Grapes and raisins contain the sugars glucose and fructose, which impart the sweet flavor of these foods. The high carbohydrate content, however, may concern you in terms of the potential effect on your blood sugar level. If you don't have diabetes or pre-diabetes, grapes and raisins typically have no significant effect of your blood sugar. With impaired glucose tolerance from pre-diabetes or diabetes, there is a possibility that grapes or raisins may cause a high blood sugar level, depending on a variety of factors.
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The glycemic index is a measure researchers devised to quantify the effect of specific foods on your blood sugar level. The higher the glycemic index value for a given food, the more likely it will raise your blood sugar. The University of Sydney database rates grapes with a low to medium glycemic index; raisins have a medium glycemic index. Despite the high sugar content of grapes and raisins, the fiber in these fruits slows digestion and nutrient absorption, which blunts the effect on your blood sugar.
The total amount of carbohydrates you consume is another important factor in determining whether raisins or grapes increase your blood sugar. A cup of Thompson seedless grapes contains 27.3 g of carbohydrates; a cup of the more tart Concord grapes contains 15.8 g. Raisins contain an exceptionally high concentration of carbohydrates with 114.8 g per cup. If you eat a cup of raisins, the probability of a high blood sugar response is greater than if you ate a cup of grapes because the carbohydrate load is quadrupled.
Nutrition scientists have developed the glycemic load, a measure of the blood-sugar-raising potential of foods that takes into account the glycemic index and carbohydrate load per serving. The higher the glycemic load value, the more likely your blood sugar will rise in response to eating a specific food. Grapes have a low to medium glycemic load rating; raisins have a high rating.
Pre-existing Blood Sugar Control
If you have good blood sugar control, it is unlikely that a serving of grapes or raisins will spike your glucose level. With poor blood sugar control, any significant carbohydrate intake may provoke an abnormal glucose elevation. In this case, the problem is not so much the food as it is poor control of the underlying disease. Your doctor will work with you to adjust your pre-diabetes or diabetes treatment plan if your blood sugar levels are unstable.
Eating protein or fat along with raisins or grapes slows digestion and reduces the likelihood of an abnormal increase in your blood sugar. A small serving of mixed nuts and raisins, for example, is less likely to increase your blood sugar than raisins alone. A slice of cheese with a serving of grapes has a similar effect — and tastes fabulous.