WHEN it comes to matters of diet and nutrition, my rule of thumb is that no-one wants to hear my opinion on what they eat. But gosh, don’t a lot of people like to talk about other folks’ breakfasts, lunches and dinners? And not just in a general, making-conversation sense, but quite often remarking negatively, unsolicited, on what someone else is eating – while they’re eating it.

The constant stream of conflicting expert advice doesn’t help. The latest warning from Glasgow University scientists – that fixating on sugar content while ignoring fat is misguided – might sound like a statement of the obvious, but it will doubtless by merrily ignored by those with degrees from the University of A Documentary I Watched One Time.

Many seem convinced there’s a wide-ranging conspiracy at play, and that scientists are working in league with the food industry to give us a bum steer. So they confidently disregard these fancy schmancy randomised controlled trials while pointing out that lard-soaked sausages didn’t do their 110-year-old grandmother any harm. They’re perfectly entitled to their opinions – but if they could find a way to stop expressing them loudly at every opportunity, that would be great.

Diet policing takes many forms, from the casual comment to something bordering on obsession. Someone might remark: “I don’t know how you can eat all that junk and stay so slim!” (top tip: if the explanation to your puzzler might be “bulimia”, perhaps don’t pose it at full volume in a restaurant), or cut the compliment altogether with something along the lines of: “Oh, I could never eat that processed rubbish” (Never? I tend to wonder, or just never in public where someone else might shame you for it?).

But that’s nothing compared to expert-level diet scrutiny. A female friend used to work with a man who, on a daily basis, would come into her office some time after lunch, look in the bin, and list aloud its contents. This isn’t a scene from The Office I’m describing – it was real life. I was particularly boggled by the idea of an older man doing this to a young woman. Is there some unwritten agreement that the ordinary rules of good manners may be set aside when there’s faux-concern to be expressed about someone’s Doritos habit? Is it fine to pretend eating disorders don’t exist when you’re on a vital mission to educate your colleagues about the calorie content of a BLT?

As with any form of bullying, the simplest explanation for all of this is surely jealousy: it’s not a stretch to imagine the self-righteous packed-lunch nibbler would really quite fancy a bite of any smoked sausage supper, four-cheese pizza or double-chocolate-chip cookie that’s being wafted under their nose. Some of those struggling with restrictive diets are probably completely unaware of how much they talk about other people’s food. But there’s another dimension too: a class dimension.

Diet obsession of the non-pathological kind is a very middle-class hobby. While there are excellent reasons to seek out locally sourced and ethically farmed fare, things get a little excessive when you start adding organic, non-GM and additive-free to your must-have list. Each to their own – I have no interest in peering into their shopping baskets, and don’t presume to know their individual medical needs – but the self-righteous commentary on other people’s choices is hard to stomach.

The wholegrainier-than-thou brigade can wax for hours about how the evil food industry has tricked people dimmer than themselves into eating three meals of poison and a side serving of cardboard every day. Listening to them, it’s a wonder those of us without Waitrose loyalty cards are even alive. Otherwise sensible adults are quick to talk about “good” and “bad” foods, as if a little of what you fancy will, contrary to the conventional wisdom, inevitably give you cancer.

I don’t doubt the sincerity of many diet evangelists who believe they are valiantly battling at the front line against the ever-advancing obesity crisis, but all too often there’s a sneering subtext to their prescriptions. There’s an insinuation that poor people are too lazy or foolish to cook meals from scratch, or even to read the nutritional information on the food they buy.

Yes, everyone should have the opportunity to learn to cook, but not everyone who learns will want to come home and do it every night. It also shouldn’t be assumed that a homemade meal is always a healthy one, even if the eggs contained in it were laid by pampered chickens and the vegetables were grown by Prince Charles. If the chef reckons calories don’t count and that portion control is for killjoys, then the diners will pile on the pounds and struggle to lose them again.

Life’s too short to obsess over every mouthful, and for most people life’s also too busy to swear off any and all forms of convenience food. It’s sensible to take a critical look at ingredient lists – and it’s worthwhile campaigning for better supply-chain scrutiny, to ensure we aren’t tricked into eating any more arbitrarily taboo meats – but not every advance in food technology is part of a secret plot to put us in early graves. We have more choices than ever before, but also more information and increasingly tight regulation of food advertising. So unless someone’s asking for their opinions, the busybodies should learn to bite their tongues.