Butter lovers may have felt vindicated recently when a study in the New England Journal of Medicine reported that a type of fat found in margarine may increase the risk of heart disease. But some medical experts are cautioning that while the study was well designed, the results do not give a green light either to eating only butter or forsaking margarine.
"This is a provocative new finding, but before it's applied to the public, it needs to be repeated and confirmed. People shouldn't go overboard with a single study," said Alan Chait, professor of medicine at the University of Washington and chairman of the nutrition committee of the American Heart Assn.
"I can't get too excited about this," said Kenneth Fisher, director of the life-sciences research office of the Federation of American Societies for Experimental Biology. Fisher said Americans should still focus on reducing the total amount of fat they eat, which would automatically lower the intake of trans fatty acids, the type found in margarine.
Trans fatty acids are formed when fats and oils are hydrogenated. During this process, hydrogen molecules are added to polyunsaturated or monounsaturated oils, which turns the liquids into semi-solid shortenings or margarines. Hydrogenation not only imparts a creamy, firm texture to the products but prevents them from turning rancid.
The more solid the fat, the more trans fatty acids it contains. Thus, stick margarines have slightly more trans fatty acids than those sold in tubs, while diet margarines and vegetable-oil spreads, which have more water and less fat than margarine and are also sold in tubs, have the least amount.
On average, liquid "squeeze" margarines contain fewer trans fatty acids than any of the other choices, according to the National Assn. of Margarine Manufacturers. Sales of stick margarines have declined in the past 10 years, while sales of lower-fat vegetable-oil spreads and diet margarines have skyrocketed, from 15% to 41% of total sales.
While Americans consume most trans fatty acids in margarines and hydrogenated oils found in processed foods, such as cookies, crackers and cakes, lesser amounts are contained in meat and dairy products.
Scientists have been concerned about the possible adverse health effects of trans fatty acids for years, according to Robert Nicolosi, a professor in the department of clinical science at the University of Lowell in Massachusetts. Aside from their effect on blood cholesterol levels, other possible adverse effects include their impact on the immune system, fat metabolism and a link with some kinds of cancer.
Nevertheless, there is "very poor evidence and very few substantiating studies" that trans fatty acids have any detrimental health effect on humans in the levels consumed, said Nicolosi. These same conclusions were drawn in several recent reports, including one by FASEB.
The study in the New England Journal of Medicine was the first to find that trans fatty acids lowered the "good" cholesterol in the blood, known as HDLs, or high-density lipoproteins, and raised the level of "bad" cholesterol in the blood, called LDLs, or low-density lipoproteins. Epidemiological evidence suggests that a high level of HDL in the blood is strongly correlated with a low risk of coronary heart disease.
According to the New England Journal article, which used the FASEB study as its source, Americans consume between eight and 10 grams of trans fatty acids a day, or 6% to 8% of their daily fat consumption. Mary Enig, a research associate at the University of Maryland, believes that the daily average is more like 13 grams but that it can go as high as 38.
In fact, another FASEB report stated that it is difficult to maintain a reliable estimate of the fatty-acid contents of foods, in part because manufacturers change the types of fats and oils they use, depending on price and availability.
The New England Journal study, conducted in The Netherlands, placed 59 people on three three-week diets that varied in their fatty-acid composition. Based on their cholesterol profiles, participants' risk of heart disease was the greatest while following the diet containing a high level of trans fatty acids.
Margarine manufacturers and other scientists have pointed out that the amounts of trans fatty acids and calories consumed in the study are far greater than those actually eaten by Americans.
In their initial investigations, scientists frequently use large quantities of a given substance so that they can observe an effect, if there is one. Furthermore, the margarine eaten by participants was a formulation manufactured specifically for the study and is not commercially available.