Sorry to break it to you, but there’s bad news. Your brain is shrinking (probably). None of us is getting any younger and time is increasingly taking its toll on our brains; our neurons are getting smaller and we’re losing connections between them. But should we accept this pattern of degeneration as inevitable? Is there anything we can do now to optimise our brains and protect them against the ravages of ageing?

When we talk about healthy brain ageing we are really discussing one of two things: how to minimise ongoing damage to the hardware of the brain, mostly by keeping its blood supply as good as possible; or how to improve the operation of the brain’s software. Many ways of doing this have been suggested, but few have scientific weight behind them. There is currently no magic bullet to protect the brain, but one area that has been best researched, and about which we can say with reasonable confidence, “this will help”, is mental activity.

There is plenty of evidence that older people who stay mentally active, by learning a new language, doing crosswords or taking part in other intellectually challenging activities, preserve full cognitive function for longer.

A back-up ‘reservoir’ of cognitive function can protect us from the effects of brain damage

However, we need to be careful about the direction of causality. It may be that people who are cognitively intact get more pleasure from cognitively challenging activities than people whose faculties are starting to fail. For this reason, it’s difficult to run rigorous studies to test the effectiveness of brain-training programmes, which use increasingly challenging but enjoyable puzzles or games designed to build up cognitive function. People will choose and adhere better to a regime of activities that they find more enjoyable, so it is tricky to determine scientifically whether any particular brain training package or cognitive pursuit is really actively supporting healthy brain function.

What is clear is that people who have spent more time doing cognitively demanding activities over a lifetime are, to some extent, buffered from the physical effects of brain ageing and degenerative diseases. We call this buffer “cognitive reserve” – a back-up reservoir of brain function that can protect from the consequences of brain damage, allowing us to continue to perform well. For example, people with a higher IQ, longer education or cognitively challenging employment have been found to have a lower risk of developing dementia. This is despite the fact that their brains actually show normal amounts of age- and disease-related damage. In fact, postmortem studies have found that people with higher cognitive reserve who do get dementia exhibit less severe symptoms even when they have more brain damage than those with lower cognitive reserve.

Yet there’s still much to discover about the potential of cognitive reserve for optimising the brain’s resilience. The more we understand about its role in protecting our brain and how to boost our reserve, the more effective we will be in designing interventions to keep the human brain healthier for longer.

The good news is that cognitive reserve isn’t exclusive to those who have the IQ of a genius or who’ve devoted their life to theoretical physics. We think it can be built up throughout life, so taking part in cognitively challenging activities, learning new skills and continuing to “use it or lose it” probably applies no matter how old you are – crucially, it’s never too late to start.

How Much Brain Do We Really Need? by Alexis Willett and Jennifer Barnett is published by Constable & Robinson on 7 December at £13.99. To buy a copy for £11.89, visit guardianbookshop.com