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

University of Melbourne researchers followed 387 Australian women from the Women’s Healthy Ageing Project for two decades. The women were aged 45 to 55-years-old when the study began in 1992.

The research team made note of their lifestyle factors, including exercise and diet, education, marital and employment status, number of children, mood, physical activity and smoking.

The women’s’ hormone levels, cholesterol, height, weight, Body Mass Index and blood pressure were recorded 11 times throughout the study. Hormone replacement therapy was factored in.

They were also asked to learn a list of 10 unrelated words and attempt to recall them half an hour later, known as an Episodic Verbal Memory test.

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 of the words.

Study author Associate Professor Cassandra Szoeke, who leads the Women’s Healthy Ageing Project, said once dementia occurs, it is irreversible. 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,” Associate Professor Szoeke said.

“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. Asoc Prof Szoeke said that the best effects came 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 said, was 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. 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.”

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