Regular exercise in middle age is the best lifestyle change a person can make to prevent cognitive decline in their later years, a landmark 20-year study has found.

Recent research shows abnormalities in brain tissue begin several decades before the onset of cognitive decline, but little is known about the lifestyle factors that might slow the onset of decline in middle age.

As the incidence of Alzheimer's disease diagnosis doubles every five years after 65, most longitudinal studies examining risk factors and cognitive disease recruit participants over the age of 60 or 70.

The new University of Melbourne study, published in the American Journal of Geriatric Psychiatry, tracked 387 Australian women from the Women's Healthy Ageing Project for two decades. The women were aged 45-55 when the study began in 1992.

Associate Professor Cassandra Szoeke is director of the Women's Healthy Ageing Project and led the study. She says researchers were interested to find out how lifestyle and biomedical factors — such as weight, BMI and blood pressure — impacted memory 20 years down the track.

"There are few research studies which have data on participants from midlife and have assessed cognition in all their participants in later life," Associate Professor Szoeke says.

"This research is really important because we suspect half the cases of dementia worldwide are most likely due to some type of modifiable risk factor.

"Unlike muscle and vessels, which have the capacity to remodel and reverse atrophy and damage, neuronal cells are not nearly so versatile with damage and cell loss is irreversible."

Over two decades, Associate Professor Szoeke and her team took a wide range of measurements from the study participants.

They made note of their lifestyle factors — including exercise and diet, education, marital and employment status, number of children, physical activity and smoking.

And they measured hormone levels, cholesterol, height, weight, Body Mass Index and blood pressure at 11 points throughout the study. Hormone replacement therapy was factored in.

The women were given a Verbal Episodic Memory test in which they were asked to learn a list of 10 unrelated words and attempt to recall them 30 minutes later.

When measuring the amount of memory loss over 20 years, frequent physical activity, normal blood pressure and high good cholesterol were all strongly associated with better recall.

Associate Professor Szoeke says once dementia occurs, it is a slow moving freight train to permanent memory loss.

"In our study more weekly exercise was associated with better memory. We now know that brain changes associated with dementia take 20 to 30 years to develop," she says.

"The evolution of cognitive decline is slow and steady, so we needed to study people over a long time period. We used a verbal memory test because that's one of the first things to decline when you develop Alzheimer's disease."

Regular exercise of any type, from walking the dog to mountain climbing, emerged as the number one protective factor against memory loss.

In fact, the beneficial influence of physical activity and blood pressure together compensates the negative influence of age on a person's mental faculties.

Associate Professor Szoeke says the best effects come from cumulative exercise, that is, how much you do and how often over the course of your life.

"The message from our study is very simple. Do more physical activity, it doesn't matter what, just move more and more often. It helps your heart, your body and prevents obesity and diabetes and now we know it can help your brain.

"It could even be something as simple as going for a walk, we weren't restrictive in our study about what type."

But the key, she says, is to start as soon as possible.

"We expected it was the healthy habits later in life that would make a difference but we were surprised to find that the effect of exercise was cumulative," she says.

"So every one of those 20 years mattered.

"If you don't start at 40, you could miss one or two decades of improvement to your cognition because every bit helps. That said, even once you're 50 you can make up for lost time. There is no doubt that intervention is better late than never, but the results of our work indicate that an intervention after 65 will have missed at least 20 years of risk."

The study was funded by the National Health and Medical Research Council and the Alzheimer's Association.

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