Sydney Diet Heart Study

Sydney Diet Heart Study (SDHS) is a study on fat and cholesterol, which was downplayed because it showed that mortality increased when replacing saturated fat with polyunsaturated fat.

The study was conducted between 1966 and 1973, but has recently received significant attention for its unexpected findings, and for how the results were originally presented. It took a full 40 years before all the findings were made available.

The Sydney study was a randomized controlled study, with the aim of investigating the effect of replacing saturated fat with polyunsaturated fat from linoleic acid (omega-6) on heart disease and mortality.

Randomized controlled trials are considered to be of high quality, in contrast to weak observational studiesand self reporting, which nutrition research has been drowning in over the last 50 years.

Background and method

The study included 458 men aged 30–59 who had either had coronary events or were at high risk for heart disease. The participants were randomly divided into two groups:

• The intervention group: They were advised to reduce their intake of saturated fat (from butter, animal fat and whole milk) and replace it with polyunsaturated fat from safflower oil and safflower oil margarine, which is very rich in omega-6 fatty acids.
  
• The control group: They continued their usual diet without specific changes.

The study measured mortality from all causes, cardiovascular disease, and coronary heart disease as its primary outcomes.

Results and the reanalysis in 2013

In 1978, the original results were published, but with a focus on how polyunsaturated fats lowered cholesterol levels in the blood. This was interpreted as a positive effect, as lower cholesterol was considered beneficial for heart health.

However, important mortality data not clearly reported. In 2013, researchers led by Christopher Ramsden resumed their analysis of the original data and uncovered shocking findings:

Increased mortality in the intervention group:

• Mortality from all reasons was 17,6% in the intervention group, compared to 11,8% in the control group.
  
• Mortality from cardiovascular diseases was 17,2% in the intervention group, compared to 11,0% in the control group.
  
• Mortality from coronary heart disease was 16,3% in the intervention group, compared to 10,1% in the control group.

Although linoleic acid lowered cholesterol levels, the increased mortality was concerning. This raised questions about whether reducing cholesterol is beneficial at all, and whether higher intakes of omega-6 fatty acids could have negative health consequences.

In recent times, we see studies that say that cholesterol is associated with increased life expectancy, and specifically that omega-6 fatty acids, especially from vegetable oils, are associated with reduced health and an increased risk of cardiovascular disease.

Why was the study watered down?

There is reason to believe that the findings from the study were significantly under-communicated, and this may be due to several factors:

• Dominant narrative: In the 1960s and 70s, the hypothesis that saturated fat causes heart disease was almost uncritically accepted. Findings that contradicted this hypothesis did not fit well with the prevailing ideology.
  
• Industry impact: The margarine and vegetable oil industry had a strong interest in promoting polyunsaturated fats as a healthier alternative, which may have influenced how studies were reported.
  
• Lack of transparency: The critical mortality data were not published in full until almost 40 years later, suggesting that important findings were deliberately withheld.

Influence on dietary advice

The Sydney Diet Heart Study has had a significant but indirect impact on nutrition research and dietary advice. In the decades following the study, recommendations to reduce saturated fat and replace it with polyunsaturated fat were continued in global guidelines, despite a lack of evidence for their effectiveness.

This study, along with other similar findings, has subsequently contributed to a reassessment of the role of saturated fat and omega-6 in the diet.

The findings have highlighted the need for a better balance between omega-6 and omega-3 fatty acids, rather than focusing solely on the total amount of polyunsaturated fat.

It has also raised questions about whether reduction in cholesterol levels alone is a good indicator of heart health, since lower cholesterol in this case was associated with increased mortality.

Lessons and significance today

The study is one of several examples of how nutrition research is manipulated to influence contemporary ideology and not least economic interests. It highlights the importance of transparency in research and the need to evaluate health effects based on hard outcomes, such as mortality, rather than biomarkers alone.

SOURCES

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