Diet composition of carbon and nitrogen (C:N) could affect diet-tissue isotopic discrimination and elemental turnover rate in consumers but studies that test the nature of these changes are scarce. We compared carbon and nitrogen isotopic discrimination and turnover rates in individuals of Pallas' long-tongued bats Glossophaga soricina fed diets with protein soya isolate or amaranth grains as their main source of protein. Diets were of similar protein biological value but the soya diet had higher nitrogen content (2.2%N) and lower C:N ratio (39.6) than the amaranth diet (1.3%N, C:N=40.5). Most bats on the soya diet gained body mass whereas most bats on the amaranth diet lost body mass. Half-lives of carbon (24.3±3.8 days) and nitrogen (25.6±4.4 days) in bats switched to the soya diet were very similar. In contrast, in the bats switched to the amaranth diet, carbon half-life (39.7±3.4 days) was longer than that of nitrogen (25.0±6.0 days). The enrichment in 15N between diet and blood was higher when bats were fed the amaranth diet (4.4±0.2‰) than when they were fed the soya diet (3.3±0.2‰). Similarly, bats on the amaranth diet had higher 13C enrichment (2.0±0.2‰) than bats on the soya diet (0.1±0.1‰). Our results support recent hypotheses of the effect of nutrition on diet-tissue isotopic discrimination and turnover rate, and further shows that blood stable isotope analysis is an adequate approach to track seasonal dietary shifts in wild bats.