A landmark, 20-year study by Australian researchers has found that regular exercise in middle age can prevent cognitive decline later in life. 

The study, undertaken by the University of Melbourne, tracked 387 Australian women from the Women's Healthy Ageing Project aged between 45 and 55 from 1992. 

The women's hormone levels, cholesterol, height, weight, blood pressure and Body Mass Index (BMI) were recorded 11 times throughout the two decades. 

The women also took part in an Episodic Verbal Memory Test where they were given a list of 10 words and attempted to recall them half an hour later. 

Those participants who had normal blood pressure, good cholesterol and engaged in frequent physical activity demonstrated a much better ability to remember the words, the study found. 

Author of the research, which was published in the American Journal of Geriatric Psychiatry, Professor Cassandra Szoeke said the nature of cognitive decline meant such a lengthy examination was required. 

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

"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 now we know it can help your brain." 

Szoeke said it is essential to start exercise 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," she said. 

"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 when you're 50 you can make up for lost time."