Jenny Craig, Uh, Gains on Net News

There aren’t many “sure things” in this world, but here’s one: To spark a surge in a beaten-down stock, just announce that the company has a new Internet venture.

Shares of La Jolla-based Jenny Craig Inc., the “weight management service” company, soared 31% on Monday--making it the second-biggest gainer on the New York Stock Exchange--after the company announced that it redesigned its Web site and that it will sell a new line of nutritional supplements via the site.

Jenny Craig stock jumped $1.06 to $4.50. The shares had sunk to $3.25 recently from $6.38 in late January, when the firm announced disappointing fourth-quarter results, including a drop in sales to $74.3 million from $78.8 million a year earlier, and an operating loss of $1.2 million.

Jenny Craig’s new line of supplements will be made by Natural Alternatives International--whose stock, however, did not get a lift on Monday. The price closed unchanged at $4.94 on Nasdaq. It has fallen from a 52-week peak of $26.63.