"People who eat a lot and eat frequently -- especially if they eat a lot of high-carb foods or sugars -- really spike their insulin levels very high. The cells of the body grow tolerant to insulin, as an alcoholic grows tolerant to alcohol, and begin ignoring "normal" levels of insulin. Normal levels of insulin don't cause cells to accept glucose. The pancreas detects that blood sugar has not decreased due to its release of insulin, and releases more insulin.
This is the start of a syndrome called "insulin resistance." Cells are exposed to higher-than-normal levels of insulin and won't take in glucose; the pancreas responds by spiking insulin even higher."