GLUT4

GLUT4 is the glucose transporter used by muscle and fat tissue. Unlike the transporters that sit permanently in the membrane of other cells, GLUT4 is stored inside the cell in vesicles and moved to the surface only when instructed.

Two signals do that instructing, and this is the detail that matters. Insulin triggers it, through the signalling cascade that follows insulin binding its receptor. Muscle contraction triggers it too, through an entirely separate pathway that does not require insulin at all.

That second route is why exercise improves blood glucose even when insulin signalling is impaired. A single session increases glucose uptake into muscle for hours afterwards, and regular training increases the total amount of GLUT4 the muscle has available.

It also explains why muscle mass matters metabolically. Muscle is the largest destination for glucose in the body, so having more of it, and using it often enough to keep its transporters plentiful and responsive, changes where glucose goes after a meal.

In insulin resistance the problem is partly here. The signal is being sent and the transporters are not being deployed as they should be, which is why insulin levels rise while glucose disposal falls.

There is nothing to supplement. This is a mechanism to understand rather than a target to hit, and the interventions that act on it are exercise, muscle mass and weight loss.

In practice, it is the cellular explanation for why a walk after a meal blunts the glucose rise, and why regular training is the most reliable non-pharmacological glucose intervention there is.