Philip Hensher: You can trust me, I'm a doctor...

It's always been a great puzzle why the Home Secretary likes to be referred to as 'Dr John Reid'

There is a constant danger lurking on Channel 4 on week nights. You decide that it might be rather nice to give your nearest and dearest a little bit of a treat, and go to your nearest organic butcher to buy a piece of well-hung ribeye steak. You might even stretch to cooking your own chips, deep-frying them twice. You slow-roast some tomatoes with garlic and parsley in the oven. You embark on a classic accompaniment, Béarnaise or au poivre, and open a nice bottle of wine. The pair of you stuff yourselves, and go and flop down on the sofa in front of the television - and what do you find?

If you're unlucky, you happen upon that atrocious harridan who presents 10 Years Younger, a programme devoted to the proposition that if you drop £30,000 on plastic surgery and cosmetic dentistry, it might improve your appearance. If you're fantastically unlucky, however, you get You Are What You Eat fronted by Ms Gillian McKeith.

These programmes tend to be statements of what ought to be perfectly obvious to anyone but an idiot. In Ms McKeith's case, she takes some obscenely unhealthy member of the public, and replaces their horrifying diet of chips and pizzas and fizzy drinks with a lot of raw vegetables and seeds. To general astonishment, the fat idiot loses some weight and generally gets a little bit healthier.

To me, Ms McKeith's proposed diets always seem a very steep price to pay, relying as they do on dishes which you would only invite someone to share if you deeply disliked them. Personally, I prefer to eat food that has some kind of cultural solidity, not some mess of pumpkin seeds and tofu dreamed up in some Hackney kitchen in 1976. It seems distinctly impractical to think that anyone is seriously going to start drinking miso soup for breakfast.

Ms McKeith's authority for advising anyone to subsist in this ghastly fashion has rested, until now, not just on her name, but on the fact that she styles herself "Dr Gillian McKeith". On the front page of her website, for instance, it refers to "Dr Gillian McKeith" or "Dr Gillian" some 30 times. A disgruntled member of the public was rather puzzled by the scientific basis of some of Gillian McKeith's claims, and investigated her doctorate.

It turns out that she is not a medical doctor - in fact, if you look closely, her publications are quite clear about this - and her doctorate was acquired from a college called the American Holistic College of Nutrition.

No doubt it is a very excellent institution, though perhaps not as well known as the American College of Nutrition; it appears that she acquired the doctorate through distance learning, and, at the time she took the degree, the college was not accredited by any recognised educational authority. The Advertising Standards Agency, when this question was referred to them, came to the provisional conclusion that the title of doctor was likely to confuse the public, and Ms McKeith has agreed to drop it in future. Yesterday, however, her website was still emphasising her doctorate in holistic nutrition from the American Holistic College of Nutrition. I rather want to quote the mid-18th century satirist Charles Churchill at Ms McKeith, who observed in rather a different context: "'Tis not thy title, Doctor though thou art/'Tis not thy beauty which hath won my heart."

One of the curious facts about adult life is what a very negative impression tends to be left by people who use their non-medical doctorate in anything but an academic context. I'm sure if Ms McKeith had just come on our screens and told people to stop eating lardy buns and start eating a bit more salad, we would enjoy her work just as much without the reminder of her holistic-nutrition PhD.

The reason is that, conventionally, only employed academics and medical doctors are permitted to call themselves "doctor" without looking a bit of an idiot. It rather goes with the tendency to wear a bow tie as a signifier of berk-dom. It's always been a great puzzle why the Home Secretary likes to be referred to as "Dr John Reid"; in a career of his distinction, you would have thought the doctorate was the least of it, and Gordon Brown is wiser not to use his in any circumstances.

Of course, when you get to the end of the hideous labour of a PhD, there's quite a temptation to make some kind of use of it - I took six years over it and it was pretty clear at the end that it wasn't likely to be any kind of help to me. But the temptation should be avoided. I don't think, in reality, anyone really does get asked to carry out emergency tracheotomies on airplanes if they've only got a PhD in mediaeval ship-building techniques.

But the indiscriminate use of the title creates an impression quite other than what is intended. To be honest, with the number of doctorates awarded every year by institutions even better than the American Holistic College of Nutrition, the best thing to do is to accept the achievement and never mention it to anyone ever again.

It seems rather a shame. Whatever Ms McKeith's scientific eminence, or lack of it, one rather regrets that the advice to lay off an unbroken diet of pies, chips and fizzy pop might be disregarded. I can't say that Ms McKeith's idea of a delicious family dinner has ever struck me as anything but somewhat grisly. But her no doubt generally sound advice might stand more chance of being listened to if she weren't the sort of person who insisted on being called Dr McKeith.