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The glycemic index (GI) is a ranking of carbohydrates on a scale
from 0 to 100 according to the extent to which they raise blood
sugar levels after eating. Foods with a high GI are those which
are rapidly digested and absorbed and result in marked
fluctuations in blood sugar levels. Low-GI foods, by virtue of
their slow digestion and absorption, produce gradual rises in
blood sugar and insulin levels, and have proven benefits for
health. Low GI diets have been shown to improve both glucose and
lipid levels in people with diabetes (type 1 and type 2). They
have benefits for weight control because they help control
appetite and delay hunger. Low GI diets also reduce insulin levels
and insulin resistance.
Recent
studies from Harvard School of Public Health indicate that the
risks of diseases such as type 2 diabetes and coronary heart
disease are strongly related to the GI of the overall diet. In
1999, the World Health Organisation (WHO) and Food and Agriculture
Organisation (FAO) recommended that people in industrialised
countries base their diets on low-GI foods in order to prevent the
most common diseases of affluence, such as coronary heart disease,
diabetes and obesity.
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American
Diabetes Association 2006
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Practical
Use of the GI
Johanna Burani, MS, RD, CDE
(2.2 MB)
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To
determine a food's GI rating, measured portions of the food
containing 10 - 50 grams of carbohydrate are fed to 10 healthy
people after an overnight fast. Finger-prick blood samples are
taken at 15-30 minute intervals over the next two hours. These
blood samples are used to construct a blood sugar response curve
for the two hour period. The area under the curve (AUC) is
calculated to reflect the total rise in blood glucose levels after
eating the test food. The GI rating (%) is calculated by dividing
the AUC for the test food by the AUC for the reference food (same
amount of glucose) and multiplying by 100 (chart below). The use
of a standard food is essential for reducing the confounding
influence of differences in the physical characteristics of the
subjects. The average of the GI ratings from all ten subjects is
published as the GI of that food.
The
GI of foods has important implications for the food industry. Some
foods on the Australian market already show their GI rating on the
nutrition information panel. Terms such as complex carbohydrates
and sugars, which commonly appear on food labels, are now
recognised as having little nutritional or physiological
significance. The WHO/FAO recommend that these terms be removed
and replaced with the total carbohydrate content of the food and
its GI value. However, the GI rating of a food must be tested
physiologically and only a few centres around the world currently
provide a legitimate testing service. The Human Nutrition Unit at
the University of Sydney has been at the forefront of glycemic
index research for over two decades and has tested hundreds of
foods as an integral part of its program. Jennie Brand Miller is
the senior author of International Tables of Glycemic Index
published by the American Journal of Clinical Nutrition in 1995
and 2002.
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