Animals in the workforce, including lawn-mowing goats and therapy cats, help humans in a number of ways, and dogs, especially, have long assisted their owners with seeing, hearing or sniffing tasks. Taking best-friendship to a new level, medical detection dogs can alert their owners to imminent danger, sometimes even beating technology to the chase.
In a recent study published in PLOS ONE, specially trained dogs were found to help their owners with type-1 diabetes by sniffing blood sugar levels and indicating when they were too high or too low.
The study measured self-reported experience of 17 people with type-1 diabetes and their dogs, trained by the UK charity Medical Detection Dogs, and all participants reported positive outcomes such as fewer paramedic calls and unconscious episodes as well as improved independence.
From the study:
Dogs have been shown to respond when their owners’ sugars are low or high, but as yet we cannot be sure as to what they are actually responding. Odour cues are the most plausible explanation  especially as dogs show “alert” behaviours when their owners are asleep (e.g.) and presumably emitting few behavioural cues (although changes in breathing rate may occur).
It is likely that dogs detect changes in the chemical composition of their owners’ sweat, or breath (including products of ketosis), using their acute sense of smell.