What research is this based on? These headlines are based on a recent article, published in the BMJ on 5th February 2013, in which researchers have reanalysed the data from a study conducted in the late 1960’s and early 1970’s, the Sydney Diet Heart Study. The original data was reanalysed to investigate the effects of omega-6 linoleic acid on all-cause mortality (death from all causes) and mortality from cardiovascular disease (CVD) or coronary heart disease (CHD). This dietary intervention study recruited 458 males, aged 30-59 years, who had recently suffered a heart attack or coronary event.
The intervention study group were given dietary advice to reduce their saturated fat intake to below 10% of energy intake and increase their n-6 polyunsaturated fatty acid intake to 15% of energy intake by using safflower oil and safflower margarine. The control group were given no dietary advice. The original researchers followed the participants for an average of 39 months. The recent analysis found significantly higher rates of all-cause mortality and mortality from both CVD and CHD in the intervention group. Including the data in a linoleic acid intervention meta-analysis did not show a significant increase in mortality from CVD and CHD, although a trend towards significance was reported.
Can polyunsaturated fatty acids increase the risk of mortality?
This is an interesting study and it highlights that the science behind the effects of different fatty acids on heath is complex. The data in this study are from the late 1960s and 1970s when total fat and saturated fat intakes were much higher than they are currently. Total fat, saturated fat and polyunsaturated fatty acid (PUFA) intake of the study group at the start of the study was around 40, 16 and 6% of energy intake, whilst current average intakes for an adult aged 19-65 are 35, 13 and 6% of energy intake, respectively.
The intervention study group was instructed to reduce their saturated fat intake to less than 10% of energy whilst increasing their n-6 PUFA intake to 15%. This is well beyond the current average n-6 PUFA intakes and it is worth noting that current UK public health recommendations are to maintain an intake below 6.5% of energy for total PUFAS as a population average and 10% of energy as an individual maximum, meaning that the intervention group in this study had intakes of n-6 PUFA that were significantly higher than the current maximum recommended in the UK. Recommendations for total fat and saturated fat are to consume no more than 35% and 11% of energy intake, respectively.
Cholesterol is a well-known risk factor for cardiovascular disease and the intervention group did show a decrease in blood cholesterol which is consistent with previous studies. In addition, more recent prospective cohort studies have shown a reduction in cardiovascular disease risk with a higher n-6 PUFA or n-6 and n-3 PUFA diet. It is currently unknown what the relative contributions of n-6 and n-3 PUFAs are to the beneficial effects of PUFA but many spreads in the UK are formulated to contain both n-6 and n-3 PUFAs.
It is also worth noting that the study was conducted in men who had a high risk of mortality, so although the findings warrant consideration, the results may not be applicable to the general population in the UK today. The study participants were aware of which group they were in, intervention or control group, and as such it is possible that participants in the control group made more of an effort to make lifestyle changes such as increasing physical activity. Dietary intake of other nutrients and food groups, such as trans fatty acids and fruit and vegetable intake, were unfortunately not reported.
In summary, although this is an interesting study, more clinical evidence is needed before any generalisation of the findings are made. The advice for consumers should still be to choose foods with a lower saturated fat content and to choose fats and oils that supply primarily unsaturated fats.