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.

Insulin helps open up blood vessels in your muscles, letting more blood through so nutrients like creatine can get to the muscle cells more easily.

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.

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.

Taking nitroglycerin while insulin is active might lower how much sugar the heart uses, even if blood flow to the heart stays the same.

When the body can't make nitric oxide, blood flow to the heart drops, but the heart still takes in sugar normally when insulin is around — meaning nitric oxide isn't needed for insulin to help the heart use sugar.