Hi,

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Jason Fung has a lot of videos on this topic https://www.youtube.com/watch?v=r0d5lJzMXnM is one.

BAsically your cells are already full of enough sugar and so they start ignoring insulin signals . If you exercise you use up this store and they become more willing to accept sugar so they become more sensitive to insulin.

Diet has a similar effect if you use foods that do not convert to sugar or do so slowly ( LowCarb/Keto) since sugar availability becomes less , and cells start to tend to use up their stores they begin to become more sensitive to insulin to get more sugar.

Intermittent fasting, is an attempt to force the body to use up its stores and cause the same sort of reactions.

HHope this helps.