General Nutrition Centers Inc. continued to ride strong e-commerce sales in the first quarter, on the heels of its initial public stock offering.

For the quarter ended March 31, GNC reported:

• Online sales of $20.1 million, a 34.1% increase from about $15.0 million in the first quarter of 2010.
• Total retail sales of $383.7 million, up by 9.4% from $350.8 million.
• Total sales, including franchise and wholesale segments, of $506.0 million, up by 8.8% from $465.0 million.
• Comparable-store sales, which include e-commerce, increased 7.5% in domestic company-owned stores.
• Net income of $9.9 million, a 61.5% decline from net income of $25.7 million for the first quarter of 2010. The drop in net income included non-recurring costs related to refinancing and the IPO, the company says.

Internet Retailer calculates the web accounted for 5.2% of total retail sales for the quarter, compared with 4.3% in Q1 of 2010.

“Our retail store performance is complemented by solid results from other high-growth businesses like GNC.com and international franchise,” says Joe Fortunato, CEO of GNC, No. 227 in the Internet Retailer Top 500 Guide. “The manufacturing business continues to add new third-party contract sales, and we are starting to see contribution from our brand extension efforts with PetSmart and Sam’s Club.”

On April 6, the company completed an IPO of 25.875 million shares of common stock at $16 per share, for a total of $414 million. The company issued and sold 16 million shares and certain of its shareholders sold 9.875 million shares in the IPO, GNC says.