By Lisa Baertlein LOS ANGELES (Reuters) – The former Iowa science teacher and McDonald's Corp “brand ambassador,” who preached the virtues of walking and near-daily french fries in presentations to youth, is no longer visiting U.S. schools, the fast-food chain said on Friday. The program presented by John Cisna, author of the book “My McDonald's Diet: How I lost 37 pounds in 90 days and became a viral media sensation,” was sharply criticized by teachers, parents and public health advocates who accused McDonald's of trying to hook youngsters on unhealthy food. Cisna's program included a documentary and discussion guide edited by McDonald's, which hired him in 2015 and provides him with a stipend for time and travel related to his speaking engagements.