Low-fat diets are not as effective as low carbohydrate or Mediterranean diets for losing weight, according to a major new study, but it also concludes that no diets work particularly well in the long-term.

Fat became the enemy decades ago – blamed for heart disease and then the steady rise of obesity. The logic of a low-fat diet was that it should work faster, since fat contains twice the calories per gram of carbohydrates.

The low-fat craze led to the formulation of thousands of low-fat products, from yoghurts to ready-meals. But invariably, sugar – now increasingly blamed for obesity – was used to bulk up the product instead and improve the taste.

A major analysis published in the Lancet Diabetes and Endocrinology journal, which looks at 53 long-term studies carried out since 1960 comparing diets – involving more than 68,000 people – says the old advice to cut the fat was wrong.

Courgette pasta. Photograph: Brett Stevens/Getty Images/Cultura RM

“There is no good evidence for recommending low-fat diets,” said lead author Dr Deirdre Tobias from Brigham and Women’s Hospital and Harvard medical school, Boston.

“Behind current dietary advice to cut out the fat, which contains more than twice the calories per gram of carbohydrates and protein, the thinking is that simply reducing fat intake will naturally lead to weight loss. But our robust evidence clearly suggests otherwise.”

Supporters of high-fat, low-carb diets, such as the Atkins diet, can take little comfort, however, from the research, which was funded by the National Institutes of Health and the American Diabetes Association.

Although they worked better than low-fat diets, the researchers found that no diets worked particularly well in the long term, as defined as more than a year. There is sustained weight-loss for six months, but then many people stop losing weight and even regain it if they give up dieting. In the trials, people on low-carb diets only lost 1kg (2.2lbs) more, on average, than those on low-fat diets and the overall average weight loss after a year in the trials was 3.75kg.

In a commentary in the journal, Keith Hall, of the National Institute of Diabetes and Digestive and Kidney Diseases in Bethesda, US, said one key reason why people failed to lose more weight was probably that they had given up well before the year was over. Much more research on how to help people lose weight and keep it off is needed, he wrote.

“What seems to be clear is that long-term diet adherence is abysmal, irrespective of whether low-fat or other diets, such as low-carbohydrate diets, are prescribed,” he said.

Mediterranean diets are often high in polyunsaturated fats, as found in olive oil. Photograph: Alamy

Naveed Sattar, professor of metabolic medicine at the University of Glasgow, said the paper showed all diets worked, but to a modest extent. “We need more work on sustaining weight loss,” he said.

“The results also indirectly suggest more emphasis should be placed on helping individuals stopping becoming obese in first place, and for this area we need specific trials and input (currently absent) from the government and food industry to alter food formulations, consider taxes etc, so that individuals are more easily directed to better food choices.”

Susan Jebb, professor of diet and population health at Oxford University, strongly defended dieting, saying that even small amounts of weight loss were worthwhile. “Weight losses of 5kg may be less than many people might hope for, but we know from many other research studies that this brings surprisingly large health benefits; more than halving the risk of developing diabetes in people with raised blood sugar,” said Jebb, who has conducted research funded by Weight Watchers in the past.

“These benefits are attenuated but not lost if weight is regained. Inaccurate reports that dieting is ‘ineffective’ undermine public confidence and deter health professionals from encouraging and supporting people who are overweight to lose weight.”

Low carbohydrate or Mediterranean diets are the best losing weight. Photograph: Thomas Barwick/Getty Images

Tobias agreed that small amounts of weight loss can give help people with conditions such as diabetes, but in the context of soaring weight gain and obesity, dieting was not successful. “If you are talking about the population as a whole, we need to identify a stronger intervention than one that [helps people lose] a couple of pounds here or there,” she said.

“What we really need to do is step away from a discussion about fats and carbs to whole foods and overall healthy eating patterns.” Instead of buying books and meal replacement products from the multi-billion dollar diet industry, we should aim to make and eat healthy food, she said. “We have complicated what really should be very simple and that is eating an overall healthy diet.”

Diet types

Low-fat diets: These swap high fat foods for low fats. Slimming World, for instance, offers a range of foods that can be eaten in unlimited quantities, such as fruit, vegetables, pasta, potatoes, rice, lean meat, fish and eggs, because they are all low in fat. The Rosemary Conley diet is also low-fat. Many people choose to eat low-fat products sold in supermarkets, which can be high in calories because of the sugar they contain.

High-fat diets: The best-known of these is the Atkins diet, which aims to starve the body of carbohydrates in the expectation that the body will burn its fat stores instead. Its followers eat lots of high protein foods, such as meat and fish and eggs and also full-fat butter and cheese.

Mediterranean diet: This is a high-fat diet, but polyunsaturated fats, predominantly olive oil, instead of saturated fat like butter. Followers eat a lot of fish, fruit, nuts, vegetables and cereal grains. Starchy foods such as bread and pasta are on the menu and so is meat – but not too much of it.