‘Vegetable oils’ derived from sunflower, soya, safflower and corn are generally promoted as healthy. They are rich in ‘polyunsaturated’ fats that come in two basic forms: omega-6 and omega-3. Omega-6 oils tend to be inflammatory in nature and also encourage clotting in the blood. Omega-3 fats tend to do the reverse. It’s generally well accepted, I think, that a balance of omega-6 and omega-3 fats in the diet is required for optimal health. The problem is, a typical Western diet tends to provide a glut of omega-6 (compared to omega-3), and some believe this is a potential driver of chronic (long-term) disease including cardiovascular disease.
In 2009, Health Canada (the government body which provides health advice to Canadian people) approved a health claim for polyunsaturated fats. The claim was that these fats could reduce the risk of heart disease through their ability to lower blood cholesterol levels. However, a paper published in the Canadian Medical Association Journal this week mounts a credible challenge to the appropriateness of this claim for some foodstuffs [1].
The authors of this study point out that the impact of fats modification on the diet does not always predict the health outcome of such a change. They draw our attention to the fact that omega-3 fats appear to reduce the risk of cardiovascular disease, although they generally have no effect on cholesterol levels or even increase them.
They also refer to a review published earlier this year in the British Medical Journal [2]. The authors of the review pooled together studies in which saturated fat in the diet was replaced with polyunsaturated fats. Overall, the review found no evidence of benefits when saturated fat was replaced with omega-6 fats. However, there was evidence of benefit when saturated fat was replaced with a mix of omega-6 and omega-3 fats. They raise the possibility that omega-6 fats might actually promote cardiovascular disease.
The authors quote a study that was featured in the BMJ review (The Sydney Diet Heart Study) [3]. In this study, 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 (rich in the omega-6 fat known as ‘linoleic acid’, but very low in the omega-3 fat found in plants known as ‘alpha-linolenic acid’). A similar number of men got no dietary instruction and acted as controls.
In the men eating the ‘heart-healthy’ diet, risk of death was elevated by 62 per cent. Risk of death from cardiovascular disease was increased by 70 per cent. The authors of the review concluded:
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.
The authors of the CMAJ analysis conclude:
In summary, much confusion has arisen from unclear nomenclature with regard to polyunsaturated fatty acids and the lack of reporting of food sources in prospective cohorts. Data from prospective cohort studies and randomized controlled trials generally support the replacement of saturated fats with mixed polyunsaturated fatty acids to reduce the risk of death from coronary artery disease. However, it is unclear whether oils rich in omega-6 linoleic acid but low in omega-3 alpha-linolenic acid also reduce this risk. We suggest that the health claim be modified such that foods rich in omega-6 linoleic acid but poor in omega-3 alpha-linolenic acid be excluded.
I feel these recommendations are supported by the science. I note that one of the authors of this review has received fees or financial support from Bunge (an agribusiness) and the food companies Unilever and Kraft. The summary of the paper closes:
Future research is needed to examine the dietary methodologies used to measure polyunsaturated fatty acid intakes in observational studies and the metabolism of polyunsaturated fatty acids, as well as their role in coronary artery disease and other diseases.
This last part is important, and in particular we need to know what the effect of fats on actual risk of death and disease is. Too often in the past have we been recommended to make changes to our diet based on the impact this has on our cholesterol levels.
As the authors of this study make clear, basing recommendations on this criterion can sometimes be misguided. And as the Sydney Diet Heart Study shows, judging a foodstuff by its impact on cholesterol levels rather than its impact on health can even be the death of us. Food companies please take note.
References:
1. Bazinet RP, et al. Omega-6 polyunsaturated fatty acids: Is a broad cholesterol-lowering health claim appropriate. CMAJ online before print 11 November 2013
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
