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-management services specialist Weight Watchers International (NYSE: WTW) plummeted 16% on Thursday after its quarterly results and guidance came in below Wall Street expectations.

So what: Weight Watchers' first-quarter EPS managed to squeak past estimates, but a miss on the top line -- $484.8 million versus the consensus of $492.5 million -- coupled with a downbeat outlook is forcing analysts to lower their growth estimates yet again. Last week, close rival Nutrisystem (Nasdaq: NTRI) also posted a weak quarter and issued disappointing guidance, suggesting that weight-management companies are being hit particularly hard by a pullback in discretionary spending.

Now what: Management now sees full-year EPS of $4.00-$4.20, well below the Wall Street consensus of $4.58. "Since June we have seen a weakening in our trends so we are taking a more cautious view of our business for the second half of the year in light of difficult macro-economic trends, particularly in consumer sentiment," CEO David Kirchhoff said in a statement. Of course, with the stock hitting a new 52-week low today and trading at a single-digit forward P/E, much of that uncertainty might already be baked into the price.

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