Vegetable oils are often recommended as a heart healthy replacement for saturated fat in the diet. A 2011 a report by the USDA Center for Nutrition Policy and Promotion concluded: “The evidence shows that a decrease in (saturated fat) equivalent to 5 percent of calories, replaced by (polyunsaturated fat)…decreases risk of CVD”. This view, based on epidemiological evidence, is particularly perplexing given the existence of a superior form of evidence, randomized controlled trials, which have not only evaluated this very subject but also frequently contradicted the quote above.
Such studies have been conducted which sought to replace saturated fat with omega-6 predominated polyunsaturated fat and observe the effect of this dietary change on cardiovascular disease deaths. However, a number of these studies introduced other between group differences, like the amount of sugar, trans fats, fruits, and/or vegetables consumed, or the amount of medically prescribed drugs used in one group relative to the other. This includes the St. Thomas Atherosclerosis Regression study, the Oslo Diet-Heart trial, the Finnish Mental Hospital study, and the Helsinki Businessman trial. While no study is perfect, these studies should probably be excluded from an analysis of this issue to eliminate the influence of serious confounding.
The remaining studies had essentially the same simple set-up: one groups eats how they they normally eat while another group replaces a lot of the saturated fats in their diet with polyunsaturated fat from vegetable oil. These studies varied primarily on the demographic of the subjects used (age, CVD history, gender, etc.).
To start we have the Diet and Reinfarction Trial (1), which reported absolutely no difference in the rates of heart disease death between the two participating groups. A small study, the St. Vincent’s Hospital study (2), also noted essentially no difference in CVD deaths between groups.
The British Medical Research Council Trial (3) noted a small decrease in heart disease deaths in the group consuming vegetable oil, while the Minnesota Coronary Survey had the opposite finding (4). The results of both of these studies may easily have been due to statistical chance.
The Sydney Diet Heart Study (5), the Anti-Coronary Club Trial (6), and the Rose Corn Oil Trial (7) all observed an increase in the amount of heart disease deaths in the groups consuming vegetable oil. It should be pointed out that the latter two of these studies have been criticized for having poor randomization.
Finally, the Multiple Risk Factors Intervention Trial, or MRFIT (8), and the LA Veterans Administration Trial (9,10) both found a decrease in the risk of CVD death when replacing saturated fat with polyunsaturated fat from vegetable oil. However, in both of these studies there was a large disparity in the rates of smokers between both groups. In the MRFIT study there were almost 11% less smokers in the group using vegetable oils. In the LA Veterans Trial, the amount of moderate and heavy smokers were both greater in the group not using vegetable oils (roughly 100% and 60% higher respectively). This may have affected the lower risk of CVD death in the vegetable oil groups. Furthermore, the total mortality between both groups in each of these studies was the same, meaning that even if the smoking rates in these studies didn’t matter, they still only show that polyunsaturated fat in the form of vegetable oil, in place of saturated fat, will decrease your risk of death by heart disease, but not your risk of death.
It bears highlighting that the only randomized, controlled trials supporting the idea that omega-6 predominated polyunsaturated fat will decrease the risk of heart disease also found that this type of dietary change does not decrease risk of death. So you still die, just from something other than heart disease.
And this of course says nothing of the favorably lower smoking rates in these studies, or the studies mentioned earlier which refute such an idea in the first place.
1. Burr ML, Fehily AM, Gilbert JF, Rogers S, Holliday RM, Sweetnam PM, Elwood PC, Deadman NM. Effects of changes in fat, fish, and fibre intakes on death and myocardial reinfarction: diet and reinfarction trial (DART). Lancet. 1989 Sep 30;2(8666):757-61.
2. Bierenbaum ML, et al. Modified-fat dietary management of the young male with CD. A five-year report. JAMA. Dec 25 1967;202(13):1119-1123
3. Controlled trial of soya-bean oil in myocardial infarction. Lancet. 1968 Sep
4. Frantz ID Jr, Dawson EA, Ashman PL, Gatewood LC, Bartsch GE, Kuba K, Brewer ER. Test of effect of lipid lowering by diet on cardiovascular risk. The Minnesota Coronary Survey. Arteriosclerosis. 1989 Jan-Feb;9(1):129-35.
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6. Christakis G, Rinzler SH, Archer M, Kraus A. Effect of the Anti-Coronary Club program on coronary heart disease. Risk-factor status. JAMA. 1966 Nov 7;198(6):597-604
7. Rose GA, et al. Corn oil in treatment of ischaemic heart disease. Br Med J. Jun 12 1965;1(5449):1531- 1533
8. Multiple risk factor intervention trial. Risk factor changes and mortality results. Multiple Risk Factor Intervention Trial Research Group. JAMA. 1982 Sep24;248(12):1465-77
9. Los Angeles Veterans Administration diet study. Nutr Rev. 1969 Nov;27(11):311-6. Review.
10. Pearce ML and Dayton S.Incidence of Cancer in Men on a Diet High in Polyunsaturated Fat. Lancet. 1971;297(7697):464-467