Like many people who struggle with dieting, shares of Weight Watchers (WTW) - Get Reporthave been yo-yoing.

On Thursday, following news that its most famous customer, shareholder and spokeswoman has used its weight loss program successfully, shares of the company shot up nearly 20%. However, year to date, the stock is still down nearly 50%.

Talk show queen Oprah Winfrey has had a powerful effect on Weight Watchers' stock since October 2015, when the company said that she had bought a large stake in the company and become a board member, as well as its spokeswoman.

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At that time, she picked up 10% of Weight Watchers' shares. And in return, the company got a breath of fresh air.

Weight Watchers had been struggling for years as more dieters turned to free online weight loss programs such as SparkPeople, as well as high-tech health gadgets such as Apple's iPhone applications and Fitbit.

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By the time Winfrey came on board, the company carried $2.2 billion in debt. And its debt-earnings ratio had doubled from 2009.

Its debt-asset ratio, at 1.61, is the highest in the specialized consumer services industry, ahead of Ascent Media (0.78) and Sotheby's (0.48).

From the start, Winfrey was a boon to the company and its stock. In fact, on the day that it was announced that she had come on board, Weight Watchers shares soared by more than 100%.

And though the stock has fallen this year, she can still boost shares.

A video that she posted on Twitter last December about the program sent the company's stock up 27%. And in January, when the entertainment maven said that she had lost more than 25 pounds while still eating bread, that drove the stock higher, too.

Weight Watchers is preparing for a post-Christmas marketing push, just in time for people to make their New Year's resolutions. Historically, this has been a big membership drive period for the company, and it has picked up about 40% of subscribers in the first months of the year.

This year's campaign will feature Winfrey, who said Thursday that she had successfully used the company's system to lose 40 pounds.

However, Weight Watchers is still a risky stock that could easily plunge. The company's massive pile of debt certainly doesn't help its health.

And Weight Watchers' future is uncertain as it searches for a new chief executive. Jim Chambers left the post in September, after three years of pushing for a turnaround at the company.

In the meantime, Weight Watchers is led by a triumvirate of executives.

Just as the majority of diets aren't successful, with dieters piling the weight back on, Weight Watchers will likely face more woes in the short to medium term. Investors should continue to be wary of this bloated stock.

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The author is an independent contributor who at the time of publication owned none of the stocks mentioned.