What is Glycemic Index?
The glycemic index or glycaemic index (GI) is a measure of how quickly blood glucose levels (i.e., blood sugar) rise after eating a particular type of food. Glucose (the defining standard) has a glycemic index of 100. The effects that different foods have on blood glucose levels vary considerably. The glycemic index estimates how much each gram of available carbohydrate (total carbohydrate minus fiber) in a food raises a person's blood glucose level following consumption of the food, relative to consumption of pure glucose.
Pure glucose is given a GI of 100, and all other carbohydrates are compared with this. A food with a GI of under 55 has a low GI. Food low in GI tends to keep us feeling fuller for longer, helping to keep weight down. It also contributes less than high-GI food to heart disease and helps keep diabetes under control. GI labeling is widely used in Australia, and is being seen increasingly in other countries too. The more processed a food is, the higher its GI tends to be, because, in effect, the processing partly digests the carbohydrate, making it easy for the body to finish the job and convert it into blood sugar. Breakfast cereals like cornflakes, for instance, have a high GI. In the same way, cooking can increase the GI of food, which is why long-baked potatoes have a higher GI than just boiled new potatoes. Fibre, however, tends to lower GI because it slows digestion down.
GI labeling can be misleading, though, to how fattening a food is – because it takes no account of just how rich in calories a food is. Crisps, for instance, have a relatively low GI of around 54, but the fat used to fry them is so rich in calories that they are actually a fattening food.
Weight Control
Recent animal research provides compelling evidence that high-GI carbohydrate is associated with increased risk of obesity. In one study, male rats were split into high- and low-GI groups over 18 weeks while mean body weight was maintained. Rats fed the high-GI diet were 71% fatter and had 8% less lean body mass than the low-GI group. Postmeal glycemia and insulin levels were significantly higher, and plasma triglycerides were threefold greater in the high-GI-fed rats. Furthermore, pancreatic islet cells suffered "severely disorganized architecture and extensive fibrosis." However, the GI of these diets was not experimentally determined. Because high-amylose cornstarch (the major component of the assumed low-GI diet) contains large amounts of resistant starch, which is not digested and absorbed as glucose, the lower glycemic response and possibly the beneficial effects can be attributed to lower energy density and fermentation products of the resistant starch, rather than the GI.
In humans, a 2012 study shows that, after weight loss, the energy expenditure is higher on a low-glycemic index diet than on a low-fat diet (but lower than on the Atkins diet).
Several lines of recent [1999] scientific evidence have shown that individuals who followed a low-GI diet over many years were at a significantly lower risk for developing both type 2 diabetes, coronary heart disease, and age-related macular degeneration than others. High blood glucose levels or repeated glycemic "spikes" following a meal may promote these diseases by increasing systemic glycative stress, other oxidative stress to the vasculature, and also by the direct increase in insulin levels. The glycative stress sets up a vicious cycle of systemic protein glycation, compromised protein editing capacity involving the ubiquitin proteolytic pathway and autophagic pathways, leading to enhanced accumulation of glycated and other obsolete proteins.
Determining the Glycemic Index of a Food
Foods with carbohydrates that break down quickly during digestion and release glucose rapidly into the bloodstream tend to have a high GI; foods with carbohydrates that break down more slowly, releasing glucose more gradually into the bloodstream, tend to have a low GI. The concept was developed by Dr. David J. Jenkins and colleagues in 1980–1981 at the University of Toronto in their research to find out which foods were best for people with diabetes. A lower glycemic index suggests slower rates of digestion and absorption of the foods' carbohydrates and may also indicate greater extraction from the liver and periphery of the products of carbohydrate digestion. A lower glycemic response usually equates to a lower insulin demand but not always, and may improve long-term blood glucose control and blood lipids. The insulin index is also useful for providing a direct measure of the insulin response to a food.
The glycemic index of a food is defined as the incremental area under the two-hour blood glucose response curve (AUC) following a 12-hour fast and ingestion of a food with a certain quantity of available carbohydrate (usually 50 g). The AUC of the test food is divided by the AUC of the standard (either glucose or white bread, giving two different definitions) and multiplied by 100. The average GI value is calculated from data collected in 10 human subjects. Both the standard and test food must contain an equal amount of available carbohydrate. The result gives a relative ranking for each tested food.
The current validated methods use glucose as the reference food, giving it a glycemic index value of 100 by definition. This has the advantages of being universal and producing maximum GI values of approximately 100. White bread can also be used as a reference food, giving a different set of GI values (if white bread = 100, then glucose ˜ 140). For people whose staple carbohydrate source is white bread, this has the advantage of conveying directly whether replacement of the dietary staple with a different food would result in faster or slower blood glucose response. A disadvantage with this system is that the reference food is not well-defined.
Glycemic Index of Foods
GI values can be interpreted intuitively as percentages on an absolute scale and are commonly interpreted as follows:
Low GI 55 or Less
beans (white, black, pink, kidney, lentil, soy, almond, peanut, walnut, chickpea); small seeds (sunflower, flax, pumpkin, poppy, sesame); most whole intact grains (durum/spelt/kamut wheat, millet, oat, rye, rice, barley); most vegetables, most sweet fruits (peaches, strawberries, mangos); tagatose; fructose
Medium GI 56–69
not intact whole wheat or enriched wheat, pita bread, basmati rice, unpeeled boiled potato, grape juice, raisins, prunes, pumpernickel bread, cranberry juice, regular ice cream, sucrose, banana
High GI 70 and Above
white bread (only wheat endosperm), most white rice (only rice endosperm), corn flakes, extruded breakfast cereals, glucose, maltose, maltodextrins, potato, pretzels, parsnip, bagels
The glycemic effect of foods depends on a number of factors, such as the type of starch (amylose versus amylopectin), physical entrapment of the starch molecules within the food, fat and protein content of the food and organic acids or their salts in the meal — adding vinegar, for example, will lower the GI. The presence of fat or soluble dietary fiber can slow the gastric emptying rate, thus lowering the GI. In general, coarse, grainy breads with higher amounts of fiber have a lower GI value than white breads. However, most breads made with 100% whole wheat or whole meal flour have a GI not very different than endosperm only (white) bread. Many brown breads are treated with enzymes to soften the crust, which makes the starch more accessible (high GI).
While adding fat or protein will lower the glycemic response to a meal, the relative differences remain. That is, with or without additions, there is still a higher blood glucose curve after a high-GI bread than after a low-GI bread such as pumpernickel.
Fruits and vegetables tend to have a low glycemic index. The glycemic index can be applied only to foods where the test relies on subjects consuming an amount of food containing 50 g of available carbohydrate. But many fruits and vegetables (not potatoes, sweet potatoes, corn) contain less than 50 g of available carbohydrate per typical serving. Carrots were originally and incorrectly reported as having a high GI. Alcoholic beverages have been reported to have low GI values; however, beer was initially reported to have a moderate GI due to the presence of maltose. This has been refuted by brewing industry professionals, who say that all maltose sugar is consumed in the brewing process and that packaged beer has little to no maltose present. Recent studies have shown that the consumption of an alcoholic drink prior to a meal reduces the GI of the meal by approximately 15%. Moderate alcohol consumption more than 12 hours prior to a test does not affect the GI.
Many modern diets rely on the glycemic index, including the South Beach Diet, Transitions by Market America and NutriSystem Nourish Diet. However, others have pointed out that foods generally considered to be unhealthy can have a low glycemic index, for instance, chocolate cake (GI 38), ice cream (37), or pure fructose (19), whereas foods like potatoes and rice have GIs around 100 but are commonly eaten in some countries with low rates of diabetes.
The GI Symbol Program is an independent worldwide GI certification program that helps consumers identify low-GI foods and drinks. The symbol is only on foods or beverages that have had their GI values tested according to standard and meet the GI Foundation's certification criteria as a healthy choice within their food group, so they are also lower in kilojoules, fat and/or salt.
References:
Armstrong, Alexander. The Glycemic Index Diet Simplified: A Concise and Easy to Read Guide on the G.I Glycemic Index Diet. Amazon Digital Services, Inc. ASIN: B007Z37LF2
Brand-Miller, Jennie; Foster-Powell, Kaye. 2005. The Low GI Diet Revolution: The Definitive Science-Based Weight Loss Plan. Marlowe & Company. ISBN 978-1-56924-413-5.
DietGrail Publisher. 2011. Glycemic Index and Glycemic Load of Foods. CreateSpace Independent Publishing Platform. ISBN-10: 1463799713
Freeman, Janine. 2005. The Glycemic Index debate: Does the type of carbohydrate really matter?. Diabetes Forecast.
Glycemic Index and Diabetes. American Diabetes Association
Glycemic Index Defined. Glycemic Research Institute.
Miller, Jennie M.D., Kaye Foster-Powell B.SC. M. Nutri. & Diet. 2013. The Low GI Shopper's Guide to GI Values 2014: The Authoritative Source of Glycemic Index Values for More than 1,200 Foods (New Glucose Revolutions). Da Capo Lifelong Books. ISBN-10: 073821714X
Reffetto, Meri. 2014. Glycemic Index Diet For Dummies. For Dummies; 2 edition. ISBN-10: 1118790561
Thompson, Rob. 2006. The Glycemic-Load Diet: A powerful new program for losing weight and reversing insulin resistance. McGraw-Hill; 1 edition. ISBN-10: 0071462694
Shari, Lieberman. 2006. Glycemic Index Food Guide: For Weight Loss, Cardiovascular Health, Diabetic Management, and Maximum Energy (Transitions Lifestyle Systems). Square One Publishers. ISBN-10: 0757002455
Warner, Joan M.S. R.D., and Lucy Beale. 2010. The Complete Idiot's Guide to Glycemic Index Weight Loss, 2nd Edition. ALPHA; 2 edition. ISBN-10: 1592578551
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