Conventional nutritional ‘wisdom’ has it that saturated fat (found in foods such meat, eggs, dairy products, coconut and palm oil) is bad for heart health, while ‘polyunsaturated’ fats (including ‘vegetable’ oils) are good. These concepts emerged decades ago and were mostly based on the impact of these fats on cholesterol levels. There’s a big potential problem here, though: the impact that foods (or anything else) have on cholesterol is not the important thing, it’s the impact that they have on health that really counts.

In 2010, a team of research based in the US reviewed the evidence in which polyunsaturated fats were given to individuals and the effects on cardiovascular disease risk (e.g. heart attacks) were assessed [1]. This review made some attempt to distinguish between the effects of the two main forms of polyunsaturated fat: omega-6 (found in ‘vegetable’ oils such as sunflower oil and safflower oil) and omega-3 (found most abundantly in oily fish such as salmon, trout, mackerel and herring).

This review found that upping omega-6 intakes did not lead to beneficial outcomes for health. However, a mix of omega-3/omega-6 did. In fact, there was a trend (though not statistically significant) for omega-6 supplementation to increase cardiovascular disease risk. The authors concluded: “Advice to specifically increase [omega-6 polyunsaturated fat] intake….is unlikely to provide the intended benefits, and may actually increase the risks of [coronary heart disease] and death.

This evidence was revisited this week by the same team in a study published in the British Medical Journal [2]. The authors added to their previous review with new data that they had extracted from an old study. The study in question, which took place between 1966 and 1973, is known as the ‘Sydney Diet Heart Study’. In this research, about 220 men aged 30-59 were instructed to reduce saturated fat intake and increase polyunsaturated fat intake. The men were supplied with safflower oil and safflower oil-based margarine (both rich in the omega-6 fat known as linoleic acid). A similar number of men got no dietary instruction and acted as controls.

The results of this study were reported in 1978. The men who cut back on saturated fat and boosted their omega-6 intake were found to be at increased risk of death. Effects of the diet on risk of death from cardiovascular disease (including heart disease) were not reported. In this week’s review, the researchers go back to this data to extract this information.

Their analysis confirms the original report of increased risk of death in men on the omega-6-rich diet. Risk of death was elevated by 62 per cent.

However, new findings previously unearthed (or undeclared) were that the men eating the ‘heart-healthy’ diet were at increased risk of dying from cardiovascular disease and heart disease (increases of 70 and 74 per cent respectively).

So, the very diet designed to reduce the risk of heart disease and fatal heart attack was found to have the opposite effect: It killed men, and specifically from heart disease.

The inclusion of this data in the wider evidence base made difference to the original finding from 2010 that omega-6 fat supplementation did not benefit health. Here’s the authors’ conclusions:

Advice to substitute polyunsaturated fats for saturated fats is a key component of worldwide dietary guidelines for coronary heart disease risk reduction. However, clinical benefits of the most abundant polyunsaturated fatty acid, omega 6 linoleic acid, have not been established. In this cohort, substituting dietary linoleic acid in place of saturated fats increased the rates of death from all causes, coronary heart disease, and cardiovascular disease. An updated meta-analysis of linoleic acid intervention trials showed no evidence of cardiovascular benefit. These findings could have important implications for worldwide dietary advice to substitute omega 6 linoleic acid, or polyunsaturated fats in general, for saturated fats.

You may wish to take this study and the authors’ conclusion and wave them under the face of anyone suggesting that you need to swap ‘deadly’ butter for ‘healthy’ margarine or vegetable oils. There is no good evidence that this is beneficial to health, and there never was. If fact, we have at least some evidence that this oft-touted nutritional strategy actually jeopardises health and life itself.

References:

1. Ramsden CE, et al. n-6 fatty acid-specific and mixed polyunsaturate dietary interventions have different effects on CHD risk: a meta-analysis of randomised controlled trials. Br J Nutr 2010;104:1586-600

2. Ramsden CE, et al. Use of dietary linoleic acid for secondary prevention of coronary heart disease and death: evaluation of recovered data from the Sydney Diet Heart Study and updated meta-analysis BMJ 2013;346:e8707