Intertidal benthos link tertiary predators and primary producers in marine food webs as well as directly contribute to sediment CO2 emission. However, current methods for studying food sources of marine benthos are time-consuming and does not allow direct estimates on feeding regime-related (including different diets, active versus dormant) CO2 production. We examined the food sources of mangrove crabs and gastropods as well as their corresponding CO2 production using cavity-ring down spectroscopy to measure the δ13C–CO2 respiration for consumers, considering the effects of feeding regime, benthos taxa, and dominant feeding habit. Benthos taxa and feeding habit have significant impact on δ13C–CO2 respiration. Particularly, the δ13C–CO2 respiration for crabs (−23.9 ± 0.4‰) was significantly lower than that for gastropods (−17.5 ± 1.3‰). The δ13C–CO2 respiration for deposit-feeders was significantly higher than that for detritivores. There are significant differences in the amount of CO2 emitted and δ13C–CO2 respiration for crabs under different feeding regimes. The differences reflect diet-switching and fuel-switching by the crabs, i.e. ‘you breathe what you eat’. Significant differences in CO2 production of crabs also exist between those feeding on microphytobenthos in the laboratory (0.13 ± 0.02 mmol g−1 day−1) and on field collection (i.e. just collected from the field) (0.31 ± 0.03 mmol g−1 day−1). CO2 production of crabs is strongly related to carapace width and length. The δ13C–CO2 respiration for mangrove crabs reflects their diet while crab-respired CO2 flux is related to crab size. These relationships enable partitioning the feeding habit and food sources of key benthos, and help incorporate their contribution into the overall sediment-atmosphere CO2 fluxes in mangrove forests.