Have you ever wondered how French women stay so miraculously thin? Me neither. However, given the regularity with which this question comes up, it may not be an issue we can dodge much longer.
Supposing French women are, indeed, thinner than women from other affluent nations, is it because a) they don't eat too much or b) because so many are followers of the Dukan diet? The high-protein regime which promotes itself as "the French medical solution" and "the real reason the French stay thin", catchphrase: "Five million French people can't be wrong."
You might, pedantically, argue that it has not been unknown for at least five million French people to be less-than-reliable role models. Even now, the National Front's Marine le Pen remains ahead of Sarkozy in the polls.
Again, if French practice provides an irrefutable argument for lifestyle change, we ought, surely, to be emulating their enthusiasm for enemas and nuclear power stations.
But in the fat department, Dr Pierre Dukan has, it turns out, made an inspired marketing decision and one that might, in retrospect, help explain the shaky reputation of a fellow protein-gobbler: what does American obesity tell us about the Atkins diet?
By declaring his regime the answer to the perennial question about thinness and the French, Dukan has enlisted every non-obese French woman as a testimonial, regardless of whether she has ever been persuaded by his super-scientific approach.
"Dear readers, be on your guard," he writes, in the new English edition of his bestseller. "The fight against excess weight has to be carried out by doctors, this is crucial."
Be on your guard, dear readers: Dr Dukan refers to medics of a particular persuasion and not, for example, doctors such as Dr Jean-Michel Cohen, a critic of the Dukan method, whom he is suing for libel.
Nor does he allude to the doctors working for France's Agence nationale de securite sanitaire, which last year included his diet in a list of 14 popular regimes which it alleged - Dr Dukan is very sensitive on this point - are ineffective and potentially dangerous.
Dr Lecerf, who led the study, was satisfied that 95 per cent of dieters regain weight as soon as they stop dieting.
So what of the leading British Dukanians, Carole Middleton - the Duchess of Cambridge's mother - and BBC radio presenter Jenni Murray - the author of a Dukan diary in the Daily Mail? Are they at risk? We note, at any rate, that Mrs Middleton has yet to pose in the accepted manner of triumphant slimmers, inside the gargantuan, elastic-waisted jeans worn by her obese self, alongside the headline: "I cried when I realised I would not fit through the door of the Abbey".
As for her fellow Dukanian, after an inspirational loss of 19kg, recent lapses involving chips, chocolate, wine and ice-cream now threaten to make Ms Murray less the pin-up for a protein-fuelled transformation than a reminder of this eternal truth: the diet industry is built on the repeated, utterly predictable and necessary failure of its products.
The peculiar commercial brilliance of high-protein wheezes is that once any initial, dramatic, carb-starved weight loss has given way to normality, as it must for all but the most lonely or obsessed, and to the gradual regaining of weight and fresh breath, its euphoric adherents will have already invested in the required library of diet book, recipe book, lifelong rule book - none of which can add much to what was said, with far greater modesty and concision, by William Banting in his Letter on Corpulence, published in 1863.
"The great charm and comfort of the system," wrote Banting, after he lost 22kg on an eccentric but effective protein diet, "is that its affects [sic] are palpable within a week of trial, which creates a natural stimulus to persevere for a few weeks more, when the fact becomes established beyond question." Or not. To be fair to Ms Murray, she is not the only intelligent woman to publicly confess to a private habit some would find embarrassing.
Where weight loss is concerned, the most respectable journalism will suspend scepticism, offer promotional services gratis and, in extreme cases, accommodate claims that would, if made in a South African village, prompt reflections on the stubborn nature of superstition.
Who would believe, for example, in singing fat away? Step forward a woman introduced in the latest Elle magazine as the Fat Whisperer. "I talk to the fat," she explains. "And I just tell it to go. I listen to the emotion in the cell membrane and I tell the cell which way to move out of the body." And the cell has to go somewhere, right?
It's easy to be cynical, but how else, other than migrating fat cells, do you explain the plight of people who gain without overeating?
Moreover, from a first do no harm perspective, no one could say of "whispering", as Dr Cohen has said of Dukan, that it causes "veritable alimentary destruction which leads to serious health problems among certain patients such as a strong rise in cholesterol, cardiovascular problems, breast cancer".
Last week, the libel case came to court. In Cohen's defence, his lawyer has cited a study of 5000 Dukanians, indicating - allegedly - that 80 per cent of those who followed the diet regained weight in four years. Dukan's lawyer says the figure is 40 per cent.
The only reason to doubt the purity of Cohen's intentions as the enemy of "malbouffe" is his connection with another French diet: "Savoir Maigrir avec Jean-Michel Cohen." For Dukan's part, the great carb-hater is not "seeking profit or fame". He wants to bring people "slimming methods capable of bringing them ways of fighting obesity for their whole life".
I am no doctor, but if that is so, would not his profits be best dedicated to nutritional education, so women won't fall for regimes like the recent frankfurter diet?
A British Heart Foundation dietician explained that a dinner of two frankfurters, half a cup of carrots, a cup of broccoli or cabbage followed by vanilla ice-cream "lacks a number of nutrients we need to stay healthy".
How could anyone fall for it? Possibly because it looked so varied after two weeks dining, in the Dukan manner approved by Grazia, Carole and the Daily Mail, on a couple of slices of meat. Why are French women so thin? From sprinting away from Dominique Strauss-Kahn.