Except this week it wasn't there. The shiny little gold cylinder I've used to store important documents contained all sorts of things. Their graduation certificates from daycare, a couple of old school reports, even several photocopies of my long irrelevant marriage certificate. One child's birth certificate was there but not the one I needed. I was stumped. Where was it? When was the last time I used it? Why couldn't I remember what I had done with it?

I pride myself on my memory. I'm great with faces and names. Just this week a woman who I last played hockey with more than 30 years ago turned up at training and I recognised her and remembered her name. My uncanny ability to see actors in films and instantly recall their filmography is almost a party trick. I remember a lot of useless information.

But every time something like the misplaced birth certificate happens these days, I'm starting to worry that I might indeed be losing my mind.

More than 55 million people worldwide are living with dementia, writes Dr Ginni Mansberg in her book Save Your Brain: Simple Steps and proven strategies to reduce your risk of cognitive decline before it's too late. (Murdoch Books, $34.99.) Dementia remains the number one cause of disability in Australians over 65 and the second biggest cause of death overall. It is the number one cause of death in women over 65 and two out of three people with dementia are women.

The book arrived over my recent break and while it didn't seem like particularly chirpy holiday reading at first, it was a fascinating insight into brain health and the lifestyle changes we can make to protect ourselves from cognitive decline.

"It's normal to forget where you left your keys," Mansberg says. "Start worrying if you're not quite sure what to do with your keys once you get them."

Dementia, she says, is an umbrella term for symptoms caused by a loss of brain function that goes beyond what might be expected from the usual consequences of biological aging. Symptoms include problems with memory, communicating, the ability to understand things like symbols and maps, problem solving and other comprehension issues.

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Am I just panicking, at 56, that this is where I'm headed?

Mansberg says recent research suggests that it's never too early to start thinking about your brain health. She quotes Dr Loren Mowszowski, who leads the Cognitive Training research stream within the specialised Healthy Brain Ageing Program at the Brain and Mind Centre at Sydney University.

"If we can target people in midlife, we have the best chance of being able to stop it before it starts or at least delay the onset of that decline," she says.

So what can we do? Mansberg said every expert she spoke to agreed on one thing - exercise. It can be any form of movement, not necessarily sweating it out at the gym, things such as dancing, gardening, even housework can be beneficial.

As an avid puzzler - my Wordle streak is still going strong - I was interested to read that it was more about regularly challenging your brain in different ways.

Social engagement is crucial. Socialising is actually a brain workout. Even short periods of isolation wreak havoc with your brain's cognitive skills. If people annoy you, owning a pet has also been linked to a slower cognitive decline.

There's much in the book. She discusses diet, the effect of depression, how quality sleep plays an important part, the role of menopause and the "brain fog" that comes with it.

She urges everyone to make three appointments today. Contact your GP for a blood pressure test, cholesterol, thyroid and blood sugar test and to discuss giving up smoking if you're a smoker. Get your hearing checked. Research shows that losing your hearing is the single biggest risk for cognitive impairment. Go visit your dentist too.

There's so much handy stuff in this book for those of us in midlife who are starting to think about the coming years and how we want to spend them and how, crucially, our brain health will affect it all.