Type of Dietary Fat Key to Heart Risk
January 02, 2008
The value of hardened (trans) unsaturated fats in our food supply is probably best exemplified by the glazed doughnut. At room temperature, a glazed doughnut can be easily eaten with one hand, but warmed up it requires two hands and a napkin.
That's what trans fats can do for us: they keep our pastries firm and our margarine stiff at room temperature. Trans fats are generally found in absolutely unnecessary foods like doughnuts and margarines.
Trans fats are produced when polyunsaturated vegetable fats are artificially hydrogenated, a process that increases both their firmness and their resistance to oxidative spoilage. About 5 to 10 percent of the fat in our American diet and about 5 percent of the fat stored in our American adipose tissue is trans unsaturated fat
It is the type of dietary fat and not the total amount of fat consumed that affect a person's risk of coronary heart disease.
The findings may have an impact on current recommendations by United States health officials that the daily total dietary intake of fat in general not exceed 30% of total calories consumed.
This study focused on the cardiac risk of several types of fat: saturated fat, found in meats and dairy foods; trans unsaturated fat, the "hardened" fat found in margarine and fast foods; monounsaturated fat, as occurs in olive and canola oils; and polyunsaturated fat, as found in corn and soybean oils.
The main finding is that it's the type of fat that's most important for the risk of heart disease -- that it's not the total amount of fat because there are 'good' fats and 'bad' fats, much like 'good' cholesterol and 'bad' cholesterol. Higher intake of trans unsaturated fat is associated with increased risk of heart disease.
This is the first major epidemiologic study to look at all the major fats and total fats -- to put all the fats together in one prospective study. The researchers calculated a 17% greater risk of coronary disease from dietary saturated fat compared with the same caloric intake from carbohydrates.
BUT, trans unsaturated fats were associated with the highest heart risk -- almost twice that (93%) of carbohydrates.
This large effect is probably explained by the impact of trans unsaturated fat on blood lipid levels, its interference with fatty-acid metabolism, and its ability to elevate triglycerides -- a type of blood fat.
But the risk percentages of the other fats ran in the opposite direction. As compared with the equivalent energy intake from carbohydrates, the heart disease risk was 19% lower for monounsaturated fats and 38% lower for polyunsaturated fats among the nurses followed in the study.
In addition, the researchers note that the high-carbohydrate diet recommended by some heart disease prevention programs, which are intended to lower LDL ("bad") cholesterol levels also lower the ("good") HDL levels.
According to the report, replacing 5 percent of energy from saturated fat with unsaturated fat leads to a 42% lower risk of heart disease. And replacing 2 percent of energy formerly eaten in the form of trans unsaturated fat with unhydrogenated, unsaturated fat drops heart risk by 53%.
The New England Journal of Medicine ( November 20, 1997;337:1491-1499)