitamin D deficiency are associated with a higher risk of dementia.
“The key word here is ‘associated’. While this study was thoroughly conducted, an association, or statistical correlation, like this does not imply that any of these factors CAUSE dementia. There could be another factor, related to the two, that is the cause. For example, in primary school, students’ shoe sizes correlate strongly with their scores on reading exams. But it isn’t their larger feet that makes them better at reading, it is their higher age and experience levels.
“So while the current paper is informative to scientists studying factors that cause dementia, more work needs to be done to establish causal relationships.
Prof. Tom Dening, Professor of Dementia Research at the University of Nottingham, said:
“It is a fascinating study and a well conducted review. The conclusions drawn seem appropriate although most of the associations are relatively weak and the studies not always of high quality either. There is sometimes a limited range of countries investigated.
“What is difficult is to tell whether the environmental exposures are themselves contributing to dementia or whether they are in fact acting as proxies for some underlying variable. For instance, many unpleasant environmental exposures (traffic fumes, living near power lines, poor water quality) are related to socio-economic deprivation, which itself is related to poor diet, low education, higher stress and worse health; so we really cannot easily tell what is causing what. But of course, socio-economic deprivation is less fashionable for the media than sexy risks like electro-magnetic fields or trace elements.
“Also, some environmental factors may be effects rather than causes. For example, mobile phone use is likely to be higher in those without dementia – it seems unlikely that buying your mother a mobile phone will in itself stop her getting dementia.
“This study gives the current state of play in this area and does suggest some areas of research which urgently need addressing as well as indicating that there are some possible risks that are not supported by current evidence.”
‘Environmental risk factors for dementia: a systematic review’ by Lewis Killin et al. published in BMC Geriatrics on Wednesday 12 October 2016.
Dr Spires-Jones: I work at the University of Edinburgh, the same institution as the authors of this paper