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The bloodstream carries glucose—a type of sugar produced from the digestion of carbohydrates and other foods—to provide energy to cells throughout the body. Unused glucose is stored mainly in the liver as glycogen.

Insulin, glucagon and other hormone levels rise and fall to keep blood sugar in a normal range. Too little or too much of these hormones can cause blood sugar levels to fall too low (hypoglycemia) or rise too high (hyperglycemia).

Normally, blood glucose levels increase slightly after you eat a meal. When blood sugar rises, cells in the pancreas release insulin, causing the body to absorb glucose from the blood and lowering the blood sugar level to normal. When blood sugar drops too low, the level of insulin declines and other cells in the pancreas release glucagon, which causes the liver to turn stored glycogen back into glucose and release it into the blood. This brings blood sugar levels back up to normal.