Hard Fats, Soft Fats—Which are Good and Which are Bad?
Synopsis: Reducing the intake of saturated fat should continue to be a high medical priority, but taking steps to reduce the level of trans fats in our diets now seems to be an appropriate additional goal.
Source: Hu FH, et al. N Engl J Med 1997;337:1491-1499.
Many reports have suggested that low-fat, high-carbohydrate diets reduce the risk of coronary artery disease (CAD) because they reduce high-density lipoprotein cholesterol (HDL-C) levels and increase low-density lipoprotein cholesterol (LDL-C) levels.1,2 More recently, the value of replacing fat with carbohydrates has been questioned because the resulting low levels of HDL cholesterol and high levels of triglycerides may independently increase cardiovascular risk.3
Hu and associates from the Department of Nutrition of the Brigham and Women’s Hospital and the Harvard Medical School evaluated the relationship between dietary intake of specific fats and the risk of developing symptomatic CAD. They prospectively studied 80,082 women who were 34-59 years of age and who had no known history of CAD, stroke, cancer, hypercholesterolemia, or diabetes. They obtained specific dietary information, repeated laboratory studies, and documented 939 cases of non-fatal myocardial infarction or death from CAD. They found that each increase of 5% of energy intake from saturated fat was associated with a 17% increase in the development of symptomatic CAD. when compared with the equivalent energy intake from carbohydrates. Total fat intake was not significantly related to an increased risk of CAD development. The replacement of 5% of energy from saturated fats with energy from unsaturated fats reduced risk by 42% (P < 0.001), and the replacement of only 2% of energy from trans fat with energy from unhydrogenated, unsaturated fats reduced risk by 53% (P < 0.001). The authors conclude that replacing saturated and transunsaturated fats with unhydrogenated, monosaturated and polyunsaturated fats is more effective in preventing CAD in women than was the effect of reducing overall fat intake.
COMMENT BY HAROLD L. KARPMAN, MD
Transunsaturated fats are produced when polyunsaturated vegetable fats are artificially hydrogenated, a process that increases both their firmness and their resistance to spoilage. Approximately 5-10% of the fat in the average American diet and about 5% of the fats stored in the adipose tissue of U.S. citizens is transunsaturated fat.4 In recent years, evidence has been published which suggested that an excess intake of transunsaturated fats can adversely effect cholesterol profiles and can therefore lead to an increase in the risk of developing CAD.4,5 The first report of the Nurses’ Health Study linking together fat in the diet to CAD was published in 1993. News reports of this study that were published in The Lancet suggested that "butter is better than margarine," although neither the authors of the study nor nutritionists in general have ever supported that point of view.6
The Hu et al study differs from the 1993 Nurses’ Health Study in two important ways. First, it extended the follow-up period for six additional years, thereby providing significantly more cases of non-fatal myocardial infarction and/or death. In addition, it incorporated into the dietary measure changes in diet reported over the 14-year follow-up period. It should be noted that, in this study, total fat intake as a percentage of energy intake declined by approximately 19% from 1980 to 1990, but the authors carefully took into account changes over time in dietary habits and food composition leading to the updated analysis. The report clearly provides evidence in support of the hypothesis that a higher dietary intake of saturated fat and transunsaturated fat is associated with an increased rate of CAD, whereas a higher intake of monounsaturated and polyunsaturated fats is associated with reduced risk of developing symptomatic CAD. These findings provide further support for metabolic studies which recommend that replacing saturated fat and transunsaturated fat in the diet with unhydrogenated monounsaturated and polyunsaturated fats favorably alters the lipid profile but that reducing the overall fat intake has little effect.
In summary, reducing the intake of saturated fat should continue to be a high medical priority, but taking steps to reduce the level of trans fats in our diets now seems to be an appropriate additional goal. The level of trans fats in foods should be reduced to a minimum, as many manufacturers are attempting to do by providing aerated "tub" margarines. However, they should, at the same time, avoid the mistake of replacing the trans fats with saturated fats in baked goods and in cooking fats. Over the past 17 years, our nutritional approach to changes in fat intake has been appropriate, but a little tweaking of the intake of specific fat intakes is now indicated, and further tweaking may be necessary in the future.
References
1. Grundy SM, et al. Circulation 1982;65:839A-854A.
2. Mensink RP, Katan MB. Arterioscler Thromb 1992; 12:911-919.
3. Katan MB, et al. N Engl J Med 1997;337:563-566.
4. Trans fatty acids and coronary heart disease risk: Report of the Expert Panel on Trans Fatty Acids and Coronary Heart Disease. Am J Clin Nutr 1995;62 Suppl 655-708.
5. Willett WC, et al. Lancet 1993;341:581-588.
6. Editor’s note. J Am Diet Assoc 1994;94:1097-1101.
Gemfibrozil treatment of post-CABG patients with isolated low HDL has been associated with marked reduction in subsequent clinical events despite lack of any demonstrated angiographic benefit.
Which of the following is true?
a. Low fat-high carbohydrate diets reduce the risk of developing coronary artery disease.
b. Replacing fat with carbohydrates may increase cardiovascular risk.
c. In women, the replacement of only 5% of energy from saturated fats with energy from unsaturated fats may reduce the risk of developing symptomatic coronary artery disease by 42%.
d. Trans fats are produced when polyunsaturated vegetable fats are artificially hydrogenated.
e. All of the above.
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