By Dr Ananya Mandal, MD
Blood sugar fluctuations
Blood sugar levels are tightly regulated by a variety of stimulations and mechanisms. This is important for metabolic homeostasis. Levels may fluctuate after fasting for long periods of time or an hour or two after consumption of food. Despite this, the fluctuations are minor. Normal human blood glucose levels remains within a remarkably narrow range.
In most humans this varies from about 82 mg/dl to 110 mg/dl (4.4 to 6.1 mmol/l). The blood sugar levels rises to nearly 140 mg/dl (7.8 mmol/l) or a bit more in normal humans after a full meal. 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).
In other words in a healthy adult male of 75 kg (165 lb) with a blood volume of 5 litres (1.3 gal), a blood glucose level of 100 mg/dl or 5.5 mmol/l means a total of about 5 g (0.2 oz or 0.002 gal, 1/500 of the total) of glucose in the blood.
This also means approximately 45 g (1½ ounces) in the total body water. Total body water includes more than merely blood and will be usually about 60% of the total body weight in men. 5 grams of glucose is about equivalent to a small sugar packet or a teaspoon full of sugar.
To be considered a non-diabetic the American Diabetes Association recommends a post-meal glucose level less than 180 mg/dl (10 mmol/l) and a pre-meal blood glucose level of 90-130 mg/dl (5 to 7.2 mmol/l).
Molarity and mass concentration
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.
Reviewed by April Cashin-Garbutt, BA Hons (Cantab)