Glycemic Index (GI) is a measure of how much a food raises blood glucose (blood sugar). A food with a higher Glycemic Index raises the blood sugar more than a food with a lower Glycemic Index. The Glycemic Index value is based on a comparison with a reference food, normally either white bread or glucose.
[[File:Glycemic Index Fellrnr.png|none|thumb|500px|A graph of blood glucose (sugar) for high and low glycemic index foods.]]
=Calculating the Glycemic Index=
The Glycemic Index is calculated by measuring blood glucose periodically after the consumption of the food. Typically the measurements are taken just before consuming the food, then at 15, 30, 45, 60, 90, and 120 minutes<ref name="Brand-MillerStockmann2008"/>. These measurements provide a blood glucose curve, and the area between the curve and the baseline measurement taken just before consuming the food is the "incremental Area Under the Curve" (iUAC). The iUAC is compared with the iUAC for a reference food and the value given as a percentage. So a Glycemic Index of 50 means that the food raises the blood sugar by 50% of the reference food. Generally the portion of test food consumed contains 50 grams of carbohydrate. (The iAUC is sometimes called the Post Prandial Glycemic Response or PPGC.)
=Glycemic Index and Health=
High blood glucose levels are linked to many health problems.
* IGT is also linked with metabolic syndrome, which is the cluster of health problems including obesity, high blood pressure, non-alcoholic fatty liver disease, elevated triglycerides, and cardiovascular disease<ref name="Grundy-2012"/>. (Note that while glucose lowering drugs may help prevent the conversion of IGT to Diabetes, but it's unclear if they will help prevent some of the health complications of diabetes<ref name="Grundy-2012"/>.)
* IGT is a risk factor for liver cirrhosis<ref name="Nishida-2006"/> and survival rates for those with liver cirrhosis<ref name="García-Compeán2014"/>.
=Simple and Complex Carbohydrates=
At one time, it was believed that "simple carbohydrates" had high Glycemic Index, while "complex carbohydrates" had lower Glycemic Indexes<ref name="ND"/>. The difference between simple and complex carbohydrates is based on the chemistry of the carbohydrate molecule, with small molecules like sugar considered "simple" and big molecules like bread considered "complex". This division into simple and complex is unfortunately crap (biochemistry term meaning 'not useful'). The digestion of carbs is a sophisticated system that does not follow this simple division. Some simple carbs (Fructose) are very slow to digest, whereas some complex carbs (maltodextrin) are very easy to digest. For instance, white bread (a "complex" carb, GI 70) has a higher Glycemic Index than table sugar (a 'simple' carb, GI 60). This is because highly refined flour in bread is more easily digested than table sugar (which is half fructose).
<ref name="Brand-MillerStockmann2008">J. C Brand-Miller, K. Stockmann, F. Atkinson, P. Petocz, G. Denyer, Glycemic index, postprandial glycemia, and the shape of the curve in healthy subjects: analysis of a database of more than 1000 foods, American Journal of Clinical Nutrition, volume 89, issue 1, 2008, pages 97–105, ISSN [http://www.worldcat.org/issn/0002-9165 0002-9165], doi [http://dx.doi.org/10.3945/ajcn.2008.26354 10.3945/ajcn.2008.26354]</ref>
<ref name="ND">http://nutritiondata.self.com/topics/glycemic-index, http://nutritiondata.self.com/topics/glycemic-index, Accessed on 13 February 2016</ref>
<ref name="Ludwig-1999">DS. Ludwig, JA. Majzoub, A. Al-Zahrani, GE. Dallal, I. Blanco, SB. Roberts, High glycemic index foods, overeating, and obesity., Pediatrics, volume 103, issue 3, pages E26, Mar 1999, PMID [http://www.ncbi.nlm.nih.gov/pubmed/10049982 10049982]</ref>
<ref name="MastersReither2013">Ryan K. Masters, Eric N. Reither, Daniel A. Powers, Y. Claire Yang, Andrew E. Burger, Bruce G. Link, The Impact of Obesity on US Mortality Levels: The Importance of Age and Cohort Factors in Population Estimates, American Journal of Public Health, volume 103, issue 10, 2013, pages 1895–1901, ISSN [http://www.worldcat.org/issn/0090-0036 0090-0036], doi [http://dx.doi.org/10.2105/AJPH.2013.301379 10.2105/AJPH.2013.301379]</ref>