Sure-to-Slim

 was supposedly a meal bar that was "low calorie". In fact, the meal bars contained blue worm-like telepathic aliens that intended to invade Earth. By controlling a few humans, they arranged for the girls in an exercise video to eat the bars on the video as an advertisement for the bars. However, the aliens were stopped by the Sixth Doctor and Peri Brown, who revealed the meal bars' true nature. (PROSE: Old Boys)