As a neurosurgeon, Henry Marsh has spent several decades making people’s brains better (and occasionally, as his memoirs document with breathtaking honesty, making them worse). But that job involved removing tumours and patching up the aftermath of serious head traumas. Keeping a brain in perfect health is an altogether different matter, not least because we understand so little about it.

A couple of years ago, the now 73-year-old Marsh agreed to have his own brain scanned. He was not overly worried about what it might show. He exercised well, stayed mentally active and was not displaying any significant signs of cognitive decline. “But it was slightly shocking, because my brain looked rather elderly,” he says. “I shouldn’t have been surprised because I was 70 years old! But it was rather scary.”

Still, there are some things that Marsh says we can all do to lessen the probability of problems in the future. “First of all, you want to avoid banging your head,” he says. “There’s convincing evidence that even minor head injuries cumulatively cause damage. You know, rugby players and football players heading the ball. So I think you should avoid sports which involve repetitive head trauma. It’s the idea of the cerebral reserve: that the more active a brain you have, the longer it takes for the batteries to run down.”

This theory can be extended to education too. “We know that the older you are when you finish higher education, the lower the risk of dementia,” says Marsh. “It’s multifactorial, of course, but the theory is that the more connections in your brain, which presumably being educated involves, the longer it takes for the brain to wear out.”

One way Marsh keeps mentally active is inventing fairy stories for his grandchildren, which he hopes to start writing up properly. “It should be my last book,” he says, before adding, grimly: “As with most people I’m horrified by the prospects that face my grandchildren in 30 or 40 years’ time. God knows where the world will be.”

I wonder if this kind of existential dread is damaging for all of our brains but he says the evidence on that is contradictory. Some of it suggests that a certain amount of stress is good for you. “On the other hand, there is some evidence that high blood pressure does cause white matter damage. When I was operating, I would have had episodes of extremely high blood pressure due to the stress, but whether that was responsible for the changes on my scan or not, I don’t know.” Marsh has worked for periods in Ukraine since the 1990s and was recently in Kyiv, arriving “at exactly the same time as the Russian cruise missiles”. That can’t have been good for the stress levels. “I wasn’t exactly frightened, but I was living very intensely. To say it was enjoyable would be a bit naff as people were being killed. But I can see why certain journalists and even some doctors become war junkies.”

The one area that Marsh does think offers fairly conclusive evidence is exercise. “That’s verging on a universal panacea,” he says. “The people who exercise regularly have lower rates of dementia and depression. Although you can’t prove causation.” Then, of course, there is luck. Marsh has removed countless tumours from brains and says there’s never been any obvious reason why some people develop them and others don’t. “Every so often you’d see a brain scan of somebody in their 80s that shows relatively little shrinkage or atrophy. We still don’t know why that’s the case. It could be genetic.”

For Marsh, it’s not his brain he is most concerned about right now but the prostate cancer he revealed he’d been diagnosed with in 2021. The diagnosis has given him a clearer perspective on life, and that applies to our conversation about brain health. “The point I would like to emphasise is that it’s about trying to live well,” he says. “And a better life is really about helping other people or helping the natural world.”

So are we ultimately looking at brain health in the wrong way, then?

“I think so. The issue is not how long you live, but the quality of what time you have. And I feel that now more strongly than ever.”