A diabetic's blood pH changes with blood sugar levels. You can smell the "sweetness" on the breath when the levels are really off (my Grandma is really bad at "tasting" things on her forbidden list and we always know). A dog's ability to smell is much greater than a humans so I'm not surprised. It is wonderful that he alerts you with no special training. Service animals are being trained now to do everything from alerting for food alergies to epileptic seizures and it is awesome. Those are special animals and I wish there was more funding/training to place more of them.