When healthy adults get both GLP-1 and insulin at the same time, it doesn’t boost blood flow to their heart or muscles any more than using just one of the hormones alone.

When healthy adults get a specific dose of a hormone called GLP-1 through an IV, it quickly boosts blood flow in the small vessels of their heart and muscles, helping deliver more nutrients and hormones where they're needed.

In male rats, a hormone called GIP can completely stop another hormone, GLP-1, from increasing blood flow to leg muscles—even if the rats eat junk food or regular food—and it does this without changing how much GLP-1 is in the blood.

When healthy adults get infusions of two hormones—GLP-1 or insulin—each one widens a major arm artery and boosts blood flow by about 30%. But when both are given together, they don’t widen the artery any more than just one alone, meaning they don’t add up.

Giving healthy adults normal levels of certain hormones like GLP-1 and insulin — either alone or together — doesn't change how stiff their main artery is over about two and a half hours.