According to a recent study by the National Center for Health Statistics, the number of adults taking dietary supplements continues to grow. I think it might have something to do with the Flintstones getting us hooked at an early age, but I can't confirm it. Regardless of the reason we're taking more supplements, it's good news for The Vitamin Shoppe (NYSE: VSI).

One-stop supplement shopping
I'll admit that the company faces a lot of competition. GNC Holdings (NYSE: GNC) has thousands of locations, including its Rite Aid (NYSE: RAD) store-within-a-store locations, and it seems that just about any place that sells toothpaste also sells supplements.  

The Vitamin Shoppe pulls ahead of this crowd by offering a huge selection at low prices. Customers can choose from roughly 8,000 products in its retail stores and an additional 12,000 items online. In contrast, GNC offers approximately 1,800. Online, Walgreens (NYSE: WAG) sells more than 1,700 different supplements, and around 700 in its stores.  The selection gets even smaller at general retailers. My local Wal-Mart (NYSE: WMT) offers just 42 supplements, and in its online store, the selection drops to 36.  

Manageable growth
I'm no fan of growth for growth's sake, but Vitamin Shoppe seems to have found a comfortable rate of expansion. The company has opened roughly 40 stores a year since 2007. Counting the 15 stores opened in the first quarter, the company currently runs 497 locations and plans to open 33 more during the fiscal year.  

When I see rapid growth like this in a retail chain, I often look at comparable-store sales. If sales plummet in the established stores, then I worry about the company's long-term viability. Vitamin Shoppe's first-quarter comps increase of 8.1% indicates that the company is thriving. And that's on top of a 6.2% gain at this time last year.

I do worry a little that the chain relies primarily on one warehouse and distribution center in New Jersey. It hasn't been an issue so far, but if the new stores overloaded the center, or the center were to shut down due to an emergency, then nearly every store in the chain would have trouble restocking its shelves. Because the company relies on its vast selection to pull in the customers, that empty space would be very bad for business.  

Fortified financials
Although I'm happy with the same-store sales, I've found a lot more in the company's quarterly report that I like. The positive same-store sales, new store growth, and a 7.4% increase in online sales brought year-over-year sales up 13.2%, to $216.8 million for the quarter. Gross margin improved 80 basis points to 34.7%, and free cash flow increased year over year from $2.8 million to $6.4 million.  

Foolish takeaway
The Vitamin Shoppe holds the enviable position of being a successfully growing company in an expanding market. I'll be watching to see whether it can continue its 22-quarter streak of increasing same-store sales as it continues to expand. If it can, then I just might make the company a welcome supplement to my portfolio.

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Patrick Martin holds no position in any of the companies mentioned here, nor has he been evaluated by the FDA. The Motley Fool owns shares of Wal-Mart Stores. Motley Fool newsletter services have recommended buying shares of and creating a diagonal call position in Wal-Mart Stores. 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 Motley Fool has a disclosure policy.