Let us begin with what diabetes is and what are the factors that harm people who have diabetes.
Diabetes manifests itself when insulin produced in the body cannot adequately metabolize the sugar in the blood, causing blood sugar levels to spike. Insulin plays a blood sugar regulatory role. It prevents blood sugar levels from either rising too high. Blood sugar levels, when these stay high, cause harm. But why do these levels rise at all? The inability of available insulin to fully break available blood sugar into energy leads to a rise in blood sugar levels.
Another reason is that the conversion process itself has become retarded. The cells of the body have become resistant to the available insulin. As a consequence, blood sugar levels rise. The medical name for such a condition is insulin resistance.
From this brief overview, we now understand that any food that causes the blood sugar level to spike rapidly will harm a diabetic individual. The rapidity at which sugar in food becomes available to the body is defined by the glycemic index. For instance, some fruits like watermelon, which otherwise may not be very sweet, are high on the glycemic index. The available sugar in watermelon becomes very rapidly available to the body, causing blood sugar levels to spike. On the other hand, Grapes lie somewhere in the middle of this index and therefore would not be harmful if consumed in moderation.
Grapes are a major source of several phytochemicals like flavonoids, polyphenols, anthocyanins, proanthocyanidins, procyanidins, and stilbene derivatives resveratrol. As expected, grapes are rich in carbohydrates; although, a lot of it is dietary fiber.
Dietary fiber, found in fruits, vegetables, and other types of food, is carbohydrates that are not digested in the small intestine. These, undigested, passing through the small intestine, reach the large intestine and colon. Fiber bulks up food. These facilitate the movement of waste down the large intestine helping in convenient evacuation.
Fructose found in fruits does not stimulate insulin secretion from pancreatic cells. The consumption of food and beverages containing fructose leads to a smaller hike in blood sugar levels when compared to sugar (sucrose) products. It also shows insulin resistance (Elliot et al.,2002).
This background, hopefully, will help you analyze any food for its suitability for people with diabetes.