There's one trait that protects the brain better than any dietary supplement, and you can't buy it at the pharmacy: curiosity. According to neurologists, people who feed their mind new things throughout life delay cognitive decline by about 7 years compared with those who stop learning.

The number isn't pulled out of thin air. An eight-year study by the medical centre of Rush University in Chicago, with 1,939 older adults, showed that those with a richer intellectual life have a 38 percent lower risk of developing Alzheimer's. Reading, learning languages, visiting museums, writing, varied interests - all of it builds what science calls cognitive reserve.

"Cognitive reserve is the brain's ability to withstand ageing or injury without showing significant symptoms," explains neurologist José Miguel Lainez. In other words, a brain that has exercised all its life finds detours when the main routes are damaged - like a city with more streets, which doesn't grind to a halt when one of them closes.

Another part of the research showed something concrete: older people who learned to play chess improved their scores on tests of reasoning and working memory - and the ones who improved most were precisely those who hadn't known how to play before. The point isn't how clever you are now, but how ready you are to learn something new tomorrow.

And here's the good news for everyone: unlike genetics, curiosity is in our hands. It doesn't cost money, it doesn't demand time at the gym, it doesn't need a prescription. It only asks that you don't stop wondering. And that, in the end, is the cheapest investment in health there is.