How much sugar is in your blood?

Where the figure comes from

The four-gram figure is the title and the opening sentence of a 2009 review by David Wasserman, a physiologist at Vanderbilt University, published in the American Journal of Physiology: Endocrinology and Metabolism. The review begins: "Four grams of glucose circulates in the blood of a person weighing 70 kg." The rest of the paper describes the control system that keeps that amount steady.

The figure can be checked with two numbers that appear in any physiology textbook:

Typical fasting blood glucose: 5 mmol/L, or 90 mg/dL
Glucose molar mass: 180g per mole, so 5 mmol/L = 0.9g per litre
Blood volume of a 70kg adult: about 5 litres
0.9g × 5 litres = 4.5g

Blood volume, body weight and the glucose reading all vary between people and through the day, so the result is an approximation. Estimates land between roughly 4g and 5g for an adult with a normal fasting level. A 4g teaspoon of sugar is the unit the calculator uses, which is why this site describes the bloodstream figure as about one teaspoon.

The same arithmetic at other levels

Blood glucose readings are given in millimoles per litre in the UK and most of the world, and in milligrams per decilitre in the United States. The table converts common reference points into grams of glucose in 5 litres of blood, using the same method as above.

To move between the two units, multiply a reading in mmol/L by 18 to get mg/dL, or divide a reading in mg/dL by 18 to get mmol/L. The factor comes from the molar mass of glucose, 180g per mole, divided by 10 to account for decilitres.

For reference, the American Diabetes Association uses a fasting level below 100 mg/dL (5.6 mmol/L) as normal, 100 to 125 mg/dL as prediabetes, and 126 mg/dL (7.0 mmol/L) or above on repeated tests as one criterion for diabetes. On an oral glucose tolerance test, 140 mg/dL (7.8 mmol/L) and 200 mg/dL (11.1 mmol/L) at two hours are the upper thresholds for normal and prediabetes. Across that whole range, the difference is a few grams of glucose in the entire bloodstream.

What happens when more glucose arrives

Starch and sugars from a meal are broken down in the gut into glucose and other simple sugars, which are absorbed through the wall of the small intestine. Blood from the intestine goes first to the liver through the portal vein, and the liver takes up part of the glucose before the rest reaches general circulation.

As blood glucose begins to rise, beta cells in the pancreas release insulin. Insulin does several things at once. It signals muscle and fat cells to take glucose in. It tells the liver to stop releasing its own glucose and to store incoming glucose as glycogen, a compact chain of glucose molecules. Muscles store glycogen too.

Glycogen storage is limited. Textbook estimates for an adult are roughly 100g in the liver and around 400g in skeletal muscle, varying with body size, activity and recent meals. When those stores are full, the liver can convert surplus glucose into fat, a process called de novo lipogenesis.

Some glucose is used straight away. The brain alone uses in the region of 120g of glucose a day in an adult at rest, and red blood cells rely on glucose for all their energy.

When blood glucose falls between meals, the pancreas releases glucagon, which signals the liver to break glycogen back down into glucose and to make new glucose from other sources. Between insulin and glucagon, the amount in the blood stays within a narrow band for most people most of the time.

If blood glucose rises above roughly 10 mmol/L (180 mg/dL), the kidneys can no longer reabsorb all of it, and glucose begins to appear in urine. That threshold varies between individuals.

Reading the two figures together

The bloodstream figure is a quantity: the glucose present in the blood at one moment. The teaspoon figures in the calculator are glycaemic load equivalents: they compare the blood glucose response to a food, measured over about two hours, with the response to a teaspoon of table sugar. A 10-teaspoon portion does not put 40g of glucose into the blood all at once, because absorption, liver uptake and insulin all operate while digestion is under way.

The calculator places both figures on the same line, one under the running total. The page what is glycaemic load explains how the food figures are calculated.

A note on measuring your own glucose

This page describes typical physiology and published reference ranges. It cannot tell you anything about your own blood glucose. Readings, targets and what they mean for an individual are questions for a doctor, nurse or diabetes educator, particularly for anyone taking medication that lowers blood glucose.