Glucagon

Glucagon is insulin's counterpart, released by the pancreas when blood glucose falls. Where insulin tells the body to store, glucagon tells it to release: the liver breaks down its glycogen and, when that runs low, manufactures new glucose through gluconeogenesis.

Together the two hormones hold blood glucose in a remarkably narrow band across feasting, fasting and exercise. It is a push and pull system rather than a switch, and both are present at all times with the ratio doing the work.

It matters during exercise and overnight, which are the two situations where you are using glucose and not eating any. Liver glycogen is the buffer that keeps blood sugar steady while you sleep, and glucagon is the signal that draws on it.

It also explains why protein does not behave like pure carbohydrate. A protein rich meal raises insulin, and it raises glucagon too, which is part of why protein has a much smaller net effect on blood glucose despite stimulating insulin release.

Clinically it appears as an emergency treatment. Injectable glucagon raises blood glucose quickly in severe hypoglycaemia, which is why people at risk of it, mainly those using insulin for diabetes, may carry a kit.

It is also a target in modern drug development. Some newer metabolic medicines act at the glucagon receptor alongside GLP-1 and GIP, which is one of several reasons the current generation of weight management drugs behaves differently from the last.

In practice, there is nothing to do about glucagon directly. It is worth understanding because it explains why the body does not collapse when you skip a meal, and why the fasted state is a managed condition rather than an emergency.