NutriSystem (NTRI) plunged 13% Tuesday after the weight-loss outfit guided below Wall Street's estimates for its first quarter.The after-hours selloff took NutriSystem down to $45 and change -- down 28% in two weeks of heavy trading. The Horsham, Pa., company said it expects to make 50 to 53 cents a share on revenue of $131 million to $133 million for the fourth quarter ended last month. That's above the Thomson Financial consensus, which calls for a 47-cent profit on sales of $128 million. But for the first quarter, NutriSystem expects to make 82 cents to 86 cents a share on revenue of $200 million to $210 million. Wall Street wanted a 95-cent profit on sales of $213 million. "With the pursuit of new market segments combined with unseasonably higher media rates, we expect to see moderately higher customer acquisition costs in first quarter 2007 on a year-over-year basis," the company said. After falling 10% in regular trading Tuesday, NutriSystem tanked $6.73 to $45.32 in late trading.