Re: 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
Can vegetable oil processing help explain the increased cardiovascular mortality in the Sydney Diet Heart Study?
We appreciate Dr. Gutierrez’s interest in our manuscript “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” 1, where we reported that: (1) patients randomized to the omega-6 linoleic acid (n-6 LA) intervention group had increased cardiovascular (CVD) mortality, and (2) the magnitude of increase in n-6 LA intake among this intervention group was associated with higher risk of CVD mortality. We extensively discussed potential limitations, including the possibility that trans fat intakes may have been modified, as noted by Dr. Gutierrez. However, we also noted that the sum of changes in both groups likely produced minimal between-group differences in trans fat intakes.
In this letter we (1) provide further context to explain why trans fats are not a convincing explanation for the observed increased CVD death; and (2) expand on our proposed mechanistic model1 to include the possibility that marked between-group differences in dietary n-6 LA oxidation products may have contributed to the unfavorable effects of the intervention.
Key considerations 1 2-16:
Consumption and displacement of trans fats:
• The primary intervention fat source was liquid safflower oil, a concentrated source of n-6 LA that contains little or no trans fat. The intervention group was advised to consume 2 to 4 tablespoons per day, displacing rich sources of saturated fat, but also some important sources of trans fat including common hard margarines and shortenings. They were also advised to avoid commercial products made with common hard margarines (for example biscuits, cakes, pastries and puddings). These substitutions would have reduced trans fat in the n-6 LA intervention group compared to the control group.
• While the safflower oil soft polyunsaturated margarine that was provided to the intervention group likely contained some trans fat, it replaced not only butter, but also common table margarines, an important source of trans fat. This safflower oil polyunsaturated margarine was selected for its high n-6 LA content (about 48% of fat), nearly 3-to-1 polyunsaturated to saturated fat ratio, and cholesterol lowering properties. Although the precise amount of trans fat in this margarine was not specified, these are characteristics of soft margarines that usually contain lower amounts of trans fat compared to commercially available margarines that it would have displaced.
• Many patients in the control group also began replacing butter with commercially available polyunsaturated margarines. This substitution helps explain the substantial (but modest in comparison to the intervention group) increase in polyunsaturated fat intake in the control group. Therefore, both the intervention group and control group likely consumed some trans fat from polyunsaturated margarines.
• The intervention group also markedly reduced their intake of dietary sources of ruminant trans fats. Trans fats of ruminant origin have been associated with increased risk of CVD death in some 17, but not all 18, published observational studies.
Collectively, these observations indicate that the two groups were not likely to have substantial differences in trans fat consumption from hydrogenated sources, and the control group likely consumed higher amounts of trans fats from ruminant sources.
Putting trans fats into context with the full trial results:
• Trans fats raise serum total and low-density lipoprotein (LDL) cholesterol. Thus, the significant reduction in serum cholesterol among the n-6 LA intervention group (-13%) compared to the control group (-5.5%) in this randomized controlled trial is not consistent with the premise that increased mortality in the intervention group was due to trans fat intake.
• Dietary trans fats are predominantly 18-carbon monounsaturated fat isomers. In our analysis of changes in polyunsaturated fat in the intervention group consuming safflower oil (a concentrated source of n-6 LA), we found a robust association between the increase in polyunsaturated fats and cardiovascular death. Adjusting this analysis for monounsaturated fat (an imperfect surrogate for trans fats, as explained in our manuscript) did not noticeably alter this association between PUFA and increased mortality.
Together, these observations and other factors discussed in our manuscript indicate that trans fats are not a convincing explanation for the increased risk of death in the n-6 LA intervention group.
Dietary linoleic acid oxidation products as mediators of cardiovascular disease
We do agree in principle with Dr. Gutierrez, that processing of high n-6 LA oils (in this case safflower oil) is a plausible factor that may have contributed to the observed increased risk of cardiovascular death. The intervention group was advised to substitute liquid safflower oil (containing nearly 75% n-6 LA by weight) for saturated fats whenever possible, including when frying or otherwise cooking. At the time it was not appreciated that thermoxidation and frying of n-6 LA enriched oils generate more oxidized linoleic acid metabolites (OXLAMs) including 9- and 13-hydroperoxy-octadecadienoic acids, and 9- and 13-hydroxy-octadecadienoic acids, than fats that are predominantly saturated or monounsaturated 19-20. Lowering dietary n-6 LA reduces circulating OXLAMs in humans 21, presumably by reducing the substrate for endogenous conversion of n-6 LA to OXLAMs. However, humans also readily absorb oxidized fatty acids from dietary sources 22. Consumption of fried n-6 LA enriched oils has been shown to increase circulating OXLAMs in humans23.
In the mechanistic model proposed in our manuscript 1 24-42, we noted that OXLAMs: (1) are the most abundant oxidized fatty acids in oxidized low-density lipoprotein (oxidized LDL); (2) are enriched in the lipid-laden macrophage foam cells, vascular endothelial cells, and migrating vascular smooth muscle cells of atherosclerotic lesions; and (3) have been mechanistically linked to cardiovascular disease pathogenesis.
We previously proposed that the combination of high n-6 LA diets and an endogenous source of oxidative stress (for example smoking or heavy drinking) facilitated OXLAM-mediated atherosclerotic progression and increased CVD mortality. Consistent with this model, the link between the magnitude of increase in LA and mortality was robust in smokers and drinkers, suggesting that diets high in n-6 LA may be particularly detrimental in the context of endogenous oxidative stress.
Here, we expand on this mechanistic model to propose that exogenous OXLAMs (from heated safflower oil) may have contributed to the increased risk of CVD death in the LA intervention group. Consistent with this model, dietary OXLAMs have been shown to increase oxidation of circulating lipoproteins and to promote atherosclerotic progression in rabbits and mice 43 44.
Importantly, LA-enriched oils are ingredients in many food products that are fried or otherwise heated (for example potato chips, french fries, crackers, numerous snack foods). However, quantitative data on OXLAM content in foods and their health effects are sorely lacking.
In addition to CVD risk, OXLAMs have recently been associated with non-alcoholic steatohepatiti 45 and mechanistically linked to physical pain 46 47. Therefore, the potential implications of processing of vegetable oils enriched in n-6 LA extend beyond cardiovascular disease.
We propose that OXLAMs should be estimated in future observational studies, and measured in future dietary intervention trials. More research into the biochemical and health effects of dietary and endogenously produced OXLAMs is clearly warranted.
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Please see the acknowledgements section of the main manuscript1. We would also like to thank Rashudy Mahomedradja and Ameer Taha for a literature review related to linoleic acid oxidation products.
The Life Insurance Medical Research Fund of Australia and New Zealand provided a grant in support of the SDHS. The Intramural Program of the National Institute on Alcohol Abuse and Alcoholism, National Institutes of Health, and the University of North Carolina Program on Integrative Medicine (National Institutes of Health grant T-32 AT003378), supported data recovery and evaluation. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.
Competing interests: No competing interests