A new analysis of data missing from a study that ended more than 40 years ago casts doubt on current dietary advice about vegetable fats and heart health.
The clinical trial published in the British Journal of Medicine shows that replacing saturated animal fats with omega-6 polyunsaturated vegetable fats (PUFAs) is linked to an increased risk of death among patients with heart disease. Nutrition epidemiologist and study co-author, Daisy Zamora, PhD, is postdoctoral researcher in the Program on Integrative Medicine in the Department of Physical Medicine & Rehabilitation at the University of North Carolina School of Medicine. She said, “These findings highlight the need to re-evaluate worldwide dietary advice to substitute omega 6 PUFAs for saturated fats.”
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, a team of researchers from the U.S. 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.
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.”
Co-authors of the study along with Zamora are Christopher E. Ramsden, U.S. National Institutes of Health (NIH); Katourah R. Faurot, UNC; Boonseng Leelarthaepin, University of New South Wales, Australia; Sharon F Majchrzak-Hong, NIH; Chirayath M. Suchindran, UNC Gillings School of Global Public Health; Amit Ringel, NIH; John M. Davis, University of Illinois at Chicago; and Joseph R. Hibbeln, NIH. Ramsden, Zamora, and Faurot also are affiliated with the Program on Integrative Medicine, Department of Physical Medicine and Rehabilitation, at UNC School of Medicine.
Funding for the study came from the National Institute on Alcohol Abuse and Alcoholism (NIAAA), a component of the U.S. National Institutes of Health; and The Life Insurance Medical Research Fund of Australia and New Zealand.