Can a dog really detect changing blood sugar?

Isoprene. It appears the body produces isoprene in relatively high doses when blood sugar drops. Isoprene is formed naturally in animals and plants and is generally the most common hydrocarbon found in the human body. The estimated normal production rate of isoprene in the human body is 0.15 µmol/kg/h, equivalent to approximately 17 mg/day for a 70 kg person, but is substantially higher for a type 1 diabetic struggling with low blood sugars.

This hydrocarbon is odourless to us humans, but to our trained diabetic alert dogs, it is (one of) the flashing red lights to indicate!