Senator Claire McCaskill -- who grilled Dr. Oz like a cheeseburger during Tuesday's hearing -- was incredulous over the Doc's "miracle pill" because she's walked the walk ... sources connected to the Senate Subcommittee on Consumer Protection tell TMZ.
McCaskill lost a ton of weight in 2011 ... actually more like 50 pounds. She did it the old fashion way -- diet and exercise -- and she was proud of it. We're told she thinks diet shortcuts are ineffective and often fraudulent.
So when Oz came to The Hill ... we're told McCaskill was locked and loaded for a showdown over Oz touting green coffee beans as a miracle drug. One of McCaskill's staffers tells TMZ ... this was not a vendetta against Oz, and others have told us the same -- that she just thinks these quick fixes are harmful, especially when they're endorsed by credible people.
For Oz's part ... we're told he and his staff privately acknowledged he went too far on the air, and pulled back on the hyperbole several years ago.