HOW SCIENTISTS MEASURE THE G.I. FACTOR
1. An amount of food containing 50 grams of carbohydrate is given to a volunteer to eat. For example, to test boiled spaghetti, the volunteer would be given 200 grams of spaghetti which supplies 50 grams of carbohydrate (we work this out from food composition tables)—50 grams of carbohydrate is equivalent to 3 tablespoons of pure glucose powder.
2. Over the next two hours (or three hours if the volunteer has diabetes), we take a sample of their blood every 15 minutes during the first hour and thereafter every 30 minutes. The blood sugar level of these blood samples is measured in the laboratory and recorded.
3. The blood sugar level is plotted on a graph and the area under the curve is calculated using a computer program.
4. The volunteer’s response to spaghetti (or whatever food is being tested) is compared with his or her blood sugar response to 50 grams of pure glucose (the reference food).
5. The reference food is tested on two or three separate occasions and an average value is calculated. This is done to reduce the effect of day-to-day variation in blood sugar responses.
6. The average G.I. found in 8 to 10 people is the G.I. of that food.
In total, eight to ten people need to be tested and the G.I. factor of the food is the average value of the group. We know this average figure is reproducible and that a different group of volunteers will produce a similar result. Results obtained in a group of people with diabetes are comparable to those without diabetes.
The important point to note is that all foods are tested in equivalent carbohydrate amounts. For example, 100 grams of bread (about 3K slices of sandwich bread) is tested because this contains 50 grams of carbohydrate. Likewise, 60 grams of jelly beans (containing 50 grams of carbohydrate) is compared with the reference food. We know how much carbohydrate is in a food by consulting food composition tables, the manufacturer’s data or measuring it ourselves in the laboratory.
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