For years, Maine nutritionists have wanted scientific evidence to show junk food is a big contributor in producing fat kids. They needed evidence to present to food-industry groups and to school administrators who hated to give up the kickbacks and free scoreboards they get from Coke and Pepsi and the rest. But scientific research costs money that has been lacking.

Now, for the first time in the nation, Maine is embarking on a controlled study of the impact of junk food on students. The money comes in a $450,000-a-year, five-year grant from the U.S. Centers for Disease Control and Prevention to the Maine Bureau of Health for a study on lowering obesity rates through physical activity and nutritional intervention.

Some schools have already switched to natural fruit juices and healthful snacks. Others have clung to the standard junk-food items. That’s where the control feature enters the picture. Professor Janet Whatley Blum, of the sports medicine department at the University of Southern Maine, will be recruiting probably 10 schools of each type for the study in September. The schools that still offer junk food in their vending machines will be asked to continue to do so during the study. She is going to recruit 800 Maine students, who, with their parents’ consent, will keep diaries of their eating and exercise habits. Half of them will be from the nutrition-practicing schools, and half will be from the junk-food schools. Full conclusions should emerge in two years. Ms. Blum, a graduate of the University of Maine, holds a doctorate in nutritional sciences from Boston University.

Meanwhile, the struggle goes on to wean students and their schools from a constant school-day diet of chips and sodas and candy bars. The Maine Nutrition Network, organized by the Maine Center for Public Health in Augusta, has put together a packet to help schools improve their vending-machine practices. It cites success stories in School Union 106 in Robbinston, Calais, Alexander, Baring Plantation and Crawford; seven schools in Maine School Administrative District 22 in Hampden, Newburgh and Winterport; and other schools in Lisbon, Old Orchard Beach and Topsham.

It tells how they enlisted the support of sometimes-resistant students, parents, teachers and administrators. Coke and Pepsi have cooperated, too, substituting spring water and fruit drinks for their standard products.

Groups like this have believed for a long time that junk food produces fat kids. Depending on how the study turns out, they soon may be able to show it.