Columnar cacti constitute the dominant elements in the vegetation structure of arid and semi-arid New World ecosystems representing a plethora of food resources for vertebrate consumers. Previous stable isotope analysis in Central Mexico showed that columnar cacti are of low importance to build tissue for frugivorous bats. We used carbon stable isotope analysis of whole blood and breath samples collected from four species of frugivorous bats (Sturnira parvidens, Sturnira ludovici, Artibeus jamaicensis, and Artibeus intermedius) to reconstruct the importance of cactus plants in their diet. Breath samples were collected within 10 min (B10) of bat capture and ~12 h after capture (B720), representing the oxidation of recently ingested food and of body reserves, respectively. We expected that bats relied primarily on non-cactus food to construct tissues and fuel oxidative metabolism. Non-cactus food strongly predominated for tissue building, whereas oxidative metabolism was supported by a moderate preponderance of non-cactus food for B10 samples, and a moderate preponderance of cactus food or an equal contribution of both sources for B720 samples. Artibeus and Surnira species appear to cover a narrow part of the diet with cactus food, confirming that the incorporation of nutrients derived from these plants is not generalized among vertebrate consumers.