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This week, the charity Alzheimer’s Research UK launched an online tool telling people they can cut their risk of dementia by making certain lifestyle changes, but the impact of such measures is uncertain and probably overstated.

The charity’s advice is based partly on a 2020 report from a respected medical journal, The Lancet, that summarised what we know about risk factors for dementia. The new tool, part of an existing campaign called the , includes some standard lifestyle advice like eating healthily, avoiding smoking and exercising regularly, as well as supposedly dementia-specific steps, such as wearing hearing aids if necessary and doing puzzles.

The tool acknowledges that risk factors such as our age and genetics can’t be changed, but says we can alter our diet and how we challenge our brain.

It has been changed since it went live on 18 January. The initial version, which was used so heavily that the website had difficulty meeting demand, didn’t mention that some of our propensity to dementia is down to lifestyle – “up to 40 per cent”, . The rest of our risk comes down to which versions we have of many genes, which are obviously immutable. This was put to Alzheimer’s Research UK and it later changed the tool to include the 40 per cent figure.

The tool also glosses over the fact that all the risk factors it highlights have emerged from observational research, not higher-quality randomised trials, the best kind of medical evidence. Observational studies can only show correlations between a lifestyle factor and a medical condition, they can’t discover if the former causes the latter.

These studies are prone to being misleading because a third factor may be behind both the lifestyle habit and the medical condition. In the case of dementia, that third factor may be income, for example. Dementia is more common in lower-income people, who also tend to have unhealthier lifestyles in a whole host of ways. This could be the real explanation for why some of the claimed risk factors – such as eating a certain diet – correlate with dementia.

Another possible explanation is that people with early cognitive decline may be less disposed to carry out some of the healthy habits, such as exercising, socialising or wearing hearing aids.

This isn’t to say that all the 12 claimed risk factors are invalid. Lifestyle probably does play some role in dementia, because although the number of people with the condition is rising due to us living longer, our individual risk of developing the condition by any particular age has been falling. Our genes haven’t altered, so this drop must be down to changing habits.

A spokesperson for Alzheimer’s Research UK says that several public health bodies have recommended some of the 12 tips for lowering our dementia risk, such as avoiding smoking and eating a healthy diet, , England and Wales’ medical guidelines agency.

But it seems unlikely that all 12 of the tips are equally relevant. We haven’t yet been able to find out which of the many lifestyle factors should take the credit for the ongoing decline in individual dementia risk.

They are probably the ones that also promote blood vessel and heart health, such as avoiding smoking and exercising. That is because there are several different kinds of dementia and the type that is caused by damaged blood vessels in the brain, vascular dementia, . The risk of developing the most common form of dementia, Alzheimer’s disease, hasn’t been falling as much.

Of these blood vessel-related risk factors, having high blood pressure seems particularly likely to be a genuine contributor to dementia, that used “Mendelian randomisation”, a relatively new way of investigating medical conditions using random genetic variation to mimic randomised trials.

That same study also suggests that high cholesterol levels and high blood sugar levels aren’t contributors to dementia and may just correlate with an increased risk. Despite this, the new Alzheimer’s Research UK tool includes recommendations to keep cholesterol levels under control and suggests that having high blood sugar could also be a risk.

Other questionable guidance in the tool is that there is no safe alcohol drinking level, .

The 12 tips are described as “simple rules for better brain health”, as if there is no doubt about the effectiveness of any of them.

In response, Alzheimer’s Research UK’s chief medical officer, , defended the tool, saying that the charity gets asked for prevention advice by the public and has therefore provided it. “Our Check-in is there to raise awareness and empower people – it’s not perfect science and we don’t claim that it is,” a spokesperson added.

It’s understandable that people want to do anything they can to avoid dementia, but surely no one wants to be misled. Sometimes, the honest answer to a scientific question must be: “We don’t know.”