Dietary fiber
The carbohydrate your body can't digest; it slows how fast glucose enters your blood.
Fiber is the part of plant carbohydrate that passes through largely undigested. Soluble fiber in particular slows stomach emptying and the absorption of sugars, blunting the post-meal rise.
It's a big reason whole foods sit lower on the glycemic index than their processed or juiced versions: the fiber that came with them is doing real work.