There’s no bullshit like dietary advice. It starts off with a dumb observation that someone hasn’t bothered to think through or an experiment so badly designed that it never stood a chance of revealing anything reliable. Then people who don’t understand it or who won’t take the trouble to try, puff it up and it ends up getting spewed out as though it were an accepted truth. But today it’s my excuse for being cheerful, for offering you a little bit of good news.

I can do so courtesy of last week’s Journal of the American Medical Association. It included a randomised trial of three different diets, fed over six-week periods to 164 people. All the diets were low in saturated fat but each one chose something different to replace the saturated fat with. The first took carbohydrates, the second protein and the third unsaturated fats.

If you think of saturated fat as being solid at room temperature and unsaturated as being liquid oil — then you won’t be far wrong. Artificially saturated fats — unsaturated ones that have been “hydrogenated” — are what Marks & Spencer in the UK plans to ban. Each diet was healthier. Blood pressure went down and fat profiles changed. But the carbohydrate diet — the traditional one that most medical authorities have been recommending — came off worst. The drop in blood pressure was biggest for those eating lots of protein but it was only a fraction behind those eating lots of unsaturated fats — and their levels of good cholesterol went up.

Six weeks isn’t a huge time for a dietary intervention, and it’s disappointing that the Americans took the short cut of looking at risk factors — blood pressure and fats — rather than the long-term events of heart attacks and diabetes. But compared to the bad level of evidence that dietary advice is usually based on, this trial was golden. And it adds weight to other evidence suggesting that the high-carbohydrate advice was just one more piece of rubbish that the media and medical profession pushed without having had the evidence to back it up. As long as you don’t put on weight, you don’t have to consume bowls of unadulterated brown rice or oil-free whole meal pasta. That’s a bit cheerful.