While omega-3 consumption was not affected, the men were also asked to lower their saturated fat intake so that it made up less than 10 percent of their diets. They did so by substituting safflower oil for animal fats, common margarines and shortening oils, salad dressings, baked goods and other products, according to the study.
The second group continued their routine nutritional habits, and both groups kept food diaries and underwent regular assessments during the three-year-plus study period.
By newly crunching all the original data the NIH team found that, compared to the no-dietary-change group, the linoleic acid group faced a higher risk of death, from both heart disease specifically as well as from all causes overall.
In turn, Ramsden's investigators included their Sydney conclusions in a new review of all studies to date exploring the impact of omega-6 consumption.
Despite the fact the American Heart Association (AHA) currently recommends that 5 percent to 10 percent of all calories come from polyunsaturated fat (principally from omega-6), the NIH team found no evidence to support the notion that linoleic acid confers health benefits. The review highlighted the possibility that boosting omega-6 consumption may actually increase the risk for developing heart disease.
Ramsden acknowledged that the Sydney study had been narrowly focused on a specific group of people: middle-aged men with a history of heart disease, who were asked to consume linoleic acid in quantities far exceeding both AHA guidelines and the dietary habits of most Americans.
"So, yes, one of the limitations of this kind of study is always the question of its generalizability to other populations," he cautioned.
But he also suggested that his team's rigorous look back offered valuable insights into how nutritional science has evolved.
"What to many people had at one time seemed pretty straightforward in the 1960s has turned out to form a much less black-and-white picture," he said. "Polyunsaturates are not just involved in cholesterol-lowering. They may also be involved in inflammation, oxidation or clotting. So, it's a very complex picture. And though our goal is not to come up with diet advice, it may be that our conclusions will have important implications for nutritional guidelines."