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: Upscale grocer Whole Foods Market (NASDAQ:WFM) has lost 10% of its value today, after issuing underwhelming guidance -- largely the same guidance, really -- for the rest of its fiscal year.

So what: Whole Foods' first quarter results showed $3.86 billion in revenue and $0.78 in earnings per share. Revenue was roughly in line with the consensus of $3.87 billion, and EPS came in $0.01 better than Wall Street's expectations. Same-store sales growth, which was an impressive 7.2%, nevertheless came in below the roughly 7.7% estimate offered by analysts.

Looking ahead to the rest of fiscal 2013, Whole Foods is holding firm in its previous guidance range of 12% to 14% revenue growth -- which roughly translates into a $13.1 billion to $13.4 billion range over 2012's result -- and $2.83 to $2.87 in EPS. Analysts were looking for $2.89 in EPS, and Whole Food's annual comparable-store sales growth guidance of 6.6% to 8% is also below its prior projections of 6.5% to 8.5%. Identical-store sales growth guidance, at 6.3% to 7.7%, is tighter, but falls below the 8% high end of Whole Foods' earlier guidance

Now what: Whole Foods is not cheap for a grocer. Today's reaction seems a little overwrought, but some large stockholders may have simply been seeking a reason to get out. The fundamentals remain strong, and nothing in Whole Foods' guidance indicates that it will cease to be a sector outperformer in 2013.

Fool contributor Alex Planes holds no financial position in any company mentioned here. Add him on Google+ or follow him on Twitter @TMFBiggles for more insight into markets, history, and technology.

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