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  • Olivia Willis

Good cardiovascular health in midlife linked to lower risk of dementia later

Dementia is an illness involving a deterioration in cognitive function.

It has been known for some time that what is good for your heart is good for your brain.

But understanding how, and to what extent, your cardiovascular health might affect your risk of developing dementia is tremendously complex. A study published on Thursday in the BMJ has found adults with good cardiovascular health at the age of 50 have lower rates of dementia later in life. The research adds to a growing body of evidence that suggests the healthier your vascular system is in middle age, the lower your risk of cognitive decline in older age. "We're quite confident we've got a body of work now that shows you ought to be targeting [people in] midlife," said study author Archana Singh-Manoux of the French National Institute of Health and Medical Research. Changes in the brain that cause dementia can start 15 to 20 years before clinical symptoms appear, so identifying factors that might prevent the onset of the disease is critical. While the main risk factor for dementia is older age, and genetic influences play a role, keeping your heart healthy — with a healthy diet, regular exercise and not smoking — can reduce your risk. "This study supports public health policies to improve cardiovascular health as early as age 50 to promote cognitive health," the authors wrote. Dementia is the second leading cause of death in Australia, where it is estimated to affect almost 450,000 people.

Heart health reduces risk

The findings are based on the data of almost 8,000 British men and women participating in the Whitehall II study, a large, ongoing study that looks at the impact of social, behavioural, and biological factors on long-term health. Each participant was assigned a cardiovascular health score at age 50 based on seven behavioural and biological metrics: smoking status, physical activity, weight, diet, blood glucose, cholesterol and blood pressure. Their progress was followed for 25 years. "Much of the literature looks at individual risk factors," Professor Singh-Manoux said. "What's different about this study is [we] looked at the synergistic effect between all of them." Dementia cases were identified using hospital records, mental health service data, and death registers. The researchers found there was an association between higher cardiovascular health scores at age 50 and lower rates of dementia in older age. Interestingly, the effect was graded: the risk of dementia decreased as cardiovascular health increased. "[It] suggests even small improvements in cardiovascular health metrics … are likely to be beneficial in reducing the risk of dementia." Better cardiovascular health at age 50 was also associated with higher whole brain and grey matter volume in MRI scans 20 years later.

Professor Singh-Manoux said one of the most surprising findings was that the association between poor cardiovascular health and increased dementia risk was evident even for people who weren't diagnosed with cardiovascular disease. "What this suggests is that the association between cardiovascular health and dementia is not simply because of stroke or coronary heart disease," she said. Multiple contributing factors

While previous research has shown a connection between good heart health and slower cognitive decline, the evidence has been inconsistent. That is, in part, because most studies have assessed risk factors for dementia in older people (who may already be developing the disease) and over a short period of time. Neurologist Rebekah Ahmed of the University of Sydney's Brain and Mind Centre said this latest study was a "step in the right direction". "Probably the most novel thing about this study is that they've taken people in middle age," said Dr Ahmed, who was not involved in the research. But she said the results should be interpreted within the big, and very complicated, picture of dementia risk. "[Cardiovascular health] is one factor in a very complex pathway for someone getting dementia," Dr Ahmed said. "I've got lots of patients that I treat that are perfectly healthy, have exercised their whole life, and still get dementia. "The fact is, we can lower your risk, but there are other factors at play." According to the Lancet Commission on dementia prevention, intervention and care, around 35 per cent of dementia is attributable to potentially modifiable risk factors. "Dementia is by no means an inevitable consequence of reaching retirement age, or even of entering the ninth decade," the commission authors wrote. Risk factors include minimal education in early life (primary school or less), high blood pressure, obesity and hearing loss in midlife (age 45 to 65); and smoking, depression, physical inactivity, diabetes and low social contact in later life (age 65+).

Henry Brodaty, who was not involved in the study, said it was well established that cardiovascular health was one of many risk factors contributing to dementia. "There's quite a lot of evidence to show that vascular risk factors also increase the risk of Alzheimer's disease, which is the major cause of dementia," said Professor Brodaty, co-director of the Centre for Healthy Brain Ageing at the University of New South Wales. The heart-brain connection

Understanding at a physiological level exactly how cardiovascular health might contribute to dementia risk is where things get even more complicated. Scientists working in dementia research have several ideas, all of which are speculative. One theory is that damage to blood vessels such as arteries that are critical for oxygen delivery may reduce "cognitive reserve" in your brain. "If you've got a bad flu, for example, you struggle on, go to work, and you can manage. You've got some reserve and you can sort of cope," Professor Brodaty said. "But if you've got no reserve, and you lose 10 per cent of your functioning, you suddenly can't do things that you could do before."

People with Alzheimer's pathology — typically beta-amyloid plaques and neurofibrillary tangles in the brain — tend to accumulate damage to their brain over several decades. "People who have more education build up richer neural networks in their brain, and have some reserve, which means they can compensate for the damage that's happening in the brain for longer," Professor Brodaty said. "But if you've got a second hit — not just the Alzheimer's pathology, but vascular pathology — then that reduces the ability to compensate. So that's one possibility." A second possibility is that vascular damage itself may actually trigger the "pathological cascade" of Alzheimer's disease, Professor Brodaty said. "There's been a theory for some time that micro-haemorrhages in the brain may stimulate the production of the amyloid protein. "[This protein] accumulates to form clumps, which then cause death in nerve cells, inflammation in the brain, and then is finally deposited in plaques. "Whether vascular disease is unmasking the Alzheimer's pathology or whether it's actually got a causative role in accelerating the Alzheimer's pathology, we're not clear." Developing a prevention tool

In an accompanying editorial published in the BMJ, two UK researchers said that the study provided further support for policies focused on promoting cardiovascular health in middle age. "However, other evidence makes clear that vascular health at 50 is determined by factors earlier in the life course, including inequality and social and economic determinants," they wrote. Without acknowledging these factors, the research could reinforce "the limited message" that dealing with health in midlife is enough to reduce population risk of dementia, they said.

Professor Singh-Manoux said that while there was some limitations to the research, the evidence was "robust". "Of course, it was an observational study, so we did not undertake an intervention. But we've got data that spans nearly 30 years." The next step, she said, was to develop a prevention tool similar to the cardiovascular risk score used by doctors to manage cardiovascular disease, but for dementia. "It's not easy," Professor Singh-Manoux said. "If you look at prevention guidelines for dementia, there's 50 risk factors in there. "How many of them are causal? We need to figure that out first."

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