If you asked most people how to get a fitter, healthier brain they'd probably talk about doing brain training exercises or Sudoku.

But it's actually just as important to make your brain physically healthy, as it is about giving it mental stimulation.

Twenty minutes of vigorous activity on three or more days a week, or 30 minutes of moderate activity on five or more days a week, will reduce your chances of getting Alzheimer's by around 20%.

Even in the short-term it can have a huge effect on how your brain functions.

You know that feeling at the end of a really busy stressful day when your brain feels “fried”. Well, that's because it is fried. The stress hormone cortisol is released to keep you in a state of heightened alertness. It keeps your blood sugar and blood pressure up to help you escape from danger.

In the short-term it's meant to help you deal with life-threatening situations. Getting that document finished by the end of the day is hardly life-threatening, but unfortunately you can't tell your body that.

It's called the “fight or flight” mechanism and that's why exercise is so good at countering it. You don't need to imagine you're being chased by a sabre-toothed tiger (although it might help you run a bit faster); just the act of running will help reduce the cortisol in your body. Even a brisk stroll will have a similar beneficial effect.

Most people believe it's the more cerebral activities like reading, studying and discussing that improve our minds. But as well as reducing stress, exercise has actually been proved to increase the production of grey matter in the brain.

How do you prove this when there are so many factors to be taken into account when measuring an increase in cognitive performance?

Simple really: identical twins.

It's like those old soap powder comparison ads.

“We took these two identical twins, made one sit in a chair, watch TV and eat crisps all day and make the other eat only raw vegetables and spend the whole day in the gym.”

Well, that's the principle, but not something that's very practical if you want to see the effect of years of difference.

Un-Identical Identical Twins

In Finland they established something called the Finnish Twin Cohort to investigate genetic and environmental risk factors for chronic disorders. They'd created a database of over ten thousand “Finn Twins”, so it was relatively easy for the University of Jyvaskyla to track down 10 sets of identical male twins – one of whom regularly exercised and one who did not.

What they found was it was not only their bodies that were different, but also their minds. Brain scans revealed that the active twins had significantly more grey matter than the sedentary twins, especially in areas of the brain involved in motor control and coordination. Amazingly, the changes in their exercise routines had mostly begun within only three years prior to the tests.

“These differences in the young men's bodies and brains had developed during their few, brief years of divergent workouts, underscoring how rapidly and robustly exercising can affect health”, said Dr Urho Kujala, who oversaw the study.

So, the next time you go to the gym, remember you're not just exercising, you're brain training.