One of the biggest winners during the latter half of 2015 is starting to roll again after kicking off 2016 as one of the worst performers. Shares of Weight Watchers (WW 2.23%) soared 35% last week, moving sharply higher after a favorable study was published.

The American Journal of Public Health study -- conducted by researchers at Indiana University School of Medicine -- found that adults with prediabetes fared considerably better on Weight Watchers' weight management program than those on a self-initiated program using supplemental counseling materials. This is a pretty big deal, as there are 86 million Americans currently estimated to have prediabetes. If turning to Weight Watchers is the key to avoiding a future of insulin pokes and other ramifications, it's going to be a game-changer.

The study finds that the average Weight Watchers participant lost 5.5% of body weight at six months, compared to a loss of 0.8% with the control group. Just as importantly, Weight Watchers patients maintained the weight loss after 12 months. The control group had gained back most of its weight by that point.

The study validates the community-driven platform at Weight Watchers as a way to lose some excess pounds. It also broadens the offering's appeal with folks showing a tendency for type 2 diabetes, since the study showed better blood glucose control for those on Weight Watchers. 

Shares of Weight Watchers had surrendered a little more than half of their value in 2016 before last week's pop. However, that followed a wild five final months of 2015, when the stock soared 470%.  

Image source: Weight Watchers.   

The catalyst for Weight Watchers was last year's move by Oprah Winfrey: She acquired a sizable stake in the provider of weight management solutions. She disclosed an investment of $43.2 million for a 10% stake and a seat on Weight Watchers' board of directors. The position was eventually boosted to 15%. 

Wall Street isn't convinced. Analysts see flattish sales and earnings growth in 2016, and that's already coming off of a depressed 2015, with sales and earnings falling sharply. That seems fairly pessimistic, underestimating the impact of both Winfrey's persuasive pull and now the allure of Weight Watchers in a diabetes-minded nation.

This should be another busy week for Weight Watchers. It reports after the market close on Thursday. Weight Watchers has been able to beat analyst profit targets in each of the three previous quarters. Another beat in its first quarter with Winfrey on board as a key influencer shouldn't be a surprise. The real pop could happen if it's able to provide a rosy prognosis for the year ahead. We'll find out soon what the so-called "Oprah Effect" can do for a publicly traded company.