Tuesday February 5 2013
Products such as margarine may contain omega-6 fatty acids
A study published today in the British Medical Journal finds that people who followed health advice and ate certain omega-6 polyunsaturated fats instead of animal fats had higher death rates. This study has prompted media comment as it appears to contradict established health guidance. Polyunsaturated fats are commonly used in "healthy" margarines, spreads and other alternatives to butter.
However, experts say that we should not be unduly alarmed. The Science Media Centre has issued a statement that says that the research, "does not alter our understanding of the possible relationship between diet and cardiovascular risk" and that "the claims in the paper are not new or at odds with existing evidence".
There's a danger of over-interpreting this research. It focused on one, not all, omega-6 polyunsaturated fats, and the results are in a very specific group – middle-aged men who had had heart attacks.
The study suggests that not all polyunsaturated fatty acids are good for the heart. But British consumers should not panic – the safflower oil used as a source of omega-6 in this study is rarely used in this country.
Where did the story come from?
The study was carried out by researchers in the US and Australia. It was funded by the Life Insurance Medical Research Fund of Australia and New Zealand and the Intramural Program of the National Institute on Alcohol Abuse and Alcoholism, US National Institutes of Health.
The study was published in the peer-reviewed British Medical Journal.
What kind of research was this?
This was a second analysis of a randomised controlled trial performed in Australia between 1966 and 1973. A randomised controlled trial is the ideal trial design to examine cause and effect. However, the current analysis includes outcomes that were not primary outcomes of the original trial.
The original trial investigated whether replacing sources of saturated fats, such as animal fats and butter, with safflower oil (a kind of oil used for cooking and in some manufactured foods) reduced the risk of death from any cause in men with premature coronary heart disease. It only reported the risk of death from all causes and deaths due to cardiovascular disease (CVD) or coronary heart disease (CHD) were not examined.
In this new study, the researchers calculated whether eating more safflower oil affected the risk of death in people with cardiovascular or coronary heart disease (this is called "secondary prevention"). The researchers also wanted to know to what extent an increased intake of polyunsaturated fatty acids or saturated fatty acids was associated with deaths from CVD or CHD. The results of this new analysis were then used to update a meta-analysis of other trials looking at polyunsaturated fatty acids for cardiovascular risk reduction.
What did the research involve?
Researchers recruited 458 men aged between 30 and 59 who had suffered a heart attack or an episode of coronary insufficiency or angina after admission to hospital. These men were randomised to receive either a dietary intervention or no specific dietary instruction, in addition to standard medical care.
The dietary intervention consisted of instructions to:
- increase polyunsaturated fatty acid intake to about 15% of total energy intake
- reduce saturated fatty acid intake to less than 10% of energy intake
- reduce cholesterol to less than 300mg per day
To help achieve these targets, the men were given liquid safflower oil and safflower oil polyunsaturated margarine. They were told to use these items to replace animal fats, butter and margarine, shortenings, cooking oils and salad dressing, as well as taking safflower oil as a supplement. Safflower oil contains 74.6g per 100g of a type of polyunsaturated fat called omega-6 linoleic acid, and no other polyunsaturated fatty acids.
Men returned for clinical assessment every three months for the first year and then every six months for a median of 39 months. Blood samples were taken to measure the levels of cholesterol and triglyceride (fat). The men also filled in a food diary so that their diet could be assessed.
Deaths that occurred during the trial were assigned codes from the International Classification of Diseases (ICD), according to information taken from death certificates of final hospital admission records. Using survival analysis, the researchers analysed whether the risk of death from any cause or deaths from cardiovascular and coronary heart disease differed between the intervention and the control group. The researchers also examined whether nutrient intake (based on the results of the food diaries) accounted for changes in mortality.
What were the basic results?
- men in the intervention group significantly increased their intake of polyunsaturated fatty acids, and significantly reduced their intake of saturated fatty acids, cholesterol and mono-unsaturated fatty acids compared with the control group
- the level of cholesterol in the blood decreased significantly more for men in the dietary intervention group compared with the control group, although changes in the level of triglycerides (fats) in the blood, body mass index (BMI) and blood pressure were similar between groups
- men in the dietary intervention group had higher rates of deaths from any cause than controls (17.6% of the dietary intervention group died compared with 11.8% of the no intervention group, hazard ratio 1.62, 95% confidence interval 1.00 to 2.64)
- men in the dietary intervention group had higher rates of death from cardiovascular disease (17.2% of the dietary intervention group died due to cardiovascular disease compared with 11.0% of the no intervention group, hazard ratio 1.70, 95% confidence interval 1.03 to 2.80)
- men in the dietary intervention group had higher rates of deaths from coronary heart disease (16.3% of the dietary intervention group died due to coronary heart disease compared with 10.1% of the no intervention group, hazard ratio 1.74, 95% confidence interval 1.04 to 2.92)
- an increase in 5% of food energy from omega-6 linoleic acid predicted a 35% higher risk of cardiovascular death and a 29% increase in all-cause mortality in the intervention group
When these results were added to a meta-analysis of other trials that have assessed the effects of linoleic acid, it was found that linoleic acid increased the risk of death from coronary heart disease and cardiovascular disease, although these results were not significant.
How did the researchers interpret the results?
The researchers have concluded that there is no clear clinical evidence that the most common polyunsaturated fatty acid, omega-6 linoleic acid, can reduce people's risk of developing heart conditions. "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."
Contrary to received wisdom, this research suggests that not all polyunsaturated fatty acids are good for the heart (the so-called "cardioprotective effect").
This study has several strengths. It was a randomised controlled trial, using just one type of oil to increase consumption of polyunsaturated fatty acids.
However, the study also has its limitations. The dietary data collected during the original trial does not contain enough information to rule out the possibility that changes in other nutrients could have caused the effect seen.
In this trial, participants were advised to increase their intake of polyunsaturated fatty acids, mainly from omega 6-linoleic acid, to 15% of total food energy, and the results may not be generalisable to lower linoleic acid intakes.
As this trial was performed on men aged between 30 and 59 who had premature coronary heart disease, it may not be possible to generalise the results to men who do not have coronary heart disease, men of different ages, and women.
It is worth noting that vegetable oils have very different characteristics in terms of the proportions of omega-3 or omega-6 content and the types of polyunsaturated, monounsaturated and saturated fatty acids that they contain. Oleic acid and linoleic acid are likely to have different properties, and so it cannot be assumed any effect seen here is typical of all vegetable oils.
Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on Twitter.