Diets or supplements that aim to increase omega-6 fatty acids have no clear effect on the risk of cardiovascular events like heart attacks or stroke, overall deaths or deaths from cardiovascular disease. Omega-6 fatty acids are poly-unsaturated oils largely derived from seeds and nuts such as sunflower oil, corn oil, soybean oil, walnuts and pumpkin seeds.
The results of this Cochrane review can be looked at alongside another recent Cochrane review, which found no benefits of fish oil in omega-3 fatty acid supplements in reducing cardiovascular events or deaths.
These findings highlight the limited benefits of modifying one dietary component at a time. A balanced diet includes a variety of foods, any or all of which can affect health, meaning that studying the effects of diet on health is complex. The results of this review are unlikely to change guideline recommendations.
Why was this study needed?
Cardiovascular disease describes a range of conditions caused by atherosclerosis (stiffening and furring of the arteries) including coronary heart disease and strokes. It is responsible for a quarter of deaths in the UK. Cardiovascular disease is estimated to cost the NHS almost £9 billion and £4 billion in costs to society.
A diet high in omega-6 polyunsaturated fatty acids is believed by some to protect against heart disease and stroke, although evidence is inconsistent. The most important type of omega-6 fatty acid is linoleic acid, an essential fatty acid that is obtained from food.
This review aimed to address this inconsistency by pooling the evidence to assess the effect of increasing omega-6 fatty acid intake by dietary modification or supplementation on mortality, cardiovascular events and weight.
What did this study do?
This updated Cochrane review found 19 randomised controlled trials that compared interventions to increase omega-6 fatty acids in diets to usual or lower omega-6 fatty acids diets for at least 12 months. Overall, 6,461 adults were included. Participants included people without cardiovascular disease (primary prevention) and people already diagnosed with cardiovascular disease (secondary prevention). The trials took place in North America, Europe, Australia and Asia, with nine from the UK.
Three trials had a low risk of bias; the rest had a moderate to high risk of bias. Many studies were published more than 30 years ago, with one over 50 years old. Worldwide public-health campaigns since that time to lower consumption of saturated fats may have increased the baseline level of omega-6 fatty acids consumption. This means that more recent studies may have been less likely to show an effect than older studies.
What did it find?
- Higher omega-6 fatty acids intake showed no effect on overall risk of death (risk ratio [RR] 1.00, 95% confidence interval [CI] 0.88 to 1.12; 10 trials with 4,506 participants,). In both groups about 17% of people died, during follow-up of up to eight years and some studies included only people older than 65 years. The lack of effect was consistent across trials.
- Higher omega-6 fatty acids intake had no effect on deaths from cardiovascular disease (RR 1.09, 95% CI 0.76 to 1.55; 7 trials with 4,019 participants. About 13% of people on a higher omega-6 fatty acids diet and about 11% of people on lower omega-6 fatty acid diets died from cardiovascular diseases.
- Higher omega-6 fatty acids intake had no effect on cardiovascular events (RR 0.97, 95% CI 0.81 to 1.15; 7 trials with 4,962 participants). About 30% of people on each diet had a cardiovascular event such as myocardial infarction or stroke.
- There was no effect on weight, and inconsistent effects on cholesterol, with total blood cholesterol lower with high omega-6 fatty acid intake in 10 trials but no effect on blood levels of high-density lipoprotein (‘good’ cholesterol) or low-density lipoprotein (‘bad’ cholesterol).
What does current guidance say on this issue?
NICE’s 2016 guideline on risk assessment and reduction for cardiovascular disease has no recommendations on the omega-6 fatty acids content of the diet. However, it recommends replacing saturated fats with mono-unsaturated or polyunsaturated fats. It also recommends consumption of some foods high in omega-6 fatty acids such as nuts, seeds and rapeseed oil, or spreads based on rapeseed oil.
An update of this guideline is planned, which will look at the wording of recommendations on dietary advice.
What are the implications?
The results of this systematic review provide little if any evidence that a higher intake of omega-6 fatty acids improves cardiovascular disease risks. Though this review set out to settle the debate, it was hampered by small studies of short duration with insufficient cases of deaths or cardiovascular events.
This limits the precision in the results. Any biases might be expected to increase any effect of diet so are unlikely to explain the lack of benefit seen here.
As foods high in omega-6 fatty acids are important parts of a balanced, healthy diet. Dietary advice is unlikely to change as a result of this study.
Citation and Funding
Hooper L, Al-Khudairy L, Abdelhamid AS et al. Omega-6 fats for the primary and secondary prevention of cardiovascular disease. Cochrane Database Syst Rev. 2018;(11):CD011094.
Cochrane UK and the Cochrane Heart Group are supported by NIHR infrastructure funding. WHO provided funding to the University of East Anglia to support this Cochrane review.
Abdelhamid AS, Brown TJ, Brainard JS et al. Omega‐3 fatty acids for the primary and secondary prevention of cardiovascular disease. Cochrane Database Syst Rev. 2018;(7):CD003177.
NHS website. Cardiovascular disease. London: Department of Health and Social Care; updated 2018.
NICE Cardiovascular disease: risk assessment and reduction, including lipid modification. CG181. London: National Institute for Health and Care Excellence; 2014.
Public Health England. Action plan for cardiovascular disease prevention, 2017 to 2018. London: Public Health England; 2017.
Produced by the University of Southampton and Bazian on behalf of NIHR through the NIHR Dissemination Centre