Understanding the glycemic index

Using the glycemic index as a tool for weight management could prove to be beneficial; explore what it is and how to use it.

Blood sugar, cholesterol, weight – chances are you’re interested in one of these three health topics. The quality of our diet, and in particular the quality of the carbohydrate we eat, has the ability to affect all three of these areas. The glycemic index is a tool that can help you understand how different carbohydrates affect your body.

The glycemic index (GI) ranks carbohydrates on a scale from 0 to 100 according to the extent to which they raise blood sugar levels after eating. Foods with a high GI (70 or more) are those which are rapidly digested and absorbed and result in rapid spikes in blood sugar and insulin levels. Low-GI foods (0-55), though still carbohydrates, are more slowly digested and absorbed, producing gradual rises in blood sugar and insulin levels.

GI figures are assigned to a food after standardized testing. The test consists of giving measured portions of a food containing 10 to 50 grams of carbohydrate to 10 healthy people after an overnight fast. Finger-prick blood samples are taken at 15 to 30 minute intervals over a two hour period after eating. These results are used to construct a blood sugar response curve and GI rating for each subject. The average of the GI ratings from all ten subjects is assigned as the GI of that food. The Glycemic Index Foundation has tested approximately 1600 foods.

Glycemic load (GL) is the big picture of how both quality and quantity of carbohydrate affects the body. In other words, GL helps predict the blood glucose response to specific amount of a specific food.

To figure glycemic load, use the following formula:

GL = GI/100 x CHO (grams) per serving.

Example: the GL of an apple = 40/100 x 15g = 6g.

Over the course of a day, glycemic load is low if below 80, moderate if less than 100, and high if greater than 120. The downside of using glycemic load is the amount of time and effort required to figure the GL .

What do we know about how glycemic index affects health? Diets rich in high GI foods, which cause quick and strong increases in blood sugar levels, have been linked to an increased risk for diabetes, heart disease and obesity. There is preliminary work linking high-glycemic diets to age-related macular degeneration, ovulatory infertility and colorectal cancer.

Some studies show reducing glycemic load may also reduce Resting Energy Expenditure (REE) – which makes rebound weight gain very possible. Other studies show low glycemic load diets affect REE less than low fat diets. Some studies show that triglycerides, blood pressure and C-reactive protein improves with a low GL – other studies do not.

There is growing, but not definitive evidence that low GI diets improve both glucose and lipid levels in people with diabetes (Type I and Type II). There may be benefits for weight control because low GI foods help control appetite and delay hunger. Research on low GI diets is also showing some improvement in insulin levels and insulin resistance – an important consideration in preventing Type II diabetes.

In general, studies have been small. Their methodology hasn’t been tight. Figuring glycemic index is in itself somewhat questionable because so few individuals are tested for only 2 hours after eating to calculate the GI of a food. Overall health benefits have been modest; it appears that the benefits to using GL/GI are on a continuum. You might receive little or no benefit, moderate benefit or you may see better-than-average results. Without balance, a low glycemic diet can actually be high fat, low-fiber and calorically dense. Aim for a well-balanced diet that includes low glycemic index carbohydrates. Use glycemic load as a guide for controlling portions.

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