When Mandy Lovell's husband Garry was diagnosed with younger onset dementia seven years ago, it changed everything but her love for him.

"I was madly in love with him, and there was no way that I was going to change my mind," she says.

Garry already knew the road that lay ahead. He had helped nurse his 50-year-old mother Jan, who later died of the disease.

"Once she got it she said to me 'I hope you don't get this' and I asked her why she is saying that, and she said that her dad had the same thing, his two brothers, their sister, their mother and it went on and on," Garry says.

"When she told me it was a bit of a punch in the head."

Sorry, this video has expired Mandy Lovell talks about her husband Garry's bucket list

Now Mandy has given up her work as a psychologist to help care for her 54-year-old husband.

"What keeps me going I think is just love," she says. "He's my best friend. And I made a commitment to cherish him and care for him. I really want to see that through."

A growing killer

Dementia is the second biggest cause of death in Australia and is predicted to become the number one killer as rates of heart disease decline.

What would you do if you knew you only had a few years before you spiralled into advanced dementia? What memories would you want to try to hold on to? And how could you slow down your decline?

Four Corners explores these intimate questions as it follows the lives of those living with dementia, including two just in their 50s.

For Garry, the answers to some of these questions were found in a bucket list.

"Garry wanted to learn to fly a plane, learn to ride a horse, and travel in a hot air balloon, and he did all of those things," Mandy says.

An answer to chaos

Brian Fischer is grateful for his diagnosis of Alzheimer's disease, which came last year after a traumatic 10-year journey.

The 79-year-old's life had become chaotic and he didn't know why. The academic, who once was in charge of 300 people, now finds it impossible to choose what clothes to put on in the morning.

Sorry, this video has expired Brian Fischer talks about what's on his bucket list

"Now I would say it's a pity that all that didn't happen 10 years earlier and I was, I should have been far more receptive," Brian says.

"My diagnosis has helped me come to grips with some of the things that I've always put off for tomorrow."

Brian has sporadic Alzheimer's, the most common form of dementia, making up nearly 70 per cent of all cases.

'It could take over your whole life'

Supporting Brian all the way has been his partner Heather.

"On one occasion, he actually hit me across the face which was just devastating for him because he's a kind, gentle man who would not approve of that in anybody whatsoever," she says.

"The diagnosis was our diagnosis … it's not just him": Heather Fischer with her husband Brian. ( Four Corners: Ron Foley )

"Every now and again, when Brian feels bad about himself and says, 'Oh maybe I should go somewhere because it's not fair on you', I just remind him that the diagnosis was our diagnosis, that it's not just him.

"It's something in our life that we need to deal with. I remind him constantly that he is Brian Fischer who happens to have Alzheimer's, he is not Brian Alzheimer's. Because Alzheimer's could take over your whole life if you let it."

The quest for delaying tactics

There are two forms of Alzheimer's disease. Garry Lovell has a rare form of younger onset Alzheimer's which was passed through his family. The second and more common form of Alzheimer's, usually diagnosed in people over 65, is not inherited.

The disease is usually random, according to Bernadette Milsted, a dementia counsellor with Alzheimer's Australia.

"Even with two parents with a diagnosis you will not have a significant increase in your risk," she says.

Studies suggest that exercise may in some cases delay its onset, along with increased intellectual and social engagement.

"We do a lot of work on risk reduction," Mrs Milsted tells Four Corners. "There aren't a lot medications available, but you can do lots of things in your lifestyle."

"So, all the classic things: not smoking, limit your alcohol intake, healthy diets, and every piece of research that comes out re-emphasizes that physical exercise is probably the number one thing that is going to lower your risk."

David Ames, an old age psychiatrist, says many people ask him what can be done to prevent or delay dementia.

"There's a little bit of evidence that control of vascular risk factors, pick things like hypertension, high cholesterol, cigarette smoking may have a little bit of an impact, and studies that follow older people through over time seem to find those who are physically active, intellectually engaged and socially engaged may do better over time than those who are not," Professor Ames says.

Forget Me Not aired on Four Corners on Monday night, and is now available on iview.