Departments of Nutrition and Epidemiology, Harvard School of Public Health; The Channing Laboratory, Department of Medicine, Brigham and Women's Hospital --
Nov 15, 1999 --
On November 12, 1999, the Food and Drug Administration announced its proposal to include the trans-fatty acid (trans fat) content of foods on the standard food label. A 90-day period in which the public may comment then began. At present, only saturated fats are listed. Because many persons will be unfamiliar with trans fat and its health effects, we have prepared the following review.
BACKGROUND AND SCIENTIFIC REVIEW by Alberto Ascherio, Meir J. Stampfer, and Walter C. Willett
Departments of Nutrition and Epidemiology, Harvard School of Public Health; The Channing Laboratory, Department of Medicine, Brigham and Women's Hospital
What are trans fatty acids? Trans unsaturated fatty acids, or trans fats, are solid fats produced artificially by heating liquid vegetable oils in the presence of metal catalysts and hydrogen.
This process, partial hydrogenation, causes carbon atoms to bond in a straight configuration and remain in a solid state at room temperature. Naturally-occurring unsaturated fatty acids have carbon atoms that line up in a bent shape, resulting in a liquid state at room temperature.
Which foods contain trans fatty acids? Trans fats are produced commercially in large quantities to harden vegetable oils into shortening and margarine. Food manufacturers also use partial hydrogenation of vegetable oil to destroy some fatty acids, such as linolenic and linoleic acid, which tend to oxidize, causing fat to become rancid with time. The oils used to cook french fries and other fast food are usually this kind of partially hydrogenated oil, containing trans fats. Commercial baked goods frequently include trans fats to protect against spoilage. A small amount of trans fat is also produced in the gastrointestinal tract of cattle, so that low levels of these isomers are found in dairy and beef fat.
Commercial production of partially hydrogenated fats began in the early 20th century and increased steadily until about the 1960s as processed vegetable fats displaced animal fats in the diets of the U.S. and other Western countries. Lower cost was the initial motivation, but health benefits were later claimed for margarine as a replacement for butter.
Although the average level of trans fat in margarines has declined with the advent of softer versions, per capita consumption of trans fatty acids has not changed greatly since the 1960s because of the increased use in commercially-baked products and fast foods.
What are the health effects of trans fats? Concerns have been raised for several decades that consumption of trans fatty acids might have contributed to the 20th century epidemic of coronary heart disease.
Metabolic studies have shown that trans fats have adverse effects on blood lipid levels--increasing LDL ("bad") cholesterol while decreasing HDL ("good") cholesterol. This combined effect on the ratio of LDL to HDL cholesterol is double that of saturated fatty acids.
Trans fats have also been associated with an increased risk of coronary heart disease in epidemiologic studies. Based on the available metabolic studies, we estimated in a 1994 report that approximately 30,000 premature coronary heart disease deaths annually could be attributable to consumption of trans fatty acids.
In response to these reports, a 1995 review sponsored by the food industry concluded that the evidence was insufficient to take action and that further research was needed.
Since that time many more metabolic studies have been conducted and additional prospective epidemiologic studies have been reported.
Because of the weight of the evidence, the FDA has recently issued a proposal for including trans fatty acid content on the food label. One important issue is whether to list trans fat as a separate constituent or to combine it with saturated fat.
What are the arguments for listing trans fat separately from saturated fat? The combined results of metabolic and epidemiologic studies strongly support an adverse effect of trans fat on risk of CHD. Furthermore, two independent methods of estimation indicate that the adverse effect of trans fat is stronger than that of saturated fat. By our most conservative estimate, replacement of partially hydrogenated fat in the U.S. diet with natural unhydrogenated vegetable oils would prevent approximately 30,000 premature coronary deaths per year, and epidemiologic evidence suggests this number is closer to 100,0000 premature deaths annually. These reductions are higher than what could be achieved with realistic reductions in saturated fat intake.
What alternatives exist to trans fats? In Europe, producers have responded rapidly to the evidence on effects of trans fats by developing trans-free margarines that are also low in saturated fats. More recently, these products have also become available in the U.S., although a large share of the market is still heavily hydrogenated stick margarine.
It is thus evident that trans-free products are feasible, and that the technical constraints often invoked by the food industry can be overcome. However, out of the trans fatty acids provided by hydrogenated vegetable oil in the U.S., only 25%5 to 37%49 comes from margarines, the remainder comes from baked goods, fast foods and other prepared foods. Replacement of trans in such products by healthier fats may be more difficult than in margarines, but can be achieved.
In spite of this, many products including most baked goods and fried fast foods still are made with partially hydrogenated fat both in Europe and in the U.S. and are high in trans fatty acids. It is unlikely that this situation will change without strong federal regulations.
How important are label changes? Current regulations in the U.S. require food labels to include the amount of saturated fat, but not the amount of trans, thereby providing an incentive to manufacturers to increase the trans content while decreasing the amount of saturated fat.
Although changes in labeling are extremely important, many products, including fast food, which often contain extremely high levels of trans isomers, are exempt from labeling regulations and can carry deceptive labels such as "cholesterol-free" and "cooked in vegetable oil."
For example, a person eating one doughnut for breakfast (3.2 g)50 and a large order of french fries for lunch (6.8 g)50 would ingest 10 g of trans fatty acids, or 5 percent of the total energy of an 1,800-calorie diet. Thus, simple labeling changes alone will not be sufficient.
