9x13 The Purge

Sam and Dean investigate recent murders where the victims were hundreds of pounds lighter after their deaths. Suspecting witches, the two go undercover at a fancy day spa that all of the victims had attended for weight loss. While Sam is hired as a fitness instructor, Dean takes kitchen duty. After Dean swipes some tainted food and passes out, the guys realize someone is drugging the guests and literally sucking all of the fat out of them until they die.