By Nora Saul
M.S, R.D., L.D.N., C.D.E.
Manager of Nutritional Education at Joslin Diabetes Center
Today we’re exploring some of the factors that affect the glycemic index (GI) of a food, and how to use the index—along with carb counting—to keep blood sugars in control.
What Makes Some Carbohydrates Low GI?
All low GI foods have characteristics that prevent the carbohydrate in the food from being rapidly digested.
Some such as beans, oats and apples have soluble fibers. Soluble fiber is viscous, creating a thick soup in the intestine. This makes it harder for digestive enzymes to quickly attach to the carbohydrate particles.
In others (basmati rice, for example) the starch molecules are arranged in such a way that it’s difficult for your intestinal enzymes to quickly dissolve the bonds holding the molecules together.
And some breads and beans contain an insoluble fiber shell that acts as a sheath, protecting the carbohydrate from easily being digested.
Tweaking the GI of Some Foods
Although you can’t affect the GI by adding or subtracting the amount of food you eat (remember, that is a function of the glycemic index), or by changing the configuration of the starch molecule in your cereal (obviously), you can alter how quickly a food is digested by the enzymes in your digestive system by changing the way you prepare the food.
One of the simplest ways of lowering the GI of starches is to cook them al dente. Shorter cooking times prevent the swelling or gelatinization of the starch molecules.
Adding an acid or fat will also help lower the GI. Acids such as lemon juice and vinegar and healthy fats such as olive oil and nuts will slow stomach emptying, retarding enzyme digestion.
For example, to lower the GI of white bread, add peanut butter. To lower the GI of potatoes, make a vinegar based potato salad.
How Do I know Which Foods Are Low GI?
If you are a numbers person and want to know the exact ranking of different foods you can go to glycemicindex.com (this is the web site operated by the University of Sydney where the glycemic index was first developed). Or you can read one of the many books published on the glycemic index by Jennie Brand-Miller.
If you’re like me, and want some basic guidelines to carry around in your head:
- Choose carbohydrates that are high in fiber, especially soluble fiber such as beans, peas, apples, oats, Brussels sprouts.
- Cook starches minimally.
- Eat more non-starchy vegetables–almost all non-starchy vegetables are low glycemic index.
How Does the GI Work with Carb Counting?
You can use the GI along with carb counting to fine tune your blood sugar control.
Let’s say you decide to eat 60 grams of carbohydrate at a meal. Knowing which foods are lower GI and or knowing how to reduce the GI value of a food can mean the difference between blood glucose above goal 2 hours after a meal or within goal.
It is best to use the GI to compare foods in the same category, so that you eat a variety of carbohydrates. For example, compare starches (rice against corn or potatoes) against one another, rather than comparing fruit with bread.
Are All Low GI Foods, “Good” Foods?
Not necessarily. Fat, remember, lowers the GI. This means whole milk will have a lower GI than skim milk, even though skim milk is a better choice.
The GI can be confusing. Working with a registered dietitian can help you figure out how different GI foods affect your blood glucose. Adding this tool to your diabetes self-management skills can make a difference in your efforts at good control.
Get more information….
- On diabetes and nutrition
- On nutrition classes at the Joslin Diabetes Center
- On diabetes
- On books about diabetes and nutrition (plus diabetes cookbooks)