In vivo measurement of near maximal rates of nutrient absorption in a mammal.

The central limitation hypothesis postulates that energy budgets are limited by the capacity for energy assimilation. Contradictory evidence from in vitro studies of nutrient uptake capacity of intestinal tissues indicates a margin, however, that could allow a higher rate of energy acquisition than actually measured. As a more direct test, I measured nutrient uptake capacity in vivo. This is possible in animals such as the neotropical nectar-feeding bats (Glossophaginae: Phyllostomidae) that have both rapid gut transit times and high daily metabolic needs. Here, during the steady-state period of feeding, the rate of food intake is equal to the rate of food processing and food egestion. Therefore, the rate of food absorption can be determined directly from the rate of food ingestion. Maximal feeding rates were elicited by limiting the time period available for feeding during the 24-hr day through manipulation of the light/dark (LD) cycle. During 4-hr nights (LD 20:4), sugar intake averaged 488 J/g/hr and was 73% higher than during 12-hr nights (LD12:12). A 16.4-g bat would assimilate 96 kJ/day if feeding at this maximal rate during a regular 12-hr foraging period. This would be sufficient for maintaining a positive energy balance even during extreme physical activity under the natural conditions of its tropical environment. Nutrient assimilation capacity could thus not be identified as the single central factor setting the metabolic ceiling.