Weight Watchers gains 20% as Oprah loses 18kg

Broadcaster, who has 10% stake in firm, helps shares make their biggest rise in 10 months as she stars in ads for diet plans

Wall Street has rediscovered its appetite for Weight Watchers shares after the company's most famous customer and investor, Oprah Winfrey, revealed the results of her new diet.

Shares in Weight Watchers jumped by almost a fifth on Thursday following the news that the broadcasting titan had lost 18kg (40lb) by following the company's latest guidance.