Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What: Shares of weight-loss specialist Weight Watchers International (NYSE: WTW) soared a staggering 43% in early Thursday trading after both its quarterly results and full-year forecast blew out Wall Street expectations.

So what: Fueled by strong demand for its subscription-based products, Weight Watchers' fourth-quarter profit jumped 160% to $0.66 per share, while analysts were anticipating $0.56. Rising costs have weighed on the company's results in previous quarters, but the recent launch of its new PointsPlus weight-loss program, as well as drastically improved Web-based sales, seems to be turning things around.

Now what: Those tailwinds are likely to keep blowing in the company's favor. Thanks to booming enrollment in the U.S. and U.K., management now sees 2011 earnings of $3.50-$3.85 per share, versus the average analyst estimate of just $2.77 per share. The shares might be hitting all-time highs today, but with a reasonable 16-ish forward P/E, Weight Watchers might still have plenty of room to run.

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Fool contributor Brian Pacampara owns no position in any of the companies mentioned. Weight Watchers is a Motley Fool Inside Value recommendation. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Fool's disclosure policy always gets a perfect score.