Chipotle Mexican Grill Inc. sales dipped into negative territory for the first time in almost five years in the first quarter and the company lowered its full-year outlook, showing that economic uncertainty is prompting diners to pull back on burrito outings.

Comparable sales fell 0.4%, below the average estimate of 1.7% growth from analysts. A decline in transactions drove the decline, a reversal from recent quarters in which Chipotle had managed to attract more diners even as peers struggled.