All the sugar in your bloodstream adds up to about one teaspoon
A healthy 75-kilogram adult with five litres of blood carries only about 5 grams of glucose in circulation, roughly a teaspoon of sugar. The body defends that tiny amount relentlessly, pulling it back into a narrow band after every meal, because the brain alone burns around 60 percent of blood glucose in a resting, fasting person.
Most of the body's glucose is not in the blood at all. It is stored as glycogen in the liver and skeletal muscle and released in measured amounts during fasting to keep blood levels steady. Part of why circulating glucose stays so low is that cells tag incoming glucose with phosphate and other groups, which keeps it flowing inward. Inside cells, glucose is broken down for energy through glycolysis.
Two sets of hormones pull against each other. Insulin, made by beta cells in the pancreatic islets, lowers blood glucose by helping cells take it up; glucagon from alpha cells, along with cortisol and catecholamines, raises it. The falling glucose that insulin produces prompts glucagon release, and the cycle repeats as a negative feedback loop. Muscle activity also lowers glucose through a pathway involving the enzyme AMPK.
The system is remarkably stable. Fasting levels in people without diabetes sit around 3.9 to 5.5 millimoles per litre, lowest in the morning before breakfast, and may briefly climb to about 7.8 after eating. Units differ by country: the United Kingdom and Commonwealth use millimoles per litre, while the United States, Germany and Japan use milligrams per decilitre, with one converting to the other by a factor of about 18 based on glucose's molar mass. Alcohol can cause an initial spike followed by a fall.
Persistently high glucose, hyperglycaemia, damages cells over time and underlies the complications of diabetes mellitus, the most prominent disorder of blood sugar control; low levels are hypoglycaemia. Other animals run at quite different settings. Domestic cows, sheep and goats run lower than cats, dogs or horses, and hooded seals have been measured at roughly 7.5 to 15.7 millimoles per litre.
Source: Blood sugar level
