Health Secretary Robert F. Kennedy Jr.’s most significant effort to overhaul what Americans eat seeks to block low-income people from buying junk food with their food stamps. It’s not so easy. Since Jan. 1, five states – Indiana, Iowa, Nebraska, Utah and West Virginia – have banned soda or other unhealthy foods from the Supplemental Nutrition Assistance Program, the nation’s largest anti-hunger initiative which serves more than 40 million people.