#502: Sydney Diet-Heart Study – Is Linoleic Acid Causing Heart Disease?
Nov 21, 2023
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Discussion on the Sydney Diet-Heart Study and its reanalysis, questioning the link between linoleic acid and heart disease risk. Emphasis on the need for further evidence and other studies. Analysis of flaws in the study and evidence supporting the benefit of replacing saturated fat with polyunsaturated fat to reduce heart disease risk.
The Sydney Diet-Heart Study showed that substituting linoleic acid for saturated fats increased all-cause, cardiovascular disease, and coronary heart disease mortality, but it has several limitations and should not be relied upon as representative of the wider evidence base supporting the benefits of polyunsaturated fats in reducing cardiovascular disease risk.
The Sydney Diet-Heart Study's reported hazard ratios indicating an increased risk of coronary heart disease mortality have wide confidence intervals, suggesting imprecision in the effect estimate, and when analyzed in the context of other polyunsaturated fat intervention trials, it stands out as an outlier, with well-conducted trials consistently showing a lower risk of cardiovascular disease with increased polyunsaturated fat intake.
Deep dives
The Sydney Diet Heart Study: Examining the Controversy
The Sydney diet heart study, a randomized controlled trial conducted in the 60s, gained attention in recent years due to a reanalysis. The study investigated the effects of substituting linoleic acid for saturated fats in men with coronary heart disease. The headline finding, often cited, indicates that this substitution increased all-cause mortality, cardiovascular disease, and coronary heart disease mortality. However, the study has several limitations, including a small sample size and methodological flaws. The assumptions made about the intervention foods, such as margarine, raise questions about the interpretation of the results. Overall, the Sydney diet heart study is not representative of the wider evidence base that consistently supports the benefits of polyunsaturated fats in reducing cardiovascular disease risk.
Imprecision and Limitations of the Sydney Diet Heart Study
The Sydney diet heart study's reported hazard ratios show a significant increase in risk of coronary heart disease mortality. However, the wide confidence intervals indicate imprecision in the effect estimate. With a small number of events and potential confounders like trans fatty acid consumption from margarine, the study's conclusions are uncertain. When the study is analyzed in the context of other polyunsaturated fat intervention trials, it stands out as an outlier. Meta-analyses and pooled analyses of well-conducted trials consistently show a lower risk of cardiovascular disease with increased polyunsaturated fat intake.
Strength of Evidence from Other Intervention Trials
The Sydney diet heart study should not be relied upon as a basis for conclusions about the effects of linoleic acid or polyunsaturated fats on cardiovascular health. In contrast, other well-designed intervention trials show consistently lower risks of coronary heart disease events and mortality with increased polyunsaturated fat consumption. Trials like L.A. Veterans, Oslo Diet Heart, Medical Research Council, and Finnish Mental Hospital Studies provide stronger evidence for the benefits of polyunsaturated fats. Synthesizing this evidence together leads to a confident conclusion that replacing saturated fats with polyunsaturated fats reduces cardiovascular disease risk.
The Need to Consider Wider Evidence
Rather than relying solely on the Sydney diet heart study, which has methodological limitations, it is better to consider the wider body of evidence on the effects of polyunsaturated fats on heart disease. High-quality observational studies, metabolic ward studies, and biomarker studies all support the conclusion that replacing saturated fats with polyunsaturated fats is beneficial for cardiovascular health. The Sydney diet heart study's isolated findings should not overshadow this broader and more robust evidence base.
The Sydney Diet-Heart Study was a clinical trial conducted in the 1960s and 1970s that aimed to examine the hypothesis that reducing saturated fat intake in the diet would lead to a reduced risk of heart disease.
However, it really only gained attention after a more recent re-analysis by Ramsden et al., which in recent years has been used as supporting evidence for the idea that increased polyunsaturated fat (PUFA), and specifically linoleic acid, in addition to reduced saturated fat intake, can increase heart disease risk.
This was based on the findings that substituting linoleic acid in place of saturated fat increased all-cause, CVD and CHD mortality.
This is of course counter to prevailing consensus and guidelines in this area, which routinely show reduced risk on replacing SFA with PUFA.
Could this trial undermine the common conclusions that replacing saturated fat with polyunsaturated fat decreases heart disease risk?
In this episode Alan and Danny discuss some of the crucial aspects to understand about this study and what it means for what conclusions can be made about the impact of PUFA broadly, and linoleic acid specifically, on our health.
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