Hercule Poirot would be appalled, but according to research, vigorous physical exercise is the key to boosting ‘the little grey cells.’

A study from the Mayo Clinic in Minnesota found that people with the best cardio-respiratory fitness also had the biggest brains, which would be a blow for Agatha Christie’s detective who did not believe in working up a sweat.

In fact, for every standard deviation of difference in fitness levels, scientists showed that grey matter grew on average by 5.3 cubic cm.

So people who were super fit had a grey matter volume of around 667 cm3, compared to someone with half that level at around 652 cm3 and the least fit around 636 cm3.

Likewise total brain volume increased with fitness, ranging from around 1,160cm3 to 1,260cm3.

Although it is possible that people with bigger brains may simply exercise more or generally be healthier, scientists believe that improving fitness through aerobic exercise may actually cause the brain to grow.

And building up a store of cognitive reserve could slow down dementia. The study found that the fittest people had more grey matter volume in brain regions that decline in diseases like Alzheimer’s.

Neurologist Dr Ronald Petersen, said: “This provides indirect evidence that aerobic exercise can have a positive impact on cognitive function in addition to physical conditioning.