UNC-CHAPEL HILL (US) — A new look at data missing from a 1966-73 study casts doubt on current dietary advice about vegetable fats and heart health.
For patients with heart disease, switching to polyunsaturated vegetable fats (PUFAs) from saturated animal fats is linked to an increased risk of death, report researchers in the British Journal of Medicine,
“These findings highlight the need to re-evaluate worldwide dietary advice to substitute omega 6 PUFAs for saturated fats,” says study co-author Daisy Zamora, a nutrition epidemiologist and postdoctoral researcher at the University of North Carolina School of Medicine.
The most common PUFA in Western diets is omega-6 linoleic acid; concentrated sources of these fats are safflower oil, corn oil, soybean oil, sunflower oil, and cottonseed oil.
The missing data
An in-depth analysis of the effects of linoleic acid on deaths from coronary heart disease and cardiovascular disease has not previously been possible because data from the Sydney Diet Heart Study—a randomized controlled trial conducted from 1966 to 1973—was missing.
But now, researchers from the US and Australia have recovered and analyzed the original data from this trial, using modern statistical methods to compare death rates from all causes, cardiovascular, and coronary heart disease.
Their analysis involved 458 men aged 30-59 years who had recently had a coronary event, such as a heart attack or an episode of angina.
Participants were randomly divided into two groups. The intervention group was instructed to reduce saturated fats (from animal fats, common margarines, and shortenings) to less than 10 percent of energy intake and to increase linoleic acid (from safflower oil and safflower oil polyunsaturated margarine) to 15 percent of energy intake.
Safflower oil, a concentrated source of omega-6 linoleic acid, provides no omega-3 PUFAs, typically associated with fish oils.
The control group received no specific dietary advice.
Both groups had regular assessments and completed food diaries for an average of 39 months. All non-dietary aspects of the study were designed to be equal in both groups.
The results show that the omega-6 linoleic acid group had a higher risk of death from all causes, as well as from cardiovascular disease and coronary heart disease, compared with the control group.
Mending the gap
The authors then used the new data to update an earlier meta-analysis (a review of all the evidence). This also showed no evidence of benefit, and suggested a possible increased risk of cardiovascular disease, emphasizing the need to rethink mechanisms linking diet to heart disease.
The researchers conclude that recovery of these missing data “has filled a critical gap in the published literature archive” and that these findings “could have important implications for worldwide dietary advice to substitute omega-6 linoleic acid (or polyunsaturated fatty acids in general) for saturated fatty acids.”
In an accompanying editorial, Philip Calder, professor of nutritional immunology at the University of Southampton says the new analysis of these old data “provides important information about the impact of high intakes of omega 6 PUFAs, in particular linoleic acid, on cardiovascular mortality at a time when there is considerable debate on this question.”
The findings argue against the “saturated fat bad, omega 6 PUFA good” dogma and suggest that the American Heart Association guidelines on omega-6 PUFAs may be misguided, he says. They also “underscore the need to properly align dietary advice and recommendations with the scientific evidence base.”
Co-authors of the study contributed from the National Institutes of Health, University of North Carolina, University of New South Wales, Australia, and University of Illinois at Chicago.
Funding for the study came from the National Institute on Alcohol Abuse and Alcoholism (NIAAA), a component of the US National Institutes of Health, and The Life Insurance Medical Research Fund of Australia and New Zealand.