Insulin is the molecule that signals to cells that it is okay to take in and use sugar that's in the blood. This is a molecule that buffers sugar; if there's a high sugar concentration in the blood it binds it together and if there's a low sugar concentration it releases it. This would normally be one of the functions of the liver, but the latter part (absorbing excess sugar for storage) won't happen without insulin signaling. If you give this to a type 1 diabetic without also giving insulin, all that will happen is that it'll absorb sugar until it reaches capacity; the cells that should be using sugar for energy still won't be able to.