Weight Watchers International, Inc.'s stock got another boost from Oprah Winfrey's weight loss; she is the company's backer and most famous customer. The shares climbed as much as 19 percent after Winfrey announced that she'd lost 40 pounds using the program. The diet company, whose shares had fallen 54 percent this year before Thursday's surge, is unveiling new TV ads featuring Winfrey that tout her weight loss. The media magnate and talk-show veteran became a centerpiece of Weight Watchers' comeback plan last year when she bought a stake in the company and joined the board. In October 2015, when Weight Watchers first announced its deal with Winfrey, the shares more than doubled in a single day. Although that rally faded, Winfrey's subsequent endorsements have given the stock temporary jolts. The shares gained 27 percent over two days last December when Oprah tweeted a video about using the weight-loss program, and there was a 20 percent bump in January after Winfrey said she had lost 26 pounds while still eating bread every day. The latest marketing push begins next week, just before the New Year's holiday. The coming weeks are critical for Weight Watchers: the company typically adds about 40 percent of new customers in the first quarter, when resolutions push people to seek out diets. The stock climbed as high as $12.50 on Thursday, its biggest intraday gain since February. Heavy Competition: Weight Watchers has been hit hard by the rise of free fitness apps and a move away from strict calorie counting among dieters. But Winfrey’s wide-ranging influence has brought new life to the brand. Weight Watchers has added subscribers for three straight quarters after years of declines. The New York-based company hopes that Winfrey’s new ads, which emphasize that she still enjoys pasta and tacos, will convince dieters that its updated program is an effective weight-loss tool. “It’s vitally important that Oprah is living the program and articulating it in a way that’s authentic,” said Maurice Herrera, the head of marketing at Weight Watchers. “It really connects with prospective members.”

Despite the subscriber gains, it’s been a rocky year for Weight Watchers. The company has billions of dollars in debt, and its stock is heavily shorted. In September, Chief Executive Officer Jim Chambers announced he was leaving after about three years at the helm. That sent the shares plummeting to their lowest level in nearly a year.

Chambers had been working to turn around the company, an effort that included revamping its nutrition program and adding a social-networking component to its app. Now Weight Watchers is being run by a trio of executives who constitute the “interim office of the CEO.” The ads rolling out next week show Winfrey doing yoga and cooking pasta. As consumers have changed their perception of wellness, Weight Watchers has pivoted to emphasize that weight loss and indulgence aren’t necessarily mutually exclusive. Winfrey will hammer that message home, Herrera said: “People want to know what’s on her mind and how she’s living her life. This will definitely get people’s attention.”

Oprah is down more than 40 lbs. The media guru is celebrating her weight-loss success in the first of two new ads for Weight Watchers, shared with PEOPLE. “Since I started Weight Watchers, I’ve lost over 40 lbs,” Winfrey, 62, says in the ad. “I can honestly tell you, I struggle no more.”

Not only can she still eat her favorite food—bread—but the program allows her to indulge in other treats that would typically be off-limits on a diet. “I’m eating everything I love—tacos, pasta. I’ve never felt deprived,” she says.

Winfrey, who is a Weight Watchers shareholder, said the program is less of a diet and more of a life change. “Weight Watchers is easier than any other program I’ve ever been on,” she said in a press release shared with PEOPLE. “It’s a lifestyle, a way of eating and a way of living that’s so freeing. You never feel like you are on a diet and it works.”

“I would say to anyone who’s thinking of joining Weight Watchers: Take the leap and get about the business of enjoying a fantastic and full life,” Winfrey says in the commercial. She’ll share some of her favorite Weight Watchers–friendly recipes in her new cookbook, Food, Health and Happiness, out on Jan. 3, along with deeply personal stories about her weight-loss journey. Since joining Weight Watchers in August 2015, Oprah has been steadily dropping pounds. By January 2016, she had lost 26 lb through healthy eating. She also stepped up her exercise routine, vowing in May to log at least 10,000 steps a day. “I try to do something every day that allows me to feel active, and I don’t make myself crazy about it,” she told PEOPLE. “I just know that movement and flexibility, particularly the older you get, is what makes you feel alive. So I don’t want to just be alive; I want to feel it.”