The Glycaemic Index (GI) of a food is a classification of its ability to raise blood glucose after consumption. Only carbohydrates, which metabolise to glucose, have a GI classification. Pure glucose is the benchmark and has a GI of 100.
Low GI foods are recommended because they result in a lower glycaemic response. A low glycaemic response is characterised by slow glucose absorption, a moderate rise in blood glucose and a steady recovery to normal levels. This is desirable because it is less demanding on insulin release from the pancreas. High GI foods elicit a high glycaemic response which is characterised by a rapid increase in blood glucose, a responding flood of insulin to “mop up” the glucose and a reactive drop in blood glucose below normal levels. This may be experienced as an energy slump, shakiness, lack of concentration, lethargy and rebound sugar or carbohydrate cravings. If this pattern is repeated frequently over time, the pancreas can become exhausted, leading to insulin resistance or impaired insulin release and the onset of type II diabetes.
However, many factors alter the glycaemic response to carbohydrates. It is often suggested that Glycaemic Load (GL) is a more accurate measure of potential glycaemic response as it encompasses the amount of carbohydrate in a food (GL=GI x amount of carbohydrate/100). The combined effect of other foods eaten at the same time as carbohydrates will also affect the absorption rate of glucose. Soluble fibre, for example, swells in the stomach and slows gastric emptying, as do acids such as lemon juice and vinegar. Dietary fibre from fruit and vegetables, complex carbohydrates (whole grains) and legumes, slows digestion time, delaying the release of glucose into the blood.
Resistant starch is also an interesting case. When starch is cooked, its molecular structure is broken down, making it easier to digest (think raw potato vs baked). However, when that cooked starch is cooled, it reforms into a different matrix, becoming resistant starch which escapes digestion. Instead, it passes into the large intestine and becomes food for bacteria to generate beneficial short-chain fatty acids. So, although a baked potato has a high GI, potato salad has a much lower GI. Adding lemon juice or vinegar in a dressing will lower the overall GL further.
A healthy diet emphasises wholefoods which are generally low GI. Sugars and sweeteners vary in GI. Fructose is quite low (around 22) which makes raw honey (about 30) a low GI alternative to sucrose (usually cane or beet sugar – between 60 and 80). Agave syrup is better at 15 and stevia is the lowest with a GI of www.glycemicindex.com. Keep in mind that a food must be clinically tested by volunteers over a 3 hour timeframe to be given a GI classification so many foods do not appear on the database.