One of the classic plotlines of detective fiction has the DA, the mayor and the press all convinced that they have identified the killer – all that remains is to put him away for a long time. But a lone detective, usually plagued by his own demons, thinks they’ve got the wrong man; the real killer is still out there, ready to strike again.

This is the scenario that’s been playing out for decades in the hunt to nail the food responsible for our epidemics of heart disease, obesity and diabetes. Nearly all public health officials and cardiologists have had fat on death row since the 1970s with just a few isolated voices seeking a retrial.

But recently pressure for a pardon has been growing and an increasing number of senior figures have been highlighting evidence that exonerates fats and instead puts carbohydrates, particularly refined flour and sugar, in the dock.

Although you wouldn’t know it from official advice, the case against saturated fat isn’t actually that strong. There are many studies going way back to the 1960s that found no link with cutting it out and reducing heart disease risk, although there are some that did.

The latest witness for the defence is a meta-analysis (a study combining the results of other studies) published in March that found: “There is no significant evidence for concluding that dietary saturated fat is associated with an increased risk of CHD or CVD.” It’s very legit, published in The American Journal of Clinical Nutrition, and big – involving 21 studies and nearly 350,000 patients.

What’s more, the authors point out, replacing saturated fat in the diet with more carbohydrates, especially refined carbs, makes all the risk factors for heart disease and diabetes worse. “This is striking,” says Dr Dariush Mozaffarian of Harvard Medical School, “because it is what we have been doing for years.” Virtually all of the low-fat supermarket meals have contained added sugar to make them palatable. Dr Mozaffarian’s own research suggests that polyunsaturated omega 6 vegetable oils like soya bean may reduce heart attack risk.

Part of the case against saturated fat is that it increases total cholesterol levels. However this idea is “based in large measure on extrapolations, which are not supported by the data”, says Meir Stampfer, professor of nutrition and epidemiology at Harvard School of Public Health. The |point is that although it does raise levels of “bad” LDL cholesterol, it also increases “good” HDL cholesterol, higher levels of which are protective.

Two years ago Professor Stampfer co-authored a study published in The New England Journal of Medicine, which found that the group on a low-carbohydrate diet who ate the most saturated fat ended up with the healthiest ratio of HDL to LDL and lost twice as much weight as those on a low fat diet.

The dossier indicting sugar as a far more serious threat than fat is already pretty hefty. An addition came last month in the form of a big study (6,000 adults) which confirmed the idea that adding sugar to processed or prepared foods is not a good idea. The effect of this, remarkably, hadn’t been studied before. Researchers at Emory University found that the added sugar affected two markers for heart disease risk – it pushed up fat in the blood (triglycerides) and reduced the “good” HDL cholesterol, in contrast to saturated fat.

The people in the study were a cross section of the population and it turned out that on average this added sugar that many consumers aren’t even aware of was contributing an extra 21 teaspoons a day to their diet. This made up 15 per cent of their total calories. Thirty years ago when the low-fat drive was starting, added sugars only made up 10 per cent . “Added sugars are associated with important cardiovascular disease risk factors,” say the authors. Could low-fat diets have actually been causing heart disease and obesity?

Also last month two more studies came out linking refined carbohydrates with a raised risk of heart disease. One found that women, but not men, were more at risk of heart disease the more foods like white bread, croissants, muffins they ate. These are high glycaemic foods – carbohydrates that are easily absorbed and quickly raise blood glucose levels. The other linked the high glycaemic and widely used sweetener fructose to an increase in men’s blood pressure. Movie audiences rooting for a maverick detective would by now be convinced that the real killer was still out there.

But what exactly is carbs’ modus operandi? Step forward expert witness for the defence – Dr Michael Shechter of Tel Aviv University. He is the first researcher to study exactly happens to the lining of your arteries (the endothelium) when high glycaemic food like cornflakes gives you a rush of blood glucose. The endothelium is increasingly being seen as the place where all sorts of vascular disease start.

“Foods like cornflakes, white bread, French fries and sweetened soda all put undue stress on the endothelium, which explains for the first time why high glycaemic carbs can affect the progression of heart disease,” says Dr Shecter. His study was published in the Journal of the American College of Cardiology last year.

At this point that fat prosecutors will point out that they have recognised that there are good and bad fats and that they already recommend the good vegetable and fish oils and that they advise moderation in high-glycaemic foods. So what’s the problem?

“The authorities have been very slow in acknowledging the problem with carbohydrates,” says Professor Charles Clarke, Fellow of the Royal College of Surgeons of Edinburgh, who specialises in treating diabetes. “The official advice for losing weight, controlling diabetes and reducing the risk of heart disease is to eat what is known as a ‘healthy balanced diet’.

“This means keep your fat intake low but make sure you get a good amount of carbohydrates like pasta, bread and potatoes. Unfortunately that allows you to consume a lot of high-glycaemic foods. In fact it’s perfectly possible to follow a ‘healthy balanced diet’ and consume the equivalent of 60 teaspoons of sugar a day, which is not good.”