Glycemic index is a measure of how foods affect human blood glucose (sugar) concentration over a short time (several hours). Specifically, GI reflects how rapidly and how long human blood glucose levels rise in response to eating a measured amount of a food on an empty stomach. Foods that cause a rapid rise in blood sugar are "HIGH glycemic index" food whereas foods that cause a slow rise in blood sugar are "LOW glycemic index" foods.
Glycemic index was developed in Australia in the early 1980s as a tool to help diabetic people control their blood sugar. Before that time, the standard advice to most diabetics was simply "avoid sugar". The problem is that starches are made of sugar and some of them like potato were known to badly raise blood sugar in diabetics. Glycemic index was developed to specifically measure how much different starchy foods affected blood sugar. Since that time, glycemic index has become an extremely important tool for helping diabetic people.
The biggest problem to glycemic index is that it cannot be predicted. It has to actually be measured in human volunteers. The way it is generally done is with ten people who arrive at a testing lab in the morning without eating. Each person has a baseline blood sugar level drawn and then each person consumes EXACTLY ENOUGH OF THE FOOD UNDER STUDY TO GET 50 GRAMS OF ABSORBABLE CARBOHYDRATE. The food must be consumed in ten minutes. Blood sugar readings are then checked often over the next several hours. When the study has ended, the blood sugar readings are compiled into a graph that looks a bit like the one shown below. The key to the graph and to assigning a "glycemic index" numbers is the "area under the curve", how "fast" the curve rises and how high. There is a mathematical formula applied and a number is generated. It is a very expensive and tedious process.
Figure 10: Effect of high and low glycemic index foods upon normal (non-diabetic) fasting human blood glucose concentration over time.
Notice several things.
Normally the body reacts to increasing blood sugar by increasing blood insulin level which then causes blood sugar to fall (insulin returns blood sugar to normal in healthy people). High glycemic foods cause the body to release more insulin than do low glycemic foods.
Insulin is the hormone, made in an organ called the pancreas that tells the cells in our muscle and other places to absorb blood glucose. Insulin is secreted into the blood by the pancreas (in non-diabetic people) in response to increases in blood sugar. Insulin levels fall when blood sugar returns to normal.
Resistance to insulin is one of the primary features of type 2 diabetes and one theory for how insulin resistance occurs is the theory of "excessive production". Essentially the idea is that exposing the body to too much insulin over and over again (by eating high glycemic foods) causes "burnout" so that the body becomes resistant to insulin. The molecular mechanism that could explain this is called "feedback inhibition of the insulin receptor". Too much insulin causes insulin receptors to decline leading to resistance. That is the theory anyway.
In the graph above, I have added curves representing blood insulin levels associated with the blood sugar curves in the earlier graph. Let's focus upon the purple insulin curve which belongs to the red "high GI" curve. Notice that insulin levels rise in response to the rapid rise in blood sugar but also notice that the response takes time and is not instantaneous. This is because insulin synthesis and secretion takes time. The effect of this delay is that a bit too much insulin in produced in response to a very rapid increase in blood sugar so that blood sugar not only falls rapidly, but COULD fall too low before finally recovering to a normal level.
The best thing I can say about glycemic index is that it really has helped people with diabetes understand food choices. THAT is good.
There are two problems with trying to use glycemic index for weight loss. The first problem is that it's far too complicated and the second problem is that it doesn't work. I don't mean to sound too harsh but in my experience, patients who have attempted to follow low glycemic diets for weight loss become confused and frustrated.