Image source: United Natural Foods.

What: Shares of natural foods distributor United Natural Foods (UNFI -0.86%) tumbled on Monday after the company released preliminary second-quarter results. At 11:50 a.m. ET, the stock was down about 23%.

So what: United Natural Foods expects second-quarter net sales in the range of $2.046 billion to $2.048 billion, a year-over-year increase of 1.5%. Adjusting for the impact of the termination of a previously disclosed customer distribution contract, net sales are expected to rise by 6.4% to 6.5% year over year.

While revenue will rise during the second quarter, United Natural Foods expects earnings to decline. It also forecasts GAAP EPS of $0.43 to $0.45 for the second quarter, down from $0.55 during the prior-year period. Adjusting for various one-time charges, EPS is expected between $0.47 and $0.49, down from $0.65.

Now what: While United Natural Foods expects to grow more slowly in 2016 compared to recent years, the big news was the steep decline in earnings that the company expects for this year. CEO Steven Spinner commented on the company's lackluster outlook. "Our updated results reflect the rapidly changing near term industry dynamics facing UNFI and its constituents. We have made significant progress in developing our fresh platform, e-commerce, brands and infrastructure. UNFI's second quarter preliminary results and our view of fiscal 2016 reflect our commitment to investing in growth throughout our organization while we continue to evaluate our cost structure and customer experience."

Along with announcing preliminary results, United Natural Foods provided revised guidance for fiscal 2016. The company expects a 1.5% to 3% increase in net sales compared to fiscal 2015, with GAAP EPS between $2.27 and $2.37. That's down substantially from the $2.76 in per-share earnings United Natural Foods reported in fiscal 2015. This guidance doesn't include the impact from the acquisition of Haddon House Food Products, which the company disclosed along with its preliminary results.