And why should you care?
Because these concepts are related to some common chronic diseases. Diets – i.e., habitual ways of eating – with a high glycemic index or glycemic load increase the risk of type 2 diabetes, coronary heart disease, gallbladder disease, and breast cancer. At least in women.
Glycemic index is a measure of how much a specific food is likely to influence blood sugar (glucose) levels. Carbohydrates we eat, except for fiber, are usually converted by the process of digestion into glucose which we use as fuel. Any glucose not needed immediately for energy is converted into a storage form called glycogen, for use later. We have the capacity to store only a half days’ worth of energy in the form of glycogen. Carbohydrates are converted to fat when eaten in excess of what we can use immediately or store as glycogen.
The standard for glycemic index is set by eating 50 grams of pure glucose, a type of sugar. The pattern of bloodstream glucose levels over the next two hours is given a rank of 100. Oral glucose leads to a more rapid and higher peak in blood sugar compared to nearly all other carbohydrates. All other foods containing carbs (carbohydrates) can be ranked in relation to glucose, on a scale of 0 to 100. Test subjects are given specific foods in whatever serving size contains 50 grams of available carbohydrate, and eaten without other foods. Blood glucose levels are measured repeatedly over the next two hours, and compared to the pattern observed for ingested glucose. The comparison yields a number, the gylcemic index.
Even when they have the same grams of carbohydrate, some foods cause a higher rise in blood sugar. That is, they have a higher glycemic index.
Cashews have a glycemic index (GI) of 22, which means they don’t raise blood sugar nearly as quickly or as much as glucose. An overripe banana has a GI of 52, so it would tend to raise your blood glucose more than cashews. Watermelon’s GI is 72. Doesn’t eating watermelon remind you of drinking flavored sugar water?
With higher GI foods, your body digests and absorbs the foods’ carbs faster, leading to greater release of insulin by the pancreas to reduce the blood sugar levels back to normal.
Some of you have already figured out the the actual rise in blood sugar will depend on other factors, such as how much of the food you eat! To account for the amount of food eaten, the Glycemic Load concept was devised (at Harvard?). You calculate the glycemic load by taking the grams of carbohydrate in the serving size, multiply by the food’s glycemic index, then divide by 100. For example, a cup of white rice has a glycemic load of 33 points; a cup of brown rice has a glycemic load of 23 points.
Remember, the glycemic index is based on the observed blood sugar rise after eating the serving of a food that contains 50 grams of carbohydrate. For example, corn’s glycemic index is 53. One half cup of canned yellow corn has 15.2 grams of carbohydrate. So to get the full glycemic effect, you’d have to eat over one and a half cups of corn. Most people just eat a half cup as a serving. The glycemic load of a half cup of corn is 8 points (15.2 x 53, then divide by 100). Carrots are a classic example of a healthy food with a high glycemic index, but a good/low glycemic load of 2.8 points per half cup (slices, frozen, boiled).
(I’m ignoring for now the grams of nondigestible carbohydrate (fiber) in the corn and carrots.)
Doesn’t this sound just like something you’d like to do at every meal?
Let not your hearts be troubled. You can get most of the pertinent GI’s and GL’s free on the Internet or in various books or pamphlets. If you’re interested.
(Blood sugar and blood glucose are identical. Glucose is also a simple sugar you can eat. I’ve eschewed the term “blood glucose” today to prevent confusion.)
For additional information, see Laura Dolson’s good work at About.com.
WebMD.com has an article on factors that affect glycemic index and glycemic load.
Jenkins, D.J., et al. Glycemic index of foods: a physiological basis for carbohydrate exchange. American Journal of Clinical Nutrition, 34 (1981): 362-366. This paper from the University of Toronto introduced the concept of glycemic index to the world.