glucose
Meaning of glucose: The brain depends on it almost exclusively and stores none, which is why the body defends the blood level so aggressively.
Definition of glucose
Glucose is a simple sugar and the main fuel the body runs on. It arrives from the breakdown of carbohydrate in food, and the liver both stores it as glycogen and manufactures it when food stops coming. Cells break it down to release energy. Its concentration in the blood is held within a narrow band by a set of hormones pulling in opposite directions.
How is the blood level controlled?
Control is a tug of war rather than a thermostat, and only one of the hormones pulls downward.
| Hormone | Released when | Effect on blood glucose |
|---|---|---|
| Insulin, from beta cells of the pancreas | Levels rise after eating | Lowers it, by moving glucose into muscle and fat and storing it as glycogen |
| Glucagon, from alpha cells | Levels fall between meals | Raises it, by releasing stored glycogen and making new glucose |
| Adrenaline, cortisol, growth hormone | Stress, illness, injury | Raise it, freeing fuel for immediate use |
Brain and liver cells absorb glucose without needing insulin at all, which is why the brain keeps running when insulin is absent while muscle and fat starve in the middle of a blood sugar excess.
What happens when control fails?
In diabetes the level stays too high, either because the beta cells have been destroyed and produce no insulin, or because tissues respond poorly to the insulin available and the pancreas cannot compensate indefinitely. Excess glucose spills into the urine and drags water with it, producing thirst and frequent urination, and over years it damages small blood vessels in the retina, kidney and nerves as well as large arteries. Too little glucose is the opposite emergency and moves faster: sweating, tremor and hunger come first, then confusion and impaired consciousness as the brain runs out of the only fuel it can readily use.
How is it measured?
A fingerstick meter gives an immediate reading, a laboratory sample a more precise one. Fasting values, random values and values taken after a measured sugary drink each answer a slightly different question. Glycated hemoglobin reflects the average over the lifespan of circulating red cells, roughly the preceding few months, so it smooths out day-to-day swings but hides them. Continuous sensors worn under the skin track the trend rather than single points.
Also known as
Used in a sentence
The laboratory panel included a measurement of the patient's fasting blood glucose.
Good to know
Both directions of abnormal glucose matter, since a persistently high level damages vessels and nerves over years while a low level impairs brain function within minutes.
Word origin
Where “glucose” comes from
From Greek gleukos, meaning sweet wine or must, reflecting the sweetness of the sugar.