The body performs best when blood glucose levels are kept relatively constant. If blood glucose levels dip too low, you feel lethargic and hungry. If your blood sugars are too high, the brain signals the pancreas to release insulin, which brings your glucose levels down by allowing glucose to enter the cells of the body and converting excess glucose to stored fat. The faster the increase in blood glucose the more likely the body will "overcorrect" by releasing too much insulin, which drives glucose too low, triggering fatigue and hunger and creating a vicious cycle of high and low blood glucose swings. The glycemic index (GI) is a ranking of carbohydrates on a scale from 0 to 100 according to their effect on blood sugar levels after eating. Foods with a high GI are those which are rapidly digested and absorbed. They result in marked fluctuations in blood sugar levels. Low-GI foods are digested and absorbed slowly. They produce gradual rises in blood sugar and insulin levels. They have proven benefits for health because of these lower fluctuations in blood sugar and insulin levels. Low GI diets have been shown to improve both glucose and lipid levels in people with diabetes (type 1 and type 2). Low-GI diets reduce insulin levels and insulin resistance. They have benefits for weight control because they help control appetite and delay hunger. The Harvard School of Public Health has demonstrated that the risks of type 2 diabetes and coronary heart disease are strongly related to the overall GI of the diet. The World Health Organization recommends that people in industrialized countries base their diet on low-GI foods to prevent obesity, diabetes, and coronary heart disease common in more affluent countries.

The GI rating is determined by feeding measured portions of food to 10 healthy, fasting subjects. Blood samples are taken every 15 to 30 minutes over a 2-hour period after eating the sample. The rise in blood glucose is compared to the rise from pure glucose. The average GI rating for the 10 subjects is published as the GI for that food.

Low-GI foods have a rating of 55 and lower, medium 56-69, and high is 70 and above.

Related to the GI is the glycemic load. Glycemic load takes into consideration the amount as well as the type of carbohydrate consumed. Glycemic load of 10 or below are low and 20 or above are high.

Using the glycemic load of a food can help explain why some lower-GI foods may have a worse impact on blood glucose levels than others that are higher-GI. For example, potato chips have a GI of 54 and watermelon a GI of 72. At first glance, potato chips would be a better choice. However, a single serving of potato chips of 4 oz. has a glycemic load of 30 while a cup of watermelon has a glycemic load of 8, indicating that the watermelon has much less of a blood glucose raising effect. While not widely distributed on package labels yet, expect glycemic index and glycemic load values to be made more readily available as evidence of the importance of these values to health and weight control mounts.

Reference: http://www.glycemicindex.com/, http://www.nutritiondata.com/topics/glycemic-index.

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