Is a Low-Fat Diet Risky?

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! It used to be so simple: butter is bad. Margarine is better. Vegetable oils are best. And too much fat of any sort is not good for you. But these easy guidelines have given way to a bewildering -- and at times contradictory -- smorgasbord of dietary dos and don'ts. Salads drizzled with olive oil are O.K. Confections rich in coconut oil aren't. Fish oil gets two thumbs up. Margarine receives rancid reviews. "I swear to God, it's confusing," declares Peter Summers, a retired health-care specialist who lives in San Francisco. "First I went to margarine. But then I found out that margarine wasn't so great, and I went back to butter. I must have changed my diet six times. What I do is study what the experts say, think about it and just throw up my hands."

Now Summers has reason to throw up his hands again. The latest dispatch from the front lines of America's losing fight against fat -- one sure to fluster consumers as they frantically scan the nutritional labels on supermarket shelves -- comes from the Boston University Medical Center. Not only do some fats appear to be less harmful than others, say Dr. Edward Siguel and his colleague Dr. Robert Lerman, but diets deficient in "good" fats may actually be dangerous to human health. Backing up this startling assertion is a study Siguel and Lerman published in the journal Metabolism. In 47 patients with heart disease, they reported, blood levels of compounds known as essential fatty acids were strikingly lower than the levels found in healthy people.

Fatty acids, the building blocks for fat, are divided into three chemical classes according to their hydrogen content: saturated, mono-unsaturated and polyunsaturated. Only the polyunsaturated ones are considered "essential," meaning they cannot be manufactured by the body. Like minerals and vitamins, they must be ingested as food. "If we don't eat enough," says Siguel, "then we won't have enough." And that would be unfortunate, for these compounds -- principally linoleic acid and linolenic acid -- are vital to the maintenance of cell membranes and to the manufacture of potent chemical messengers that regulate everything from blood pressure to the firing of nerves.

"Essential-fatty-acid deficiency," Siguel sweepingly declares, "is perhaps the most important health problem in America." When essential fatty acids are in short supply, he explains, the body compensates by substituting other types of fatty acids that have a less supple biochemical structure. As polyunsaturates are replaced by these other compounds, cell membranes become more rigid, leading to progressive hardening of the arterial walls.

Fortunately, antidotes to this problem abound. Varying amounts of linoleic and linolenic acid are found in different kinds of cooking and salad oils -- among them corn, soybean, safflower and walnut oils. They are also present in seeds, nuts and green vegetables like broccoli. But don't look for polyunsaturated oils in processed grains, advises Siguel. Food manufacturers generally remove these spoilage-prone compounds from pasta, bread and breakfast cereal in order to lengthen the shelf life of their products. Thus, Siguel ventures, a slice of pizza made with soybean oil may be healthier than portions of some low-fat foods containing the same calorie count.