Basically Penn was on a ton of different medications for high blood pressure and his doctor said if he got his weight down that he wouldn't need the medication anymore. So according to People and Page Six he ended up dropping the weight from December - March by eating about 1,000 calories a day (for some reference Penn is 6'7") which had him lose nearly a pound a day. He now follows a different diet that isn't as low-calorie and has started working out which he now enjoys.
@lagranderusty I'm happier.— Penn Jillette (@pennjillette) April 8, 2015
( More tweets from Penn behind the cut including him arguing with some vegansCollapse )
Source 1, 2 and 3