A Scottish GP, Dr David Walker, has caused a furore by saying that there should be a tax on chocolate – because people are eating too much of it, he says, and getting diabetes and becoming fat and then bothering him in his surgery, puffing and wheezing away at him and preventing him from putting a sign on his door saying "Gone to play golf".

Actually, I'm not sure Dr Walker plays golf. It has never, as PG Wodehouse said, been hard to distinguish between a Scotsman with a grievance and a ray of sunshine; and Dr Walker has done his countrymen no favours in perpetuating the stereotype of the Scottish killjoy.

But the worst stereotype he is perpetuating is that of the meddling doctor. And the publicity-seeking doctor, and yes I know I am giving his publicity engine a little extra push by writing this, but when will doctors realise that we want them to look after us when we get ill, and not before? Until then, they can keep their mouths shut. It will, for one thing, stop them from saying the four most irritating words in the language – "I told you so" – when we do turn up for an appointment.

I do speak from an unusual perspective: I can, and do, eat anything I like, as much as I like, and yet never put on weight. Despite a very high-fat diet indeed, and one supplemented, moreover, by regular late-night trips to the corner shop for a Double Decker of an evening, I hover around 10 stone and that's that. And, in case you're wondering, I don't burn it off with exercise. I think jiggling my foot while sitting down is the most exercise I get. You might think this disqualifies me from pronouncing on Fat Issues, but I think that my weird metabolism illustrates a point that the medical profession chooses to ignore: that weight gain is not necessarily a matter of what you put in your mouth. It's a matter of luck. Recent scientific research suggests that a tendency to weight gain might actually be due to a virus; are we going to punish people for what amounts to little more than bad luck? One would also have thought that, were body weight merely a matter of individual willpower, obesity would be its own punishment. Personally, I don't have a problem with fat people. Shakespeare was fine with them; think of Caesar's wariness around the lean; think of Falstaff. He wouldn't work if he was thin, would he? (Actually, he might just, if you consider Withnail a Falstaffian character. But anyway.)

The worst offenders, though, are the people who give Dr Walker the oxygen of publicity, who think this jumped-up quack has anything insightful to say on the subject. Dr Walker is being ignorant and obtuse when he singles out chocolate as a cause of diabetes. There are legions of sweets and fizzy drinks out there which are just as bad, if not worse. And the slap-a-tax-on-it mentality is also hopelessly reductive. A friend of mine who is a tax lawyer is beside herself with frustration because Dr Walker does not understand the basic principle about tax, which is that people will go to some lengths to avoid paying it; in this case, you will find an entrepreneur will invent some chocolate-like substance which will speciously avoid falling under the legal definition of chocolate – whatever that will turn out to be – but will, undoubtedly, be worse for you. And he does not understand what Montaigne understood about sumptuary laws five centuries ago: "The way by which our laws attempt to regulate idle and vain expenses in meat and clothes, seems to be quite contrary to the end designed." In other words, people will start eating more chocolate. Enough of this. I'm off for a Kit Kat.