Shares of NBTY Inc. sank in trading Tuesday after the nutritional supplement maker reported a fiscal second-quarter profit that doubled but still fell far short of Wall Street expectations.

The Ronkonkoma, N.Y., company said it earned $46.7 million, or 73 cents per share, in the three months that ended March 31. That compares to net income of $23.1 million, or 37 cents per share, in the same quarter of 2009.

Sales rose 18 percent to $705.1 million.

Analysts polled by Thomson Reuters expected, on average, earnings of 93 cents per share on $681.6 million in revenue.

CL King analyst Michael W. Gallo downgraded the company's stock to "Neutral" from "Accumulate." He said in a research note the company missed estimates for the first time in a year and noted that the miss was driven by a jump in advertising expense.

"Given our long history covering this name, we believe it is prudent to step out of the way as it appears clear that margins have peaked and could be under pressure for some time," the note said.

NBTY said advertising costs increased $18 million to nearly $51 million. They made up 7.2 percent of sales for the quarter compared to 5.5 percent of sales in the same quarter last year.

The company said it boosted television advertising to support Nature's Bounty, Osteo Bi Flex and several other brands, and that level of advertising was not expected to continue for the balance of the fiscal year.

NBTY shares tumbled $9.27, or nearly 20 percent, to $37.63 in Tuesday afternoon trading.