An endorsement from Oprah Winfrey is known to bring struggling brands into peak popularity. The Oprah effect has now carried over to the stock market after she took a stake in Weight Watchers.

Shares of Weight Watchers, which offers online and group programs to help people lose weight through diet and exercise, gained 70 percent Monday after Winfrey said she would purchase newly issued shares, worth about 10 percent of the company. She is also joining the board, a statement states.

Throughout the years, Winfrey has been open about her weight fluctuations and the struggle to maintain her own ideal weight. The billionaire, whose “Oprah Winfrey Show” ran for 25 seasons until 2011, endorsed products through her annual list of favorite things.

Weight Watchers, based in New York, has been struggling with 10 quarters of declining sales in the face of a plethora of new competition, from weight-loss apps to fitness tracking devices. All but one analyst has recommended investors sell or hold the stock, which has slumped 73 percent this year through Friday.

Winfrey agreed to offer her name and face exclusively to Weight Watchers and no other weight-loss products for five years with the option to renew afterward, according to a filing with the Securities and Exchange Commission.

The company agreed to issue Winfrey 6.4 million shares of common stock for $43.2 million, with the option to purchase an additional 3.5 million shares, the regulatory filing showed.