A RE-EVALUATION of a 40-year-old study has cast doubt on the cardioprotective role of omega-6 polyunsaturated acids and underlines the importance of leaving no stone unturned in analysing clinical data, says an Australian expert.
Professor Anthony Keech, deputy director of the NHMRC’s Clinical Trials Centre, said it was important for all data collected in clinical trials to come to light.
Researchers from the US National Institutes of Health recovered unreported data from the Sydney Diet Heart Study, conducted in 1966‒1973, which showed substituting dietary linoleic acid for saturated fats increased the rates of death from all causes (17.6% vs 11.8%); cardiovascular disease (17.2% vs 11.0%); and coronary heart disease (16.3% vs 10.1%). (1)
The single-blinded randomised controlled trial of 458 men aged 30‒59 years who had been hospitalised with a coronary event used safflower oil in the intervention group, which contains high levels of linoleic acid but no other polyunsaturated fatty acids.
The researchers originally reported increased overall mortality in the dietary intervention group compared with the no intervention group, but did not report the primary survival outcomes, such deaths from all causes, cardiovascular disease or coronary heart disease.
“We’ve waited an awfully long time for that to happen with this data set”, Professor Keech told MJA InSight. “Even though a primary end point might not be significant, it’s still important to look at the rest of the data because it might generate important hypotheses about whether things are good or bad for you.”
The researchers said the findings could have important implications for worldwide dietary advice to substitute omega 6 linoleic acid, or polyunsaturated fats in general, for saturated fats.
However, Professor Paul Nestel, cardiovascular nutritionist with the Baker IDI Heart and Diabetes Institute, said the results from the small Sydney study conflicted with the results of more recent, much larger studies.
“We now have evidence over 20‒30 years from very large prospective trials, such as the Nurses’ Health Study and the Physicians’ Health Study, with collectively more than 100 000 people, and there’s no question that substituting linoleic acids for saturated fatty acids is associated with significant reductions in all cardiovascular events”, Professor Nestel told MJA Insight.
Professor Nestel said the proportion of linoleic acid used in the Sydney study (15% of food energy) was far above the maximum levels of 7%‒10% recommended by health organisations around the world.
“[This is] partly because oils are oils — they’re high-energy foods — not so much because we have other concerns”, he said.
“We all agree that increasing the long-chain omega 3s from fish oils is highly desirable, it’s not a question of either/or, it’s a recognition that both … are healthy and a reasonable ratio, such that is recommended around the world, is probably the way to go.”
Professor Peter Clifton, Baker IDI’s head of nutritional interventions, said the results may have been confounded by the presence of trans fatty acids in the margarine used for the intervention group.
“Although dismissed by the authors, trans fats are likely to be responsible for the lack of benefit. The margarine would have contained at least 20% trans fatty acids,” Professor Clifton said.
However, Professor Keech said the data suggested that all polyunsaturated fats were not alike and that oils with low concentrations of linoleic acid, such as olive oil, might be the better choice from a cardiac-risk perspective.
“Overall, replacement of saturated fats with polyunsaturated fats is still advantageous based on the overall evidence, but those that are rich in linoleic acids don’t seem to share the same benefits based on this [study]”, he said, adding that it would be useful to have more data.
An editorial in the same issue of the BMJ said the findings underscored the need to properly align dietary advice and recommendations with the scientific evidence base. (4)
“These findings argue against the ‘saturated fat bad, omega 6 PUFA good’ dogma”, the editorial said.
– Nicole MacKee
Posted 11 February 2011