I still have a letter from my doctor at MD Anderson stating that I have the need to drink and carry water and that I need Boost for nutrition. Fortunately, back in the day when I had serious swallowing issues I was able to take my thermos of soup on flights with me without issue. Of course all that changed a couple of years later when TSA disallowed liquids.
Although I have always thrown away my water bottle as I approach the TSA security, for several years I was able to carry a couple of bottles of Boost on the plane, by showing TSA my letter, usually after a short hassle and the calling over of a supervisor. One time they made me angry and I threatened to file suit on them under the Americans With Disabilities Act. Although I still would prefer to carry a couple of bottles of Boost on the plane, as I still drink it for morning and afternoon snacks, I can eat and swallow well enough that I don't really need it anymore so I have just given up the fight.
Of course it is better to be polite, but occasionally it does not hurt to mention the Americans With Disabilities Act. After a judge in a town nearby allowed his bailiff to throw me out of court for refusing to give up my water bottle,(I am a lawyer) even after I explained that it was for medical reasons, and pulled out my M.D. Anderson letter to show him, I sent a letter threatening to file suit under the Act, and received a letter of apology.