Chipotle Mexican Grill Inc. lowered its full-year outlook after quarterly sales declined for the first time in almost five years, suggesting that economic uncertainty is causing diners to pull back on burrito outings.

Comparable sales fell 0.4% in the first quarter, according to a statement Wednesday, while analysts were expecting growth of 1.7%. A decline in transactions drove the slide, a reversal from recent quarters in which Chipotle had managed to attract diners even as peers struggled.