If insulin is “the key that lets glucose into the cell,” then why don’t people need more carbohydrates and more insulin during exercise? I keep reading that exercise makes the body more sensitive to insulin, but how? Is it the increased sensitivity that makes the athlete not need more insulin? Is there some part of the muscle tissue that works like the brain, kidney, and retinal cells which do not require insulin to use glucose?

Insulin interacts with receptors on the muscle, liver and fat cells. During exercise, muscle receptors change and there is less insulin required to do the same job. In relative terms, you would always need less insulin either produced or provided with exercise and more food/carbs but you do not have a good way to measure this at home except with blood glucose readings. In research settings we can provide this quite easily.

SB
Additional comments from Delaine Wright:
In the exercising muscle, other pathways for glucose (non-insulin driven) open, allowing glucose to enter cells through more “doorways” than that opened by the insulin “key”. These doorways are called the PPAR family (peroxisome proliferator activated receptors). It was this discovery that actually led researchers to develop another class of medication for Type 2 diabetes, the thiazolidinediones or “glitazone”
class (Actos and Avandia). This occurs in addition to the increased insulin sensitivity.

DMW