A half-naked 71-year-old with 8% body fat and the testosterone levels of a boy of 18 greets me at the door of a large house overlooking a golf course near St George, Utah. He has the physique of a very fit young man and the springy, energetic demeanour of somebody who has cracked most of life’s outstanding problems. I am calling on him because, ever since I heard of him, I have begun to look uncannily well.

Two weeks earlier I had strolled into the office of the editor of this organ.

“You look 10 years younger. Why?” he barked, as editors do.

I was 11lb lighter than the last time he’d seen me. But he said “younger”, note, not just thinner.

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“Arthur De Vany,” I reply, “he’s this guy…”

“Well, you’d better go and see him, then,” he growled, as editors do.

And so here I am in the red desert of Utah saying hello to Superman’s slightly fitter grandad. Now, I know all you men will be wanting to know the secret of Arthur’s testosterone trick. I’ll come to that – it couldn’t be simpler. But first a word of explanation.

Feeling bloated and absurd after seven weeks in America, most lethally Texas, where the steaks are bigger than your head, I went on a diet. I do this periodically and lose up to 20lb over three months or so. Then, slowly, I put them all on again. I console myself with the thought that this inflate-deflate regime has kept me from the more severe middle-aged inflations I see around me. Many of my contemporaries are beginning to look like Sylvia Plath in pregnancy – melons “strolling on two tendrils”.

My usual diet is simplicity itself: eat too little and lay off the booze. All other dietary schemes struck me as over-elaborations or, in many cases, cunning excuses to keep eating. Calories in, calories out was, I thought, the only rational basis on which to lose weight. The body is a bag that gets lighter if you take out more than you put in. This time, however, I had a more sophisticated plan – Arthur’s – and boy, did it work.

I adopted the Arthur plan because the last time I was in America I had encountered Nassim Nicholas Taleb. He wrote a brilliant book called The Black Swan, superficially about risk in financial markets but, in fact, about life. He couldn’t stop talking about the Arthur diet. Nassim embarked on his regime after someone told him he looked like the writer Umberto Eco, a fatty. He had lost 20lb in three months after getting hooked on Arthur via his website (arthurdevany.com ). Nassim still looks a bit like Umberto, but not fat.

Now, as I say, I have never believed in smart diets. But for the first time, Nassim gave me a justification for a diet that made sense. (I’ll come back to that just before I get back to the testosterone.) And so here I am in Utah, now 13lb lighter, confronted by a half-naked Arthur.

I am early – I am always early, it’s a curse – and he has just got out of the shower. Unlike most 71-year-olds he feels no need – and has no reason – to cover his body. A good thing too. It’s 103 degrees and I, fully clad, am sweating like a thinnish pig. But it’s cool, tranquil and immaculate inside the house. The only signs of familial chaos are the bits of wire botched together in the garden to keep in their Yorkshire terriers, Django and Bela. Otherwise, Arthur’s world is disciplined and active. He listens to jazz on a stratospherically expensive Krell hi-fi, he has motorbikes and push-bikes in the garage, also a Range Rover that I assume is brand new, but which is in fact a couple of years old; he just looks after it. Arthur’s stuff, like Arthur’s body, is the best there is and all very carefully maintained.

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