Fruits and vegetables contain soluble and insoluble fiber in varying proportions. When you eat soluble fiber, it absorbs water to form a gel-like mass, which helps you feel full and provides other health benefits. It prevents a surge in blood sugar after you eat by slowing down the absorption of carbohydrates.
If the blood glucose level falls to dangerous levels (as in very heavy exercise or lack of food for extended periods), the Alpha cells of the pancreas release glucagon, a hormone whose effects on liver cells act to increase blood glucose levels.
These are called the Islets of Langerhans. They secrete two hormones, insulin and glucagon, which maintain blood glucose (sugar) levels. Insulin is secreted in response to high glucose levels in the blood. It lowers sugar levels in the blood by increasing the uptake of glucose into the tissues. Glucagon has the opposite effect.