The Glycaemic Index (GI) is a ranking of carbohydrate-based foods on a scale from zero (0) to one hundred (100), according to the extent by which they raise blood sugar levels after eating. High Glycaemic index foods when consumed are quickly digested and absorbed, causing increases in blood sugar levels which leads quickly to higher insulin levels. Low Glycaemic Index foods when consumed, bring about slower changes in blood sugar levels.
The reference food against which the GI is measured and ranked is glucose. Glucose is a simple carbohydrate, a monosaccharide. The other two monosaccharides are fructose and galactose. It is reasonable to expect that glucose would be the GI “reference food” rated 100. The liver is the first destination of the three monosaccharides, glucose however, is not metabolised in the liver as are fructose and galactose, which are ultimately converted to glucose and stored as glycogen, primarily in the liver and muscles; the glucose quickly passes through and is distributed to other parts of the body. Because of this, glucose is the simple carbohydrate to have the quickest impact on blood sugar levels.
The Glycaemic Load (GL) is a verry useful calculation which considers not only the carb. GI values but more importantly, the carb. quantity consumed. Watermelon for example, has a GI of about 80 but it is unlikely, that a whole watermelon will be consumed at one sitting. They both however, help determine how blood sugar and in turn insulin will react after eating.