TotallyADD quoted correctly from the book he referenced. However I believe the book is wrong and should read inulin, not insulin, as jgnat said. Insulin is a hormone secreted by the pancreas that controls the metabolism and cellular uptake of sugars whereas inulin is a carbohydrate found in plants that yields fructose in the presence of water. (Taber's Cyclopedic Medical Dictionary, Edition 20, pp. 1117, 1136)
Below is the Wikipedia article from which jgnat quoted:
The artichoke contains about 10% protein, no oil, and a surprising lack of starch. However, it is rich in the carbohydrate inulin (76%), which is a polymer of the monosaccharide fructose. Tubers that are stored for any length of time will digest its inulin into its component fructose. Jerusalem artichokes have an underlying sweet taste because of the fructose, which is about one and a half times sweeter than sucrose. [ 4 ]
Jerusalem artichokes have also been promoted as a healthy choice for diabetics. The reason for this being the case is because fructose is better tolerated by people that are diabetic. It has also been reported as a folk remedy for diabetes. [ 5 ] Temperature variances have been shown to affect the amount of inulin the Jerusalem artichoke can produce. When not in tropical regions, it has been shown to make less inulin than when it is in a warmer region. [ 6 ]
When all is said and done the Jerusalem artichoke is a valuable addition to the diabetic diet. Hope this helps to clarify.
Reopened Mind