When insulin levels go up in healthy adults, it helps blood flow better in the legs by turning on a molecule called nitric oxide — and it works about twice as well as when insulin isn’t active.

When insulin goes up in healthy people — without changing blood sugar — blood flow to the legs nearly doubles, showing insulin itself can widen blood vessels.

In healthy adults, insulin doesn't make blood vessels relax more when nitric oxide is already at work — meaning insulin probably doesn't improve blood flow by boosting the muscle's response to nitric oxide.

When healthy adults get a certain drug in their leg artery, blood flow drops by up to 45%, and insulin makes this drop even bigger — showing that insulin helps control blood flow in muscles by affecting nitric oxide.

Insulin helps blood vessels relax better when the signal comes from the vessel lining, but not when the signal bypasses it—meaning insulin probably boosts the lining's ability to release a blood-vessel-relaxing chemical called nitric oxide.