First we learned that butter was bad for our hearts. Then we switched to margarine and found that was unhealthful too. Now nutrition experts are declaring war on the fats used in almost all commercially prepared foods.
Commonly listed on food labels as "partially hydrogenated" vegetable oils, these semisolid shortenings are prized for their shelf life and less greasy taste. But they contain trans fats, which studies have associated with higher blood levels of LDL, or bad cholesterol. Elevated cholesterol is considered a major risk for heart disease.
The Food and Drug Administration is expected early next year to require the listing of trans fats on nutrition labels. And in two reports this summer, an expert panel of the National Academy of Sciences' Institute of Medicine said trans fats aren't essential to the diet and provide no known health benefit, adding that there is no safe level of trans fats.
Knowing where the federal government was headed, leaders in the snack and fast-food arenas have taken action.
Last week, Frito-Lay announced that Doritos, Tostitos and Cheetos are going "trans-fat free" in early 2003. (The company's Lays and Ruffles potato chips and Fritos corn chips never contained trans fats.)
Abelardo E. Bru, president and chief executive of Frito-Lay North America, said the company would simply replace partly hydrogenated soybean oil with corn oil, one of the polyunsaturated oils thought to lower bad cholesterol.
McDonald's announced earlier this month that beginning in October, it would halve the trans fats in its signature French fries and eventually its chicken nuggets, fried fish and fried chicken sandwiches and hash browns. McDonald's will change its frying oil to a new blend of corn and soybean oils. Eventually, it plans to zero out trans fats from all its foods.
But the health rationale behind all this activity isn't as clear as we might think, given all the public and commercial concern.
When researchers have given people various amounts and types of fat, those eating trans fats had higher bad cholesterol, lower HDL (the good cholesterol), higher triglycerides (another unhealthy blood fat) and higher lipoprotein A than test subjects given the same amount of saturated fat each day.
Each of those measures is associated with heart disease, said Dr. Walter C. Willett, chairman of the nutrition department at the Harvard School of Public Health.
Saturated fat's seeming superiority was particularly telling, experts say, because it's that type of fat--found in meat, dairy products such as butter, coconut, palm and palm kernel oils--that has long been dietary enemy No. 1 in heart disease.
Further, Willett said data from 90,000 women in Harvard's landmark Nurses Health Study showed "that trans fat was the worst type of fat in terms of future heart disease risk."
There also is some evidence that those who eat trans fats instead of more healthful polyunsaturated fats have a higher risk of type 2 diabetes, he said.
So although no one has proven that trans fats directly cause heart disease, nutrition and heart experts are making their judgments based on the evidence at hand, hammered home through the persistent lobbying of organizations such as the nonprofit Center for Science in the Public Interest.
The trans-fat message isn't an easy one to convey to the eating public. We've been conditioned to think about eating less cholesterol-laden food, although even the link between dietary cholesterol and high blood cholesterol remains controversial and far from proven.
There are three main kinds of fatty acids--saturated, monounsaturated and polyunsaturated--all made up of carbon and hydrogen. They get their names based on how those hydrogen atoms attach to the carbon atoms.
When the maximum number of hydrogen atoms are attached to carbon atoms, a fat is said to be "saturated."
When some hydrogen is missing and two carbons attach by a double bond, the fat is said to be "monounsaturated." Examples include olive, canola and peanut oils, and nutritionists encourage more consumption of these oils because they're thought to lower our bad cholesterol.
When there are several missing hydrogen atoms and multiple double bonds, the fat is said to be "polyunsaturated." These also are considered healthful fats that can lower bad cholesterol. Sources include seafood and vegetable oils such as safflower, sunflower, corn, flaxseed and canola.
Some of the polyunsaturated fatty acids, such as linoleic acid and alpha-linolenic acid, are needed by the body.
But sometimes hydrogen atoms are added back to a fatty acid to make it solidify, retain its flavor and last longer, all characteristics prized by food makers. When this happens, the fat is said to be "hydrogenated," and the extra hydrogen makes it less healthful.