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The types of fat people get in their diet may not be as closely related to their risk of heart disease as previously believed, a new review of past studies suggests.
Guidelines from the U.S. federal government and recommendations from the American Heart Association call for increased consumption of polyunsaturated fatty acids and lower consumption of saturated fats.
But researchers found people's risk of heart disease varied little based on how much of those fats they ate.
Polyunsaturated fats generally come from plant-based foods such as nuts, seeds and vegetable oils. Omega-3 fatty acids, a type of polyunsaturated fats, are found in fish.
On the other hand, most saturated fats in the American diet come from foods of animal origin, including red meat and high-fat dairy products.
The authors of the new review say uncertainties in evidence have led to considerable variation in international guidelines on fat intake. They also say the use of self-reported diet information may have resulted in problems classifying the different fatty acids that people eat.
"We intended to help resolve the existing uncertainties around fatty acids and their potential association with coronary heart disease risk," Dr. Rajiv Chowdhury told Reuters Health in an email.
Chowdhury, from the University of Cambridge in the UK, led the review that was published in the Annals of Internal Medicine.
He and his colleagues collected data from 72 previously published studies of more than 600,000 people from 18 countries.
Those included studies that measured the types of fatty acids people consumed or had in their blood, as well as those that randomly assigned people to take fatty acid supplements or not.
All of the studies followed participants to see who developed heart problems like heart attacks, heart disease or coronary insufficiency.
When Chowdhury and his team analyzed data on fatty acid intake, they found that none of the types of saturated or polyunsaturated fats had a significant impact on heart disease risk. However, consumption of trans fat - found in some processed foods and some forms of stick margarine - was tied to a 16 percent increase in risk. Guidelines call for avoiding trans fats altogether.
When the researchers examined markers of fatty acids in the blood, they also found little difference in heart risk based on levels of saturated or polyunsaturated fats. But the results varied for individual fatty acids.
The researchers found that higher blood levels of two forms of omega-3 fatty acids - docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) - were associated with a lower risk of heart disease.
They did not see a significant reduction in heart disease risk with any of the fatty acids in studies that randomly assigned some participants to take them in supplement form. Doses used in the studies ranged from 2 to 5.5 grams per day of added oils and 0.3 to 6 grams per day when capsules were used.
"The pattern of findings from this review did not support the current cardiovascular guidelines that encourage high consumption of total long-chain omega-3 polyunsaturated fatty acids and suggest reduced consumption of total saturated fatty acids," Chowdhury said.
But he said further careful research and specifically large-scale clinical trials are required before making a conclusive judgment and changing dietary guidelines.
Linda Van Horn, from the Northwestern University Feinberg School of Medicine in Chicago, told Reuters Health the study was well done and demonstrated that some fatty acids are better than others. But it's not enough to change current guidelines, she added.
Van Horn chaired the 2010 U.S. Dietary Guidelines Advisory Committee which was involved in creating federal recommendations and is a spokesperson for the American Heart Association. She was not involved in the new review.
"People need to eat as has been recommended - this paper changes nothing about the adverse impact of saturated fat," she said.
Van Horn pointed out that there is no biological need for saturated fats.
"People like their burgers and their hot dogs," she said, "but this study still doesn't make them nutritious."
"Frankly I'm really worried this will confuse consumers," Duffy MacKay told Reuters Health of the findings.
MacKay is senior vice president of scientific and regulatory affairs for the Council for Responsible Nutrition, a trade association in Washington, D.C. He was not involved in the new research.
"It may possibly be used by some as a license to ignore these decades of good advice, common sense and Grandma's advice, and go right for the cheese breads," he said.
He said the report doesn't change what's perceived as a heart-healthy diet.
"I think the concept of a diet high in polyunsaturated fat, low in saturated fat and low in trans fat still holds a lot of weight based on decades of research," he said.
MacKay also said this report does nothing to change the need to get certain fatty acids in the diet.
"It all pointed toward the contribution of EPA and DHA as maintaining heart health and preventing cardiovascular disease, which to me is promising," he said.
Van Horn said the emphasis is still on choosing plant-based foods and fish.
"It's just that now we'll have the ability to be more specific about what the better unsaturated fats choices are," she said.