Blood sugar, or, more correctly, blood glucose, simply means glucose which is found in the blood. The glucose is normally obtained from carbohydrate in food and absorbed from the intestinal tract into the bloodstream. Blood glucose rises after a carbohydrate meal and falls during fasting. The body attempts to maintain blood glucose levels between strict limits during fasting conditions because the brain relies on glucose as fuel, and a lack of blood glucose could lead to unconsciousness. A battery of hormones is responsible for this regulation.

Insulin is released from the beta cells of the pancreas in response to a rise in blood glucose, and helps the glucose to diffuse into muscle and fat (adipose) cells. It also promotes the storage of glucose as glycogen (stored sugar) in the liver and muscle cells, and enhances the uptake of glucose by fat and liver cells for conversion into fat. Effect: to lower blood sugar.

Glucagon is released from the alpha cells of the pancreas in response to low blood glucose levels, and has exactly the opposite effect. It causes a rise in blood glucose by converting glycogen back into glucose and promoting the conversion of amino acids and glycerol (from fat) into glucose, and stimulates the release of insulin from the pancreas. Effect: to raise blood sugar.

Cortisol and corticosterone (also known as 'glucocorticoids') are produced by the adrenal cortex in response to low blood glucose levels, and, like glucagon, they reduce the utilization of glucose by the tissues. They also increase the rate at which amino acids are converted into glucose. Effect: to raise blood glucose.

Thyroxine is produced by the thyroid gland in response to severely decreased blood glucose levels. Like glucagon, it promotes both the conversion of glycogen back into glucose, and the conversion of amino acids and glycerol into glucose. Thyroxine also increases the absorption of another sugar, hexose, from the intestines.Effect: to raise blood sugar.

Growth hormone, released by the anterior pituitary gland, also reduces the cellular uptake of glucose. Effect: to raise blood sugar.

Adrenaline (known in the United States as epinephrine) is produced by the adrenal medulla gland in response to anger or fear, and, like glucagon, favours the breakdown of glycogen to yield glucose. It also decreases the release of insulin from the pancreas. Effect: to raise blood sugar specifically in order to provide extra energy for dealing with stressful situations.

Also see Energy production, Glycaemic index, Hyperglycaemia and Hypoglycaemia