CardioExchange Archive

In an exceedingly strange turn of events, data from a clinical trial dating from the 1960s, long thought to be lost, has now been resurrected and may contribute important new information to the very contemporary controversy over recommendations about dietary fat composition.

The American Heart Association has long urged people to increase their consumption of polyunsaturated fatty acids (PUFAs), including omega 6 PUFAs, and reduce their consumption of  saturated fatty acids (SFAs). The recommendations are based on the simple observation that PUFAs lower total and LDL cholesterol while SFAs have the opposite effect. However, the cardiovascular effects of substituting PUFAs for SFAs have never been tested in randomized, well-controlled clinical trials, and a growing proportion of experts now suspect that simple changes in total cholesterol and LDL cholesterol may not tell the whole story.

One trial that actually tested the hypothesis was the Sydney Diet Heart Study, which ran from 1966 through 1973. In the trial, 458 men with coronary heart disease (CHD) were randomized to a diet rich in linoleic acid (the predominant omega 6 PUFA in most diets) or their usual diet. Although total cholesterol was reduced by 13% in the treatment group during the study, all-cause mortality was higher in the linoleic acid group than in the control group. However, in the original publications, and consistent with the practice at the time, deaths from cardiovascular disease (CVD) and CHD were not published.

Now, in a new paper published in BMJ, Christopher Ramsden and colleagues report that they were able to recover and analyze data from the original magnetic tape of the Sydney Diet Heart Study. The new mortality findings are consistent:

• All cause: 17.6% in the linoleic group versus 11.8% in the control group (HR 1.62, CI 1.00-2.64)
• CVD: 17.2% versus 11% (HR 1.70, CI 1.03-2.80)
• CHD: 16.3% versus 10.1% (HR 1.74, CI 1.04-2.92)

The investigators then used these data to perform an updated meta-analysis and found similar but nonsignificant trends for overall mortality and for CVD and CHD mortality. They concluded that their findings “could have important implications for worldwide dietary advice to substitute” PUFAs for SFAs.

In an accompanying editorial, Philip Calder agrees:

These findings argue against the ‘saturated fat bad, omega 6 PUFA good’ dogma and suggest that the American Heart Association advisory that includes the statement ‘higher [than 10% of energy] intakes [of omega-6 PUFAs] appear to be safe and may be even more beneficial’ may be misguided.