High blood glucose levels caused by excessive sugar consumption are detrimental to mammalian health and life expectancy. Despite consuming vast quantities of sugar-rich floral nectar, nectar-feeding bats are long-lived, provoking the question of how they regulate blood glucose. We investigated blood glucose levels in nectar-feeding bats (Glossophaga soricina) in experiments in which we varied the amount of dietary sugar or flight time. Blood glucose levels increased with the quantity of glucose ingested and exceeded 25 mmol l(-1) blood in resting bats, which is among the highest values ever recorded in mammals fed sugar quantities similar to their natural diet. During normal feeding, blood glucose values decreased with increasing flight time, but only fell to expected values when bats spent 75 per cent of their time airborne. Either nectar-feeding bats have evolved mechanisms to avoid negative health effects of hyperglycaemia, or high activity is key to balancing blood glucose levels during foraging. We suggest that the coevolutionary specialization of bats towards a nectar diet was supported by the high activity and elevated metabolic rates of these bats. High activity may have conferred benefits to the bats in terms of behavioural interactions and foraging success, and is simultaneously likely to have increased their efficiency as plant pollinators.