Nutritive (calorie containing) low-glycemic sweeteners are products like "Agave Syrup" that are made from "fructans" which are plant fibers composed of long chains of fructose molecules (starch on the other hand is made from chains of glucose). Agave root for example is rich in fructans and if the root is boiled for a long time in a pressure cooker, the fructans are broken down into individual molecules of fructose in the form of a thick syrup. This is a product that is currently being sold as a "low" glycemic sweetener.
Fructose is a sugar that humans can digest and metabolize in small doses such as the amounts found in whole fruit. Unfortunately, higher doses of fructose and especially large amounts of pure fructose absent dietary fiber is another matter. Large amounts of fructose act as a kind of metabolic poison and there is strong scientific evidence that over time, large amounts of fructose will cause leptin resistance. Since "low-glycemic" sweeteners like agave syrup are nearly pure fructose, they are especially bad and are not a suitable alternative to sugar.
This is why fructose is a "low-GI" sugar. It can't cause blood glucose levels to rise rapidly because it is not glucose. Now if one were to measure blood fructose levels after a person consumes a lot of fructose, he would see that blood fructose rise very rapidly just as blood glucose levels rise after a meal of pure starch or sugar. So fructose is low glycemic for a very technical reason: namely that fructose and glucose are different chemicals and glycemic index measures only glucose levels.
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