Glycemic index is a measurement of how a food influence blood glucose levels. High glycemic index foods, such as white breads and sugar, cause a rapid spike in blood sugar levels, while low glycemic index foods, such as vegetables and beans, which tend to be higher and fiber and contain more complex carbs, are metabolized more slowly and release a steadier, slower flow of glucose into the blood stream.  
   The glycemic index is a 100-point scale, with white bread at 100 points, that measures how quickly carbohydrates enter the bloodstream as sugar. People should avoid high glycemic foods such as white bread and potatoes because they will quickly raise a person’s blood-sugar level. Meanwhile, low glycemic foods are absorbed more slowly, making a person feel full longer and reducing cravings, which helps with weight loss. Moms-to-be who eat a low glycemic index diet have healthier babies.

Food           Glycemic index
Instant rice       90
Baked potato  85
Corn flakes      84
White bread    70
Banana            50
Spaghetti         41
Apple               36
Lentils              29
Peanuts           14
Broccoli      very low

Generally, foods with refined sugars and simple starches, like candy and white bread, have a high glycemic index, while those with more complex carbohydrates and greater fiber content, such as vegetables and whole grains, have a low glycemic index.

Acne may be improved by low glycemic diet
Dr. Robyn N. Smith, from the RMIT University in Melbourne, Australia, evaluated acne symptoms in 43 male patients, between 15 and 25 years, who were randomly assigned to a low glycemic load diet or a normal diet. The intervention diet consisted of 25 percent energy from protein and 45 percent from low glycemic index carbohydrates. After 3 months, the low glycemic diet was associated with a significant reduction in acne compared with the normal diet. In addition, the low glycemic diet produced greater reductions in body weight and body mass and a greater increase in insulin sensitivity. American Journal of Clinical Nutrition, July 2007.

High glycemic index diet and children
Children may eat fewer calories throughout the day if their first meal of the day has a low glycemic index. High glycemic index foods, like white bread and potatoes, lead to a quick surge in blood sugar, while low glycemic index foods, such as lentils, soybeans, protein, and many high-fiber grains, create a more gradual increase in blood sugar.

High glycemic foods and diabetes
Regular consumption of foods with a high glycemic index increases the risk of type 2 diabetes.

Heart disease
Am J Epidemiol. 2013 Nov 15. Dietary carbohydrates, refined grains, glycemic load, and risk of coronary heart disease in Chinese adults. High carbohydrate intake, mainly from refined grains, is associated with increased CHD risk in Chinese adults.

High glycemic diet and liver health
People who eat lots of high glycemic index foods for many years not only risk gaining excess weight, they also run a greater risk of developing non-alcoholic fatty liver disease, a condition that can lead to liver damage or liver failure.

Controversy
Although made popular by the South Beach Diet and others, the glycemic index has not been fully embraced by most dietitians and remains a point of debate among scientists. Now, diabetes researcher Elizabeth Mayer-Davis of the University of South Carolina says the use of the index should be ended altogether in favor of more traditional methods of losing weight and reducing the risk of diabetes — eating less and exercising more. The glycemic index is sufficiently flawed as an index that it is not helpful for scientists or people trying to create a healthy diet,” Mayer-Davis claims. Promoters of the diet also say that eating low-glycemic foods will result in less fluctuation in their blood sugar levels. Both the Atkins and South Beach diets have raised interest in the theory.

In recent years, researchers have taken to classifying carbohydrates based on their GI, or glycemic index -- a measure of the effects of a given food on blood sugar levels. High glycemic index foods, like white bread and potatoes, tend to produce a quick surge in blood sugar, and some studies have suggested that diets heavy in such foods can contribute to weight gain, diabetes and heart disease. But the science is mixed. Some studies have failed to find links between high- glycemic index foods and elevated blood sugar and diabetes. The new research suggests on reason is that it's hard to translate lab findings on glycemic index to the much more complicated realm of everyday eating, according to Dr. Elizabeth Mayer-Davis, the lead author of the new study. One problem is that a food's glycemic index is determined under artificial conditions where a person eats the test food after a fast, then has blood sugar tests taken two hours later. But a food has different blood sugar effects when it's not eaten after a fast, or when combined with other foods. Furthermore, the quantity of the food makes a significant difference, along a person's activity level after the meal. In on study published in the British Journal of Nutrition, there was no association between high-GI eating habits and elevated blood sugar among 813 adults who were followed over 5 years. The findings reinforce the notion that glycemic index is "simply not a good index of how food impacts blood sugar. The health benefits that some studies have attributed to low glycemic index foods may actually reflect other qualities of those foods -- like high fiber content. Fiber-rich foods like whole grains are often lower on the glycemic index scale. What's more, glycemic index is a complicated way to judge a food's value. Certain vegetables, for instance, have a fairly high glycemic index, but actually contain very few grams of carbohydrate and few calories. On the other hand, a dish of ice cream may have a lower glycemic index than a bowl of brown rice.
     Dr. Sahelian comments: I don't think we need to discard the whole concept of the glycemic index, but to recognize that it is one of the factors we should evaluate when considering which foods are appropriate to eat and in what quantity. Try to have a balance of protein, fat, and carbohydrate. Avoid extreme and fad diets. Frequent, small meals are preferable to large meals. Try to have more protein during the day, and switch to slightly more complex carbs in the evening. Protein helps with alertness while carbs induce sleep. Limit your intake of simple carbohydrates, choose carbohydrates with low glycemic index.

Glycemic Index predicts rise in blood sugar
Assessing a meal using the glycemic index, which lists the quality of carbohydrates contained in many common foods, appears to be a good way to predict the effect a meal will have on blood sugar levels. Dr. Thomas M. S. Wolever, of the University of Toronto and colleagues examined whether overall carbohydrate content and glycemic index of individual foods, as given in published tables, determined the effects of a realistic mixed meal on the blood sugar in normal subjects. The team measured the responses to six test meals in 16 subjects in Sydney and eight meals in 10 subjects in Toronto, and then pooled the results. The meals varied in amount of calories, protein, fat, available carbohydrates and glycemic index score. The blood sugar and insulin responses to the Sydney test meals varied over a 3-fold range. For the Toronto test meals, the blood sugar responses varied over a 2.4-fold range. The team found no correlation between the blood sugar levels and fat or protein content of the test meal, but there was a significant correlation with carbohydrate content and glycemic index alone, which accounted for 88 percent of the variation in the blood sugar response. American Journal of Clinical Nutrition, 2006.

Low Glycemic Index Diet and weight loss
A diet rich in carbohydrates with a low glycemic index appears to be more effective in reducing fat mass and lowering the chances of developing cardiovascular disease (CVD) than diets with a high glycemic index or high in protein.

Staying away from simple carbohydrates and eating plenty of fiber may help women avoid packing on pounds as they get older. Dr. Helle Hare-Bruun of Copenhagen University Hospital and colleagues found that normal-weight women who ate a diet with a relatively high glycemic index gained more weight, more fat, and more padding around the middle over a six-year period than women who ate a low glycemic index diet. Theoretically, a high glycemic index diet could make a person feel hungry faster and eat too much as a result. But studies of the effects of dietary glycemic index on weight loss have had mixed results. To see how dietary glycemic index might affect weight over time, the researchers evaluated 376 normal-weight men and women ages 35 to 65 years and followed-up with them six years later. A high glycemic index diet correlated with greater waist circumference, body weight, and percentage of body fat in women, and the effect was strongest among inactive women. But glycemic index had none of these effects on men. The researchers suggest that gender somehow affects the influence of glycemic index on weight gain. American Journal of Clinical Nutrition, October 1, 2006.

LGI diets are more effective than other diet strategies for helping overweight or obese people lose weight and improve their cholesterol levels. The Cochrane Library, 2007.

Questions
Q. What is the glycemic index of agave syrup?
   A. As of 2013 I have not come across reliable research regarding the glycemic index of agave syrup.