Shares of weight loss company Weight Watchers jumped nearly 19% on Wednesday after a television ad featuring Oprah Winfrey was shown in the US. In an emotional one-minute commercial, the former talk show host spoke about her renowned weight loss struggles, flashing back to videos of her working out over the years. She asked viewers to join her, saying "let's do this together". Ms Winfrey bought a 10% stake in the company for $43.2m (£28m) in October. New York listed shares of the dieting company have more than tripled in value since she bought a stake in one of the world's leading weight loss brands. Her stake is now worth more than $148m.The company, however, has been facing stiff competition from the popularity of fitness trackers and other free mobile apps which track your eating. Oprah tweeted her new advert to her 30 million followers.