Nectar-feeding bats fuel their high metabolism directly with exogenous carbohydrates

Mammals usually derive energy from metabolizing fat and glycogen stores combined with exogenous food. Nectarivorous bats mostly consume a diet low in both fat and proteins but rich in simple carbohydrates. Metabolizing exogenous carbohydrates directly to fuel their high mass-specific metabolic rate would save the energetic costs of lipogenesis and gluconeogenesis for nectarivorous bats. Therefore, we expected nectarivorous bats to switch to exogenous carbohydrates rapidly when available and use them predominantly instead of fat or glycogen. We first investigated the rate of fractional incorporation of dietary sugars into the pool of metabolized substrates in Glossophaga soricina by measuring the change in C-13 enrichment of exhaled CO2 (delta C-13(breath)) when animals were fed glucose, fructose or sucrose that was isotopically distinct from their normal diet. Second, we performed a diet-switch experiment to estimate the turnover rate of fat tissue. When fed with sugars, the delta C-13(breath) converged quickly on the isotope signature of the ingested sugars, indicating an almost exclusive use of dietary carbohydrates. The time for a 50% carbon isotope exchange in exhaled CO2 equalled 9, 13 and 14 min for fructose, glucose and sucrose, respectively. Nectarivorous bats fuelled 82% of their metabolism with exogenous carbohydrate when fed with fructose, 95% when fed with glucose and 77% when fed with sucrose. Bats depleted 50% of their fat stores each day. Although nectarivorous bats consumed most of their body fat each day, this was still barely enough to sustain their diurnal metabolism. The fractional incorporation rates of dietary sugars into the pool of metabolized substrates in G. soricina are the fastest rates ever found in a mammal.