Purple food, ping pong and life drawing could be the key to a healthy mind

That's right, from the age of 40 you can expect a 5% decrease in brain size every decade. The percentage accelerates over time, meaning the average 70 years old has lost more than a 5th of their brain capacity. The shrinkage is caused by individual nerve cells dying. This can have a huge impact on who we are as people, affecting our memory, reasoning and even our emotions. Gulp.

Some people's brains decline more than others. A Scottish study has tested over 600,000 factors in a group of 79-year-olds regularly since they were 11. It found that a quarter of brain ageing is down to genes while three quarters (75%) is dependent on our lifestyle choices. Which means we can control how our brains age (and shrink...)

The residents of the Japanese island of Okinawa live longer than anywhere else in the world, suffer from 50% less brain disease and have an 80% reduced chance of heart disease. According to its 100-year-old residents, and scientists who are studying them, it's down to the purple sweet potatoes that make up much of their diet.

The average Okinawan eats half a kilo of the colourful vegetable a day, and they are "powerhouses of nutrition." They contain compounds called anthocyanins, which are the pigments found in cells of certain plants. It's this which gives the sweet potato its rich purple colour – and also what contributes to the residents' healthy, younger brains.

These vegetables might be hard to find in the UK, but don't panic. Blackcurrants are full of anthocynanins, as are blackberries and blueberries. Aubergine and red cabbage contain less, but still a similar amount to those sweet potatoes.

Sure, those sweet potatoes are practically magic, but according to other studies highly-valued elders live longer. The residents of Okinawa credit their culture, which promotes respect towards older members of society, for their long and healthy lives.

How to Live Young carried out its own test, pitting walking against table tennis. Two groups of over 60s did either walking or table tennis for an hour twice a week. After 10 weeks they found both activities improved cognitive performance, but the walkers brain function improved significantly. They had more neurons in their hippocampus, which boosts memory and our ability to learn.

However, the table tennis group saw an increase in cortical thickness, which is especially good because it's the part of the brain which deals with complex thinking and is the bit that shrinks the most as we age. They also reported fewer negative emotions compared to the walkers, seeing a mood lift due to the combination of regular exercise with socialising.

A happy brain is the best way of having a healthy brain, concluded presenter Chris van Tulleken. "You've got to walk to your table tennis lesson."

Learning something new changes the micro-structure of your brain and sees its size increase in certain areas, rather than shrink.

If you do similar sudoko challenges every day for 10 years it won't work different parts of your brain, it's got to be something new. Life drawing is a good option, as each picture is a fresh new challenge. As is learning a new language. Whatever you choose, continuing to learn as we age can have a "dramatically positive effect."

This isn't so helpful if you're already a fully grown adult, but studies have found that our early years can have a significant impact on how likely we are to get Alzheimer's disease. A study in the US followed 600 nuns for 30 years, carrying out cognitive tests while they are alive and analysing their brains when they died. Each nun wrote a short biography when she joined the study and the scientists have found a link between this and how likely they were to develop dementia.

The nuns who wrote the most elaborate stories – full of ideas and complex grammar – were less likely to get the disease than those who expressed themselves more simply, leading the scientists to conclude that education and an enriched early life can afford us with protection from diseases of the brain.

Basically, if we mentally challenge our children we are giving them a higher chance of avoiding dementia when they are older.

Continual exercising of the brain is always beneficial and will help protect against disease. Presenter Angela Rippon suggests that dancing is the ideal activity as it pulls together much of what we've discovered: it's aerobic and social, while learning and remembering new routines will challenge your brain.

These aren't practical solutions, people. But scientists have found that electrically stimulating your brain can improve function. And there are also indications that injecting a young person's blood into an old person's body can counteract the effects of dementia. Or, at least, it works in mice. Watch this space...

How To Stay Young is available on BBC iPlayer