Understanding The Glycemic Index

The glycemic index is a rating or ranking scale developed to measure the impact of a particular carbohydrate containing food on blood glucose levels. All carbohydrates will increase blood glucose levels because they contain sugars, starches or both. However, carbohydrates also contain various amounts of fiber. Fiber works to slow down the absorption of the sugars and starches and therefore produces less of a spike in blood glucose levels.

The glycemic index, more simply known as GI, is a relatively new concept developed by diabetes researchers at the University of Toronto in Ontario, Canada. In the early 1980s Dr. Jenkins and his staff and colleagues collected the data using the blood glucose response curve at the two hour mark after subjects ingested a set amount of a particular food. This was then rated against a standard, which is usually glucose but may also be white bread, to determine the specific ranking that the food obtained. White bread or glucose, depending on the scale used, is given the score of 100 with everything ranking from this. Given glucose is uniform and white bread can have variations, most GI charts and tables use glucose as the standard.

Foods that break down slower or that contain higher levels of fiber tend to score lower on the GI. Foods that have very little fiber and high levels of sugar and starch break down more quickly and have higher GI values since they are rapidly absorbed by the body and contribute to a rapid increase in blood glucose levels. In research studies of women surveyed over an 8 year period it was found that those that consumed a diet high in carbohydrates and low in fiber had an increased risk of type 2 diabetes. This study tracked 91249 middle aged and young women who were undiagnosed at the time of the initial questionnaire. Of the group 741 developed type 2 diabetes during the eight year follow-up study, clearly linking the dietary choices and the risk for development of diabetes. 1

Low GI Foods

All of the low GI foods are considered beneficial as the bulk of the carbohydrates in the diet for people with any type of diabetes, those attempting to lose weight and people with metabolic syndrome and prediabetes. These foods are usually very high in fiber and low to moderate in their levels of carbohydrates. Foods in these groups include vegetables, fruits, legumes, nuts and whole grains.

Food preparation methods and potion control is important in consuming low GI foods. For example, an apple with the peel is considered a low GI food, which means it has score of 55 or less. An apple with the peel is typically rated at a GI of 38.  Apple juice, which doesn’t contain any fiber, will have a rating of over 41 depending on the specific added sugars or other juices found in the beverage. Applesauce, depending on the specific brand or preparation method will have a GI of 53. Cooking vegetables also breaks down the fiber content, which can increase the GI, but usually this is a minor increase and still keeps those foods within the low GI level.

A low GI diet can help to reduce the need for insulin, even in those women with gestational diabetes. In a study of 63 women diagnosed with gestational diabetes it was found that those on a low glycemic diet did better than those on a high fiber diet. The number of those requiring insulin on the low GI diet was half that on the conventional high fiber diet recommended for management of gestational diabetes.2

Medium GI Foods

Foods in the medium GI range include sweet and baked potatoes, basmati rice, whole wheat foods, fruit drinks, bananas, apricots, corn chips, pancakes and many prepared cereals. These foods have a GI of between 55 and 69 and should be consumed in small portions and infrequently in the diet. Since they do cause more rapid spikes in blood glucose levels it may also be important to eat these foods early in the day and in combination with very low GI foods.

High GI Foods

Foods high on the GI scale include anything with a ranking of 70 or higher. These foods need to be carefully limited in the diet and portion control must be considered when eating these items. As with the medium GI foods it is important to consider the time of day and the foods consumed with these high GI choices to avoid high glycemic loads which can cause changes in the body’s reaction to insulin or the need for additional blood glucose monitoring.

High GI foods are typically sweets but they can also be very starchy foods such as white bread, bagels, muffins, cookies and potatoes. Some vegetables such as beets and root vegetables can be high GI as are watermelon, pumpkin and dates. Instant foods such as rice and oatmeal are also high GI foods. Most snack foods including pretzels, chips, candies and savory crackers are all included in this group.

The GI And Diabetes

The GI is considered a good tool for diabetics but there are several factors to consider. It is important to remember that different food combination and preparation methods can change the glycemic load, which is the total energy consumed during a meal. It is the glycemic load as opposed to the total of the GI values that actually determines the blood glucose levels and the possibility of spikes after meals.

Depending on the level of insulin resistance the glycemic response of individuals varies both during the day as well as when the body is under stress. Using the GI in combination with carbohydrate counting is seen as a very effective way to manage this natural variation and provide the opportunity to use foods to regulate blood glucose levels throughout the day. It is endorsed by the American Diabetes Association in conjunction with total carbohydrate counting.

The good news for diabetics is that research has indicated that following a low GI diet can do more than help to regulate blood glucose levels. Over a six month period comparing a low-GI diet to a high cereal fiber diet in those diagnosed with type 2 diabetes it was found that those following the low GI diet had a slight decrease in HbA1c levels as well as an increase in HDL or good cholesterol levels. 3

It is important to discuss the benefits of following a low GI diet with your doctor. People with prediabetes, gestational diabetes or type 2 diabetes may be able to manage blood glucose levels without the need for insulin by adopting this lifestyle and eating plan. Those with type 1 diabetes may be able to better control blood glucose and prevent rapid increases in blood glucose by using the glycemic index in planning meals and food choices.

References

1 Schulze, M. B., Liu, S., Rimm, E. B., et al. (2004). Glycemic index, glycemic load, and dietary fiber intake and incidence of type 2 diabetes in younger and middle-aged women. The American Journal of Clinical Nutrition , 348-356.

2 Moses, R. G., Barker, M., Winter, M., et al. (2009). Can a Low–Glycemic Index Diet Reduce the Need for Insulin in Gestational Diabetes Mellitus? Diabetes Care , 996-1000.

3 Jenkins, D. J., Kendall, C. w., McKeown-Eyssen, G., et al. (2008). Effect of a Low–Glycemic Index or a High–Cereal Fiber Diet on Type 2 Diabetes. Journal of the American Medical Association , 2742-2753.

This article was originally published July 12, 2012 and last revision and update of it was 9/10/2015.