Understanding Glycemic Index

What is Glycemic Index ? Glycemic Index (GI) is a measurement carried out on carbohydrate-containing foods and their impact on our blood sugar. The glycemic index or glycaemic index is a number associated with a particular type of food that indicates the food’s effect on a person’s blood glucose. GI is a relatively new way of analysing foods. Previously, most meal plans designed to improve blood sugar analysed the total amount of carbohydrates (including sugars and starches) in the foods themselves.

GI goes beyond this approach, looking at the impact of foods on our actual blood sugar. In other words, instead of counting the total amount of carbohydrates in foods in their unconsumed state, GI measures the actual impact of these foods on our blood sugar.

How is GI measured?

Food is consumed in whatever serving size will provide 50 grams (about 1.8 ounces) of available carbohydrates. Available carbohydrates, are carbohydrates that get readily digested, absorbed, and metabolised by our body. These carbohydrates have a much greater impact on our blood sugar level than carbohydrates in general because carbohydrates in general include substances that aren’t readily digested, absorbed, and metabolised. Insoluble fibres, for example, are carbohydrates that do not have an immediate impact on our blood sugar level because they cannot be readily digested. As a very general way of estimating available carbohydrates in a serving of food, researchers take the total amount of carbohydrates and subtract out the total amount of fibre. Available carbohydrates are what’s left.

After 50 grams of available carbohydrates have been consumed, blood sugar levels are measured over a period of 2 hours. The results are plotted on a graph and summarised in what is called glucose AUC, or “area under the curve.” Glucose AUC shows the immediate impact of the food on our blood sugar.

Accuracy of GI

Glycemic index charts often give only one value per food, but variations are possible due to variety, ripeness (riper fruits contain more sugars increasing GI), cooking methods (the more cooked, or over cooked, a food the more its cellular structure is broken with a tendency for it to digest quickly and raise GI more), processing (e.g., flour has a higher GI than the whole grain from which it is ground as grinding breaks the grain’s protective layers) and the length of storage. Potatoes are a notable example, ranging from moderate to very high GI even within the same variety.

The glycemic response is different from one person to another, and also in the same person from day to day, depending on blood glucose levels, insulin resistance, and other factors.

Most of the values on the glycemic index do not show the impact on glucose levels after two hours. Some people with diabetes may have elevated levels after four hours.

Interesting thing about GI values

With respect to their GI, foods are also differently impacted by cooking. One of the most interesting aspects of GI involves its relationship to the unique features of carbohydrates. Carbohydrates are definitely not the same with respect to their immediate impact on our blood sugar. For example, non-whole grain breads and pasta noodles both contain similar amounts of starch, and their starches are similarly composed of long chains of the simple sugar, glucose. But the 3-dimensional structure of bread allows more of the starch to be exposed to enzymes in our saliva and in our digestive tract. This greater exposure to enzymes allows more of the starch to be broken down into sugar and gives non-whole grain breads a generally higher GI value than non-whole grain pastas. Similarly, two basic types of starch found in many foods – amylose (Amylose is a helical polymer made of α-D-glucose units) and amylopectin ( is a soluble polysaccharide and highly branched polymer of glucose found in plants) – also influence their GI values, even if the foods have identical amounts of total starch.

Glycemic Index of Various Sweeteners vis – a – vis Stevia;

References

For Established Glycemic Index values, We Used the Following Databases and Publications Atkinson FS, Foster-Powell K, Brand-Miller JC. International Tables of Glycemic Index and Glycemic Load Values: 2008. Diabetes Care 2008; 31(12).

Foster-Powell K, Holt HA, and Brand-Miller JC. International table of glycemic index and glycemic load values: 2002. Am J Clin Nutr2002;76:5—56.

Human Nutrition Unit, School of Molecular Biosciences, University of Sydney, Sydney, Australia. (2013). GI Foods Advanced Search Database. Online at http://www.glycemicindex.com/foodSearch.php.

http://www.wikipedia.com

National Cancer Institute (NCI). DHQ Nutrient Database. Applied Research: Cancer Control and Populations Sciences. National Institutes of Health, Bethesda, MD. Available online at: http://appliedresearch.cancer.gov/DHQ/database/http://www.wikipedia.com