Glycemic Index (GI)

Definition

The glycemic index (GI) is a measure of the relative impact of carbohydrate-containing foods on serum glucose.

  • A particular food's GI is determined by evaluating the incremental rises of blood glucose after ingestion of a food that contains 50 g of available carbohydrate compared with the same amount of carbohydrate from a reference food, usually white bread or glucose.

The following values are generally applied for defining the GI of a particular food (using glucose as reference)

  • Low GI - 55 or less
  • Medium GI - 56 to 69
  • High GI - 70 or more



Determination

The GI of a food depends upon the rapidity of digestion and absorption of its dietary carbohydrates, which is largely determined by both its physical and chemical properties.

  • Typically, foods with a low degree of starch gelatinization (e.g., pasta), and those containing a high level of viscous soluble fiber (e.g., whole-grain barley, oats, and rye), have slower rates of digestion and lower GI values.
  • Foods with a higher amylose/amylopectin ratio (such as legumes and some varieties of rice) tend to have lower GI values, likely due to the compact structure of amylose, which physically slows enzymatic reactions. By contrast, the branched structure of amylopectin is open to enzymatic attack and is thus easily digested.
  • Cooling starch after cooking (such as refrigerating boiled potatoes) can also lower the GI.

NOTE: Whole versus Refined Grains

  • Carbohydrates from grains may also be classified by whether the grain is whole or has been refined by removing all or part of the bran and germ. The majority of the fiber, vitamins, minerals, and phytochemicals reside in the bran and germ fractions, and most of these are removed with refining.
  • The intact whole grain tends to act like a slow-release capsule, reducing the surface area for enzymatic digestion of the starch, while the milling process greatly increases the surface area, thus increasing the GI of the grain.



Glycemic Load

Glycemic load (GL) is the product of the GI value of a food and its carbohydrate content.

  • The concept of the GL was developed because the blood glucose response is influenced not only by the quality of the carbohydrate consumed (i.e., the GI) but also by the quantity of carbohydrate consumed.

With white bread used as the reference standard, dietary GL quantifies the serum glucose-raising potential of dietary carbohydrates; each unit of dietary GL representing the equivalent glycemic effect of 1 g of carbohydrate from white bread or glucose.

In general, carbohydrate-rich foods with low fiber content have high GI and GL values; examples include potatoes, refined cereal products, and many sugar-sweetened beverages. By contrast, intact whole grains, legumes, fruits, and vegetables with high fiber content provide low to very low GLs per serving.

The following values are generally applied for defining the GL of a particular food:

  • Low GL - 10 or less
  • Medium GL - 11 to 19
  • High GL - 20 or more
The following values are generally applied for defining the GL per day:
  • Low GL - Less than 80
  • High GL - More than 120



Weight Control

The manipulation of dietary glycemic index (GI) or glycemic load (GL) has emerged as an important approach to weight control.

  • Since the duration of post-meal satiety is generally related to postprandial glycemia, low GI foods have been hypothesized to reduce hunger signals and delay the onset of the next meal.



Dietary Carbohydrate and Disease

Available evidence is in support of an overall low GL diet for reduction in risk of type 2 diabetes mellitus and CHD.



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