For years we have been told that we should cut down on the amount of fat we eat, inspiring many people to adopt low-fat diets.
Now it seems the story is changing, with a report saying that such eating patterns are having "disastrous health consequences".
The National Obesity Forum and the Public Health Collaboration have called for a "major overhaul" of dietary guidelines, accusing major public health bodies of colluding with the food industry.
Their report pushes for a return to "whole foods" such as dairy, fish and meat, as well as high fat healthy foods such as avocados, saying that "eating fat does not make you fat".
Saturated fats do not cause heart disease and full fat dairy foods - including milk, yoghurt and cheese, can actually protect the heart, the authors add.
They blame the focus on low-fat diets and snacking between meals for Britain's growing obesity crisis, saying that processed foods labelled "low fat", "lite" or "low cholesterol" should be avoided.
People should also stay away from sugar, stop counting calories and those with type two diabetes should eat a fat-rich diet instead of one dominated by carbohydrates.
The authors of the report say that food science has been "corrupted by commercial influences" in a way that represents a "significant threat to public health".
They said the recent Eatwell Guide from Public Health England, for example, was produced with a large number of people from the food and drink industry.
Professor David Haslam, chairman of the National Obesity Forum, said: "As a clinician, treating patients all day every day, I quickly realised that guidelines from on high, suggesting high carbohydrate, low fat diets were the universal panacea, were deeply flawed.
"Current efforts have failed - the proof being that obesity levels are higher than they have ever been, and show no chance of reducing despite the best efforts of Government and scientists."
Dr Aseem Malhotra, consultant cardiologist and founding member of the Public Health Collaboration, a group of medics, said dietary guidelines promoting low fat foods "is perhaps the biggest mistake in modern medical history resulting in devastating consequences for public health.
"The current Eatwell guide from Public Health England is in my view more like a metabolic timebomb than a dietary pattern conducive for good health.
"Eat fat to get slim, don't fear fat, fat is your friend. It's now truly time to bring back the fat."
Personal trainer and nutritional therapist Rob Suchet said: "Eating fat does not 'clog' your arteries as we've been told, this is a gross over-simplification of heart disease which is far more attributable to stress and high blood sugar caused by eating too many sugary and starchy foods.
"We should be directing our attention to insulin, the hormone which tells our bodies to store fat in response to eating sugars and starches like cereal, bread, rice, pasta, potatoes and even fruit, despite us having been told these are healthy foods due to their low fat content."
Professor Iain Broom, from Robert Gordon University in Aberdeen, agreed: "Our populations for almost 40 years, have been subjected to an uncontrolled global experiment that has gone drastically wrong."
But the report has drawn strong condemnation from other food experts and medical professionals.
Dr Alison Tedstone, chief nutritionist at Public Health England , said: "In the face of all the evidence, calling for people to eat more fat, cut out carbs and ignore calories is irresponsible.
"Unlike this opinion piece, our independent experts review all the available evidence - often thousands of scientific papers - run full-scale consultations and go to great lengths to ensure no bias."
Professor John Wass, the Royal College of Physicians' special adviser on obesity, said there was "good evidence" that saturated fat increases cholesterol.
He said: "What is needed is a balanced diet, regular physical activity and a normal healthy weight. To quote selective studies risks misleading the public."
Professor Simon Capewell, from the Faculty of Public Health, said the report was not peer reviewed and there was no indication of how it had been funded, something he said was "worrying".
Dr Mike Knapton, associate medical director at the British Heart Foundation, said the report did not have the "robust and comprehensive review of evidence" required to be taken seriously by the charity.