What is Blood Sugar?
Blood sugar, used in a physiological context, is a misnomer and misleading. Physiologically, the term means only glucose in the blood. Other sugars are present, sometimes in more than trace amounts, but only glucose serves as a controlling signal for metabolic regulation. Other sugars are, to some extent, inert. Glucose, transported via the bloodstream from the intestines or liver to body cells, is the primary source of energy for the body's cells.
Blood sugar concentration, or glucose level, is tightly regulated in the human body. Normally, the blood glucose level is maintained at a reference range between about 4 and 6 mM (mmol). The normal blood glucose level is about 90mg/100ml, which works out to 5mM (mmol/l), since the molecular weight of glucose, C6H12O6, is about 180 g/mol daltons. The total amount of glucose in circulating blood is therefore about 3.3 to 7g (assuming an ordinary adult blood volume of 5 litres, plausible for an average adult male). Glucose levels rise after meals for an hour or two by a few grams and are usually lowest in the morning, before the first meal of the day.
Failure to maintain blood glucose in the normal range leads to conditions of persistently high (hyperglycemia) or low (hypoglycemia) blood sugar. Diabetes mellitus, characterized by persistent hyperglycemia from any of several causes, is the most prominent disease related to failure of blood sugar regulation.
Although it is called "blood sugar," other simple sugars aside from glucose are found in the blood, such as fructose and galactose. But only glucose levels are used as metabolic regulation signals (via insulin and glucagon).
If blood sugar levels drop too low, a potentially fatal condition called hypoglycemia develops. Symptoms may include lethargy, impaired mental functioning, irritability, and loss of consciousness. Brain damage is even possible.
If levels remain too high, appetite is suppressed over the short term. Long-term hyperglycemia causes many of the long-term health problems associated with diabetes, including eye, kidney, and nerve damage.
Low blood sugar:
Some people report drowsiness or impaired cognitive function several hours after meals, which they believe is related to a drop in blood sugar, or "low blood sugar". For more information, see:
idiopathic postprandial syndrome:
Mechanisms which restore satisfactory blood glucose levels after hypoglycemia must be quick and effective, because of the immediately serious consequences of insufficient glucose; in the extreme, coma, but also less immediately dangerous, confusion or unsteadiness, amongst many other symptoms. This is because, at least in the short term, it is far more dangerous to have too little glucose in the blood than too much.
In healthy individuals these mechanisms are generally quite effective, and symptomatic hypoglycemia is generally only found in diabetics using insulin or other pharmacological treatment. Such hypoglycemic episodes vary greatly between persons and from time to time, both in severity and swiftness of onset. For severe cases, prompt medical assistance is essential, as damage (to brain and other tissues) and even death will result from sufficiently low blood glucose levels.
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