What is Blood Sugar?

By Dr Ananya Mandal, MD

Blood sugar or blood glucose level is the amount of glucose present in the blood of a human or animal. Blood glucose is a tightly regulated biochemical parameter in blood. It is important for metabolic homeostasis.

Blood sugar - a misnomer

The term 'blood sugar' is a misnomer since it means only glucose. Yet there are other sugars besides glucose that are present in blood.

Food contains several different types of sugars e.g. fructose from fruits, galactose and lactose from milk and dairy, sorbitol, xylose, maltose etc. These sugars are inert with regard to the human metabolism and its regulation by insulin hormone and thus blood sugar tends to refer to glucose alone.

Normal levels of blood glucose

Normal levels range between 3.6 and 5.8 mM (mmol/l). In humans normal blood glucose levels are around 90 mg/dl, equivalent to 5mM (mmol/l).

Since the molecular weight of glucose, C6H12O6, is about 180 g/mol, when calculated the total amount of glucose normally in circulating human blood is around 3.3 to 7g (assuming an ordinary adult blood volume of 5 litres).

Blood glucose is measured in terms of molarity, measured in mmol/L or millimoles per litre. In the United States, and to a lesser extent elsewhere, mass concentration, measured in mg/dL.If a mg/dL figure is to be converted to mmol/L, it is to be divided by 18 or multiply by 0.055. Similarly to convert a mmol/L figure to mg/dL it is to be multiplied by 18 or divided by 0.055.

Normal fluctuations in levels of glucose

Glucose levels in blood fall to indicate hunger. The brain is dependent on glucose as its primary energy producing substance. Low blood glucose indicates to the brain that there is need for food intake and this triggers sensations of hunger.

Glucose is absorbed from the intestines and via blood stream it reaches the liver and various body cells to provide the primary source of energy for body's cells.

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. The morning levels are the lowest since it follows around 6 to 8 hours of fasting throughout the night.

Imbalances of glucose levels in blood

If the body fails to maintain normal levels of glucose, it may give rise to several disease conditions. Persistently high blood glucose is called hyperglycemia. Diabetes mellitus is characterized by persistent hyperglycemia from any of several causes. It results from the body’s failure to regulate blood sugar. Long term hyperglycemia leads to damage to several organs like the retina, kidneys, arteries etc.

If blood sugar levels drop too low, a potentially fatal condition called hypoglycemia develops. This may be manifested as weakness, drowsiness, shaking, irritability, sweating etc. In severe cases it may lead to loss of consciousness and even brain damage.

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.

Reviewed by , BA Hons (Cantab)


  1. http://professional.diabetes.org/UserFiles/File/Make%20the%20Link%20Docs/CVD%20Toolkit/04-Blood-Sugar.pdf
  2. http://ndep.nih.gov/media/knownumbers_eng.pdf
  3. http://www.learningaboutdiabetes.org/downloads/HighBloodSugarEN.pdf
  4. https://diabetesatwork.org/_files/handouts/I_A01b_HO.pdf
  5. https://www.healthinfotranslations.org/pdfDocs/FastingBloodSugarTest_Som.pdf
  6. http://www.lincolnsurgery.com/media/ask-diabetes.pdf
  7. http://www.gov.ns.ca/health/Pharmacare/diabetes_assistance/DAPBloodSugarBrochure-1.pdf

Further Reading

Last Updated: Dec 3, 2012

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