Sounds too good to be true? Well it's absolutely kosher. Apparently.

Sitting in your armchair, taxing your brain with a sudoku puzzle or a few cryptic clues not only exercises the grey matter but gives your body a workout too.

Can you believe it? It actually burns off calories. It's all to do with neurons in the brain which produce chemicals called neurotransmitters to relay signals around the body.

This is the technical bit now, so bear with me. These neurons extract three quarters of sugar glucose, available calories and a fifth of oxygen from the blood (the neurotransmitters), so doing a difficult crossword or a challenging puzzle makes your brain crave more glucose and more calories.

Amazingly you can burn up to 90 calories an hour. So what's the point of donning Lycra, I ask? What is the point of putting yourself though all that humiliation of sweating and panting and trying to get to grips with monstrous pieces of scary-looking equipment in front of complete strangers?

Surely you don't need neurons to work out that flopping on the sofa, ruminating over five across and seven down, a KitKat Chunky to hand is a far more pleasing way to spend an afternoon than getting out of breath on the stepper?

Surely this is the news all gymaphobes have been waiting to hear?
Now you could be excused for feeling a little sceptical about this research since it has been carried out by mental agility experts at cannyminds.com in a bid to get more people to log on to their brain-training website.

But it is backed up by the British Dietetic Association. Thinking hard means the brain needs lots of energy and it has to get it from somewhere.

But – and I am sure this a question on everyone's lips – can you think yourself thinner? Sadly no. Mainly because even though the brain uses up energy, it does not burn off fat.

Still, it eats its way through those excess calories that have a nasty habit of turning into fat.

I am now wondering how many my brain might have used, just writing this column. No comments, please.

Just imagine the mental agility required to work out the answers to The Times crossword – always a killer for me.

Example: "Somehow from capitalism you'll get what oppressed people may need" (9-6.)

I mean, where do you begin to unpick that one? Or what about their fiendish sudoku? My brain is going into overdrive just thinking about it.

Perhaps a custard cream (just 56 calories in each one) or a jammy dodger (85) would help.

Both are allowed, of course, if you settle yourself down to an hour's puzzling.

Pass the newspaper. And the biscuit tin.