"The enumeration in the Constitution of certain rights, shall not be construed to deny or disparage others retained by the people."
The "people" have the "retained" right to consume the chemical of their choice, in the quantity of their choice, for the reason of their choice.
The FDA "power," delegated to it by the Congress, to ban the consumption of any chemcial from use by free people, is blatantly and unambigously unconstitutional.
Congress can delegate the power to the FDA to investigate the safety and advertised claims of benefits from manufacturers of chemicals made for human consumption for accuracy and report that information to the citizens for the purpose of assisting citizens make intelligent decisions to consume such chemicals. That action would be constitutional.
Rush Limbaugh could use the same constitutional argument in his case.