The Glycaemic Index is a ranking system for carbohydrates based on their effect on blood glucose levels in the first two hours after consumption. It compares carbohydrates gram for gram and provides a numerical index of postprandial (post-meal) glycaemia. Carbohydrates that break down rapidly during digestion rank highest on the glycaemic index whilst carbohydrates that break down slowly (thus releasing glucose gradually into the blood stream) have low glycaemic index rating. A low GI food will release glucose slowly and steadily. A high GI food will provide a rapid rise in blood sugar levels and is suitable for energy recovery after heavy exercise or for a diabetic experiencing hypoglycaemia.
A healthy adult blood glucose level between meals is around 60-110mg of glucose per 100ml. When blood glucose rises above this, after eating for example, the hormone insulin stimulates the body’s cells to take in and store it as glycogen in the muscles and liver, to lower blood sugar level. If glucose levels in the blood start to drop below the healthy level after several hours without eating, the hormone glucagon stimulates the conversion of glycogen in the liver back into glucose.
Studies have shown that individuals who followed a low GI diet over many years were at significantly lower risk for developing both type 2 diabetes and coronary heart disease than others. The ratings system uses glucose as a reference food, giving it a GI value of 100 by definition. Sometimes, white bread is used as a reference food, which will give a different set of GI values, which is important to be aware of. If white bread is used as the reference than it has a GI value of 100 whilst glucose is 140.