Wouldn't it be nifty to be able to eat ice cream by the gallon, french fries by the bushel, doughnuts, custards and pies, all without worrying about calories or cholesterol?

Well, fake fat, at least as an ingredient in ice cream, is a reality. Whether it will turn out to be as nifty as we might wish is still a matter of some debate.

One fat substitute -- Simplesse -- made by Monsanto, the giant chemical company that makes the sugar substitute NutraSweet, is already available in Simple Pleasures, a fake ice cream -- or frozen dessert -- as Monsanto prefers to call it. A four-ounce serving of Simplesse has 120 calories and one gram of fat, compared to 250 calories and 15 grams of fat contained in a super-premium ice cream, such as Ha agen-Dazs or Ben & Jerry's, according to Monsanto.

Trailblazer, another fake fat, manufactured by Kraft General Foods, is awaiting approval from the Food and Drug Administration.

Simplesse and Trailblazer, both made with specially processed protein from egg white and skim milk or whey (the thin, watery part of milk), fall under the FDA's categories of foods that are "generally recognized as safe."

Another fat substitute -- Olestra, made by Procter & Gamble, is also awaiting FDA approval, but it is a different kind of fake fat. Olestra is a sucrose-polyester, and although it sounds like a cheap suit, it is, in fact, intended to substitute for fat in food.

Olestra is an entirely new molecule, which, according to the FDA, makes it a "food additive." The FDA requires that food additives be proved safe in long-term studies involving two different animals. Olestra has already been the subject of long-term studies in rats; according to the Washington-based Center for Science in the Public Interest, mouse studies are currently under way.

Reports of suspicious changes in the livers of rats have raised some questions about Olestra's safety, so it may be months -- or years -- before the product is approved for sale, according to Lisa Lefferts, a researcher at CSPI.

Lefferts said that CSPI, a public-interest consumer organizaton, has no safety problems with Simplesse --

except that "if you're allergic to eggs or milk, you'll probably be allergic to Simplesse."

"Our concern is that people would eat more and more double-dip ice cream cones and then not have room for the really nutritious foods they need," she said.

Lefferts also questioned whether eating a fat substitute would "really substitute for fat." She cites the experience with sugar substitutes: "We're eating four times as many foods with sugar substitutes as we were in 1975, but sugar consumption has gone up as well, so clearly sugar substitutes are not substituting for sugar."

Richard J. Wurtman, a neuroendocrinologist at the Massachusetts Institute of Technology, said he doubts that the advent of fake fats will result in increased fat consumption.

The body's intake of carbohydrates and proteins, he notes, is regulated in the brain by neurotransmitters, chemicals whose synthesis is signaled by agents in the blood that send the message that fuel -- in the form of protein or carbohydrate -- is required.

Wurtman said that there is apparently no feedback system in the body that would trigger a craving for fats similar to the physiological craving for carbohydrates, including sugar.

"People like fats," he says, but historically seeds and berries were a lot more accessible, which is one theory of why the body developed internal signals for proteins and carbohydrates, rather than fats.

When primitive humans managed to kill an animal, they ate the fat-laden meat avidly, probably gained weight and lived on that stored fat.

"But you couldn't count on that every day, so the body never devised the mechanisms that exist for proteins and carbohydrates," says Wurtman. At least, he says, "that's one argument."

Simplesse is actually minuscule protein globules that give the creamy taste of fat. They do not stand up to heat, so they cannot be used in baking or frying. But they could be used in mayonnaise, salad dressings, cheese substitutes and other cold foods.

Fake French fries and doughnuts, it seems, will have to wait.

Eating Right appears on alternate Tuesdays.