There's nothing we can do about the biggest risk factor for developing dementia - getting older. But there's lots of evidence that lifestyle factors can potentially make a big difference.

Around 50 million people worldwide have dementia, according to the World Health Organisation (WHO), with around one in 14 over-65s affected, as well as a significant number of younger people. But the disease doesn't have to be an inevitable part of ageing.

Brain surgeon and neuroscientist Dr Rahul Jandial, who works at City of Hope Hospital in Los Angeles, has seen for himself in thousands of operations the difference between a young brain and an ageing one, and how the way a person lives their life can affect their grey matter - which he explores in his new bestselling book, Life Lessons From A Brain Surgeon.

"I've operated on over 5,000 skulls, and they're all different ages so you see the developing brain to the ageing brain, and everything in between," says Jandial, who also spent years trawling through scientific research in order to come up with what he believes are the best ways to help boost memory, manage stress and reduce Alzheimer's risk.

"There are habits some people have, that tend to link with them having resilience of thought and emotion as they get older. Since we don't have a medicine for dementia, it's really about lifestyle modification. And doing it early - early being your 40s, 50s, even 60s. There's always a window to make a difference," he adds.

"Fortunately, our brains ask so little of us that very manageable changes, like replacing your steak for salmon a few times a week, eating more plants, less fried food, a bit of brisk walking - all these changes add up. It would be great to have this be a new focus because there's no treatment if you get dementia," he says.

"Let's see if we can make a difference with small changes, rather than waiting for a silver bullet or a single pill or food that fixes everything."

Remember, whatever health advice you follow, right now, staying sensible and following the guidelines on minimising the spread of coronavirus is everyone's top priority. So don't adopt any lifestyle changes without considering whether it's sensible to do so at this current time.

Here are five changes Jandial recommends...

1. Eat a Mediterranean 'mind diet'

"The brain's 90 billion neurons share the garden inside the skull with supporting cells called glia. They're sort of the shrubs around the roses that protect the brain environment. Those glia create fatty insulation for the neurons, so the electricity can bounce around inside our heads faster and more organised.

"That fatty sheath at the microscopic level is the good fat that comes from fatty fish - and there are some good choices for vegans as well. That's an essential part of the Mediterranean diet, and huge studies over decades show eating mostly plants, fatty fish, nuts, and drinking occasional red wine, really makes a dent in the chances of getting dementia.

"It's not about how much you eat, it's about what you eat," he adds. "These are the nutrients that are helpful and not difficult to adhere to, to help reduce our dementia risk. So the first and most fundamental thing is the mind diet - essentially the Mediterranean diet."

Jandial warns people should be wary about other so-called 'brain foods' however, as nutrients have to get past the gut wall, into the blood, and are then filtered by the liver before passing through the blood-brain barrier. "Getting to the brain requires passing three barriers and the Mediterranean diet and its nutrients, whether it's flavonoids, antioxidants or all of them, really is an effective strategy," he says.

"So the first thing to do is to switch to more components of the mind diet. Occasional cheesecake or burger or chips isn't an issue - it's not the indulgences, it's the regular things we put inside us [that matter]."

2. Standing and walking

Right now, with coronavirus on everybody's minds, getting out and about for exercise may not be as straightforward as usual. Generally speaking though, when it's safe to do so, being active is among the brain surgeon's top tips for brain health. Until the coronavirus situation settles, could you stretch your legs with a stroll in the garden, or do some exercise in the living room?

"The second most important thing is exercise - and I don't mean becoming super-athletic, I mean simply standing and walking. The neurons and their supporting cells are floating in liquid - our brain is like densely-packed tissue in an aquarium. The tissue doesn't physically touch, it gets very close to each other and sprays chemicals at each other called neurotransmitters.

"But there's also something called brain-derived neurotrophic factor (BDNF), which the brain showers on itself, and the trigger for that is being vertical and moving. It doesn't have to be a marathon; a 30-minute brisk walk gets you to a sweet spot where your brain is showering itself with BDNF - it's a growth factor, like Miracle-Gro for the flesh of the brain. It's something anybody can do, it's free and can be just a micro-change in your week."

3. Learn something new

"The brain is thinking flesh. Life is brain training - you don't need to buy an app or pay money - but you do need to learn. Engaging the brain, learning, reading, trying to learn a new instrument or language, even if you fail miserably, just the effort of trying to learn anything will engage wider swathes of your brain, and that serves as the engagement of those brain cells," he says.

Even thinking about planning something for the future, like how you would run a company or keep up with friends, could count, he says: "Any time you're thinking.

"But it has to challenge you just a bit. If it's too easy, your brain doesn't need to think and you'll rely on habits. If it's too hard, your brain won't engage and you'll say it's impossible. So the trick is to find just that one level past your comfort zone. That's the trigger for the brain to say it's got to dial it up."

4. Socialise

Again, socialising isn't as easy right now, as the government advises everybody, particularly older age groups, to make avoiding non-essential contact and minimising the risk of coronavirus their top priority.

Now is the time to make use of our phones and laptops to keep in touch with friends we can't see in person. And when the pandemic settles, we can embrace our social lives again.

"Socialising is also considered an advantage because it's forcing you to think - about others, what you're going to wear, where you're going to go, etc. For people who are lonely, part of the risk is that they're thinking less and they're thinking negative thoughts."

5. Try to maintain a healthy heart

"If the arteries in your brain aren't open, just like those in the heart, you can have small swathes of brain tissue wither, much like a garden that doesn't get irrigated goes dry. Exercise helps with that, and good heart health with control of blood pressure, diabetes and cholesterol is fundamental to keeping the plumbing of the brain open, so it's getting the blood flow it wants. It's an amazing amount of blood it demands - 20% of our blood flow goes to our 3lb brain."

Life Lessons From A Brain Surgeon: The New Science And Stories Of The Brain by Dr Rahul Jandial is published by Penguin Life.