McDonald's new Lean Deluxe may be lower in fat than other fast food burgers {"Food, Folks, Fun and Less Fat?" Food, Dec. 12} -- but so what? That just means it's slightly less unhealthy than a regular burger.

As a nutritionist, I know McDonald's effort to offer consumers a lower-fat alternative is a small step in the right direction. But it's time to shift from our dependence on beef and chicken, both foods loaded with fat and cholesterol, to a diet that is low in fat and generous in fiber.

We don't need so-called ''lean-beef.'' We need a McMeatless burger -- and that's hardly an unreasonable request. In the United Kingdom a vegetarian burger produced by the Archer Daniels Midland Co. in Decatur, Ill., has met with real enthusiasm. Over 60 million of the burgers were sold in fast food restaurants and grocery stores last year.

But in this country nutritionists still hail weak efforts toward diet improvement, like the new ''Lean Deluxe,'' as ''good news.'' Until we have some real good news about changes in the American diet, we'll never be successful in fighting our epidemic of killer diseases such as atherosclerosis and cancer. VIRGINIA MESSINA Washington