Margarine is anything but marginal fat.

What's the leading source of calories in the U.S. diet? Whole milk -- the primary source of protein -- tops the charts, contributing 3.88 percent. Cola-flavored soft drinks -- the main sugar contributor -- provide 3.04 percent of calories. At 2.48 percent, margarine -- the leading source of fat -- captures third place. Shortening, the third leading source of fat, trails in 11th place; it provides 1.43 percent of calories.

Researchers with the U.S. Department of Agriculture (USDA) reported these rankings last week at an agency-sponsored nutrition meeting in Beltsville, Md. The findings emerge from USDA's Continuing Survey for Food Intakes of Individuals (CSFII), conducted between 1989 and 1991.

The survey gathered 3 days' worth of data on 11,912 people for foods prepared at home, bought in grocery stores, or eaten in restaurants. Though the most detailed analysis yet of the composite U.S. diet, CSFII remains an imperfect tool, notes David Haytowitz of USDA's Human Nutrition Information Service (HNIS) in Hyattsville, Md.

For instance, a person who reports having eaten lasagna doesn't provide a list of ingredients in the entree. Rather, an HNIS computer program calls up standard recipes for lasagna and then calculates how much pasta, tomato, and other foods went into the portion eaten. CSFII also collected data on the fats used in preparing foods -- for example, whether cooks used oils instead of shortening or margarine. However, Haytowitz notes, the new rankings do not yet reflect such individual adaptations.

Consumers may find the new fat data especially disturbing in light of a commentary published this week in the May AMERICAN JOURNAL OF PUBLIC HEALTH.

In it, Walter C. Willett and Albert Ascherio of the Harvard School of Public Health in Boston review recent epidemiological and metabolic data linking consumption of the partially hydrogenated fats in margarines and shortenings to increased risk of heart disease. Indeed, a new analysis by the pair indicates that at least 30,000 deaths per year in the United States may result from consumption of these processed fats.

Food processors convert oils into semi-solid fats that resemble butter or lard by adding hydrogen atoms to an unsaturated fat's chemical double bonds. Partially hydrogenated margarines and shortenings may contain up to 40 percent of their fats in this modified -- or transform. Indeed, for most people, trans fats now make up about 2 percent of calories eaten.

Manufacturers began hydrogenating edible oils nearly a century ago, and the resulting modified fats had permeated the U.S. diet long before anyone performed analyses to gauge their health implications, Willett and Ascherio observe.

Indeed, indications that these trans fats can alter the ratio of various cholesterol-carrying lipoproteins in the blood didn't begin to emerge until 4 years ago (SN: 8/25/90, p.126).

Since then, several studies have confirmed the ability of trans fats not only to increase concentrations of "bad," low-density lipoproteins (LDLs) -- as most saturated fats do -- but also to decrease concentrations of "good," high-density lipoproteins (HDLs).

The Harvard duo recommends "a regulated phaseout or strict limitation of partially hydrogenated fat in the U.S. diet." Short of that, they argue, the food-labeling law that took effect May 8 should be amended "immediately" to specify quantities of trans fat. Today, manufacturers need only list a food's total- and saturated-fat content.