Incretins

Fact: When most people eat, their stomachs (okay actually the ileum and jejunum) release incretins that cause their pancreata to make more insulin, even before blood sugar rises. That's part of how the body preboluses.

Some of the medications used to treat type 2 diabetes are incretins (Byetta and Victoza) or imitations of incretins, and substances that increase the levels of incretins in the body (the medications ending in -gliptin).

Some people with type 2 diabetes do not make normal levels  of incretins in response to eating; that is a fairly common contributor to the diabetes itself. 

Some people with type 2 diabetes as well as some forms of MODY are related to having beta cells that don't respond to incretins- if the beta cells don't respond to incretins, it's sort of like having a body that doesn't prebolus for meals, and after meal blood sugar levels are likely to be high, even though fasting levels are probably normal.

Partly because of incretins (and partly because of the speed of digestion), in non-diabetics eating sugar has a smaller effect on blood sugar levels than being injected with the same amount of sugar. In some type 2 diabetics (and in most type 1 diabetics), the effect on blood sugar level from eating sugar is the same as being injected with sugar.

However, it's pretty hard to tell whose diabetes is caused by less production of incretin, because diabetes also causes a lowered production of incretins- when people have other forms of diabetes, their bodies also make less incretins.