THE G.I. FACTOR: THE EFFECT OF SUGAR ON THE G.I. FACTOR
Table sugar or refined sugar (sucrose) has a G.I. factor of only 60-65. This is because it is a disaccharide (double sugar) composed of one glucose molecule coupled to one fructose molecule. Fructose is absorbed and taken directly to the liver where much of it is slowly converted to glucose. So, the blood sugar response to pure fructose is very small (G.I. factor of 20). Thus when we consume sucrose, in effect we have consumed only half as much glucose. This explains why the blood sugar response to 50 grams of sucrose is approximately half that of 50 grams of pure maltose (where the molecules are all glucose).
Many foods containing large amounts of refined sugar have G.I. factors close to 60. This is the average of glucose (G.I. = 100) and fructose ( G.I. = 20). This is lower than that of ordinary soft bread with a G.I. factor averaging around 70. Kellogg’s Cocopops™ which contains 39 per cent sugar has a G.I. factor of 77, lower than that of Rice Bubbles™ (83) which contains little sugar.
So, contrary to popular opinion, most foods containing simple sugars do not raise blood sugar values any more than that of most complex starchy foods like bread. The same is true of honey (G.I. factor of 58). Some types of honey have a much higher G.I. factor (87) than refined sugar (65), possibly because they are a mixture of honey and glucose syrup.
Sugars that naturally occur in food include lactose, sucrose, glucose and fructose in variable proportions, depending on the food. The overall blood sugar response to a food is very hard to predict on theoretical grounds because gastric emptying is slowed by increasing concentration of the sugars, whatever their structure.
Some fruits for example have a low G.L factor (cherries have a G.I. factor of only 22) while others are relatively high (watermelon has a factor of 72). It seems the higher the acidity and osmotic strength (number of molecules per ml) of the fruit, the lower the G.I. factor. Thus it is not possible to lump all fruits together and say that they will have a low G.I. factor because they are high in fibre. They are not all equal. See the tables in Part HI to compare fruits.
Many foods containing sugars are a mixture of refined and naturally occurring sugars. The overall effect on the blood sugar response is too hard to predict. This is why we need to test the G.I. value of sugary foods in real people before we make generalisations about their G.I. factor.
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