In male rats, insulin helps blood flow to leg muscles pretty quickly—but only if they're eating regular food. If they're on a high-fat diet, insulin doesn't do this job well anymore.

In male rats, a hormone called GIP can block insulin’s ability to increase blood flow to leg muscles, even when insulin levels stay the same — suggesting GIP directly interferes with how insulin works in the blood vessels of muscle.

When the tiny blood vessels in muscles don't respond well to insulin, it's linked to being overweight, having insulin resistance, and getting older — and this might actually happen before the muscle cells themselves become resistant to insulin.

In rats, eating a diet that's a bit high in fat or salt messes up blood vessel function in muscles before the muscle cells themselves become resistant to insulin — and that blood vessel problem alone can slow down how much sugar the muscles take in.

When healthy adults get a controlled dose of insulin for two hours, it boosts blood flow and blood volume in their heart and muscle tissues, helping more nutrients move between blood and muscles.

Insulin helps more blood flow to muscles in rats and people by using a substance called nitric oxide, which helps sugar get into muscle cells better.