The review of 45 observational studies and 27 randomized trials on coronary heart disease risks based on diet data from more than 600,000 people in Europe, North America and Asia was published in Monday’s issue of the Annals of Internal Medicine.
Polyunsaturated fats tend to be liquid (olive oil, safflower oil and canola oil). Foods high in saturated fats include coconut oil, butter, nuts, poultry and red meat.
Despite a focus on reducing saturated fat in the diet dating back to the 1970s, saturated fat by itself did not help predict heart disease risk, said study author Dr. Dariush Mozaffarian of the department of epidemiology at Harvard University in Boston.
"Current evidence does not clearly support guidelines that encourage high consumption of polyunsaturated fatty acids and low consumption of total saturated fats," the researchers concluded.
In the past five to 10 years, it’s become clear that it’s wrong to take a single nutrient like saturated fat and use that to explain heart disease, Mozaffarian said. "It’s much more complicated."
"Guidelines that focus on the nutrients, single nutrients, as targets for preventing chronic diseases don’t make a lot of sense. I think we need to move to food-based guidelines, to really talk about food, not nutrients," he said in an interview.
"What they are debunking, though, is this idea that further lowering our saturated fat is going to lead to cardio protection," said Richard Bazinet, a professor of nutritional sciences at the University of Toronto, where he studies fats.
For fatty acids, the type matters.
"The other thing they are pointing out is not all polyunsaturated fats are good for you. Some of the omega-3s are," Bazinet added.
There was some evidence that circulating levels of eicosapentaenoic and docosahexaenoic acids (two main types of long-chain omega-3 polyunsaturated fatty acids found in oily fish), and arachidonic acid (an omega-6 fat) were associated with lower coronary risk.
Omega-6 linoleic acid makes up just five per cent of oils in the Canadian diet, Bazinet noted. Mixed omega-3 and omega-6 sources like canola oil and soybean oil account for 55 per cent and 25 per cent of our dietary intake.
"The best evidence we have right now in nutritional sciences is kind of a Mediterranean-style diet, which does include some fats. Healthy foods, unprocessed foods, the fruits and vegetables, seem to be the way to go," Bazinet said.
For example, processed meat such as sausage or low-fat turkey or chicken that is low in saturated fat can have adverse effects on heart disease risk because of high sodium levels, Mozaffarian said.
The review also confirmed that trans fats aren't healthy.
The public perception that lowering saturated fat intake will lower bad cholesterol is based on an imprecise notion, said clinical pharmacologist James DiNicolantonio, who wrote an editorial earlier this month in the journal Open Heart.
In a journal podcast, DiNicolantonio said the condemnation of saturated fats has led to dietary guidelines recommending people increase their intake of carbohydrates and omega-6 polyunsaturated fats while decreasing saturated fat.
He said the authors of such guidelines should "strongly reconsider" their recommendations and called the belief that saturated fats increase total cholesterol a "flawed theory."
An increased intake of omega-3 fatty acids with a decrease in omega-6 fatty acids has been associated in previous studies to a reduction in inflammation.
Shortcomings include how participants in randomized clinical trials don't always follow instructions and a failure to take repeated looks at fatty acids in some of the studies, the researchers acknowledged.
The review was funded by the British Heart Foundation, Medical Research Council, Cambridge National Institute for Health Research Biomedical Research Centre, and Gates Cambridge. Mozaffarian has received fees from pharmaceutical advisory panels and multinational food companies. Bazinet has received funding from agribusiness and multinational food companies.