HEALTH AND nutrition groups called it a victory last week when McDonald's announced it will begin frying its potatoes entirely in vegetable oil -- abandoning the higher-fat beef tallow it has previously used and joining Burger King, Wendy's and several other chains that have seen the lite. The shift is good news for nutrition reformers and also for low-income families and families with children, who rely disproportionately on the meals the megachains have to offer. Beef fat is pretty bad for you, and french fries soak up a lethal lot of it, as anyone can ascertain by looking at the napkin. So potatoes fried in vegetable oil, which is polyunsaturated, have to help the eaters. But this is not to imply that the nutritional salvation of American cuisine is at hand.

Nutrition, for some reason, brings out the wildest of rhetoric -- and on both ends of the seesaw. (It was not especially useful either when the director of a heart program at D.C. General Hospital opined, in a now notorious 1985 press conference, that parents who take their families to fast-food restaurants that use beef fat "would be safer if they told their children, 'Go out and play in traffic.' ") But the tide of health-conscious consumption has been rising around the fast-food chains, and most McDonald's, Burger King and Roy Rogers outlets, among others, now sport salad bars and grilled meats. Several chains now offer booklets on request with nutrition information about the food being consumed. Not satisfied with this practice, McDonald's now pledges it will soon begin posting such information prominently in its restaurants, though it's the sort of step that we can imagine engendering some backlash from consumers who prefer thinking wishfully. The potato specialists at McDonald's, at least, insist that they've meant to create a healthful french fry all along but that it took them 10 months of testing to find one that would taste as good as the truly vicious ones.

Given the clout of these mega-fast-food purveyors in the overall American diet and the ever clearer awareness by consumers of the relation between animal fats in high quantities and heart disease, this kind of evolution is probably inevitable. We applaud it, but can fast food ever become the staple of a truly healthy diet? The prospect is unlikely. Would it be as popular if it did? Unlikelier still.