Blood sugar concentration, or glucose level, refers to the amount of glucose present in the blood of a human or animal. Normally, in mammals the blood glucose level is maintained at a reference range between about 3.6 and 5.8 mM (mmol/l). It is tightly regulated as a part of metabolic homeostasis.
Mean normal blood glucose levels in humans are about 90 mg/dl, equivalent to 5mM (mmol/l) (since the molecular weight of glucose, C6H12O6, is about 180 g/mol). The total amount of glucose normally in circulating human 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. Transported via the bloodstream from the intestines or liver to body cells, glucose is the primary source of energy for body's cells, fats and oils (ie, lipids) being primarily a compact energy store.
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.
If blood sugar levels drop too low, a potentially fatal condition called hypoglycemia develops. Symptoms may include lethargy, impaired mental functioning, irritability, shaking, weakness in arm and leg muscles, sweatting 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, heart disease and nerve damage.
The term 'blood sugar' has colloquial origins. In a physiological context, the term is a misnomer because it refers to glucose, yet other sugars besides glucose are always present. Food contains several different types (eg, fructose (largely from fruits/table sugar/industrial sweeteners). galactose (milk and dairy products), as well as several food additives such as sorbitol, xylose, maltose). But because these other sugars are largely inert with regard to the metabolic control system (i.e., that controlled by insulin secretion), since glucose is the dominant controlling signal for metabolic regulation, the term has gained currency, and is used by medical staff and lay folk alike. The table above reflects some of the more technical and closely defined terms used in the medical field.
In most countries, blood glucose is reported in terms of molarity, measured in mmol/L (or millimolar, abbreviated mM). In the United States, and to a lesser extent elsewhere, mass concentration, measured in mg/dL, is typically used.
To convert blood glucose readings between the two units:
- Divide a mg/dL figure by 18 (or multiply by 0.055) to get mmol/L.
- Multiply a mmol/L figure by 18 (or divide by 0.055) to get mg/dL.
Further Reading
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