The L.A. Law star plays the President of the United States in the action series, which debuted in September (10) - but he couldn't ignore his bulging belly when he reviewed the first episode earlier this year (10).
He underwent the gruelling Master Cleanse for "weeks" to get back in shape - which meant restricting his intake to nothing more than a mixture of lemon water, maple syrup and cayenne pepper.
And now Underwood's finally feeling camera ready.
He says, "A couple months ago I was 20 pounds heavier. This show I'm doing now, The Event, we did the pilot. There's five months between (shooting) the first episode and the second episode. I saw the pilot and I go, 'Oh, no. Oh, no brother, you need to lose some weight!'"