Pyler says: Glycemic index isn't the whole story

by E.J. Pyler and L.A. Gorton
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From Chapter 1: Basic Food Science — Glycemic Impact

 

Carbohydrate-rich foods find themselves at the center of today’s concern over the rising rates of obesity and diabetes in population groups consuming Western diets. Many researchers link consumption of such foods with development of these health problems, although not all foods in a given category can be proven to have such effects.

One way to differentiate carbohydrate foods is by their post-consumption impact on blood sugar levels in the body. Indeed, individuals with diabetes (type 1, type 2 and pre-diabetes) must regulate their diet to prevent rapid rises in blood sugar. Typically, they use “exchange” calculations to evaluate food choices, particularly those containing carbohydrates. The glycemic index (GI) developed as a way to assist diabetics in making those decisions (Jenkins et al. 1981).

The numerical GI measures the rise in blood sugar — the higher the number, the greater the blood sugar response. A low-GI food will cause a small rise, generally over a longer period of time, while a high-GI food will trigger a dramatic spike. Measured on a scale of 0 to 100, a GI of 55 or less is considered low, 56 to 69 is medium, and 70 or above is high, with glucose pegged at 100, the highest GI rating possible.

GI can only be measured accurately by in vivo testing. The blood of at least 10 test subjects is measured for sugar content before eating the studied food in portions containing 50 g of available carbohydrates and then tested afterward at timed intervals. The food’s GI is calculated taking the area under the curve for the test food and comparing it with that for glucose alone. (The actual test is considerably more complicated than this simple description, involving overnight fasting and a series of timed glucose challenges.) A variety of factors can influence GI measurements, including differences between individuals, use of venous vs. capillary blood samples, the food’s cooking factors and foods consumed along with the tested item in “mixed meals.”

Because GI numbers alone can’t tell the whole story, the concept of glycemic load (GL) emerged. This measurement takes into account the amount of available carbohydrate being consumed in the portion and is said to be a truer reflection of the exposure of the subject to glycemic foods than GI alone. It is calculated from the GI for a given food:

 

GL = (GI ´ amount of available carbohydrate) ÷ 100

 

Available carbohydrate, according to a definition recommended by AACC International’s ad hoc Glycemic (Net) Carbohydrate Definition Committee, is “carbohydrate that is released from a food in digestion and that is absorbed as monosaccharides and metabolized by the body” (Jones 2007).

In the GL scale, a rating of 20 or more is high, 11 to 19 is medium, and 10 or less is low. Foods that have a low GL almost always have a low GI, but intermediate GL ratings can characterize foods that range in GI from very low to very high.

As a component of foods, carbohydrates fall in and out of favor with diet “gurus” and the consuming public, depending on the temper of the times. Bakers and others who produce foods made with carbohydrates need to stay abreast of such issues. For example, the baking industry and its ingredient suppliers responded, albeit somewhat late, to the Atkins low-carb diet fad of the late 1990s and early 2000s with many interesting baked foods and ingredient applications. As the fad waned, those breads, muffins, cakes, snacks and nutrition bars disappeared from the market, but the well-prepared product developer kept his notes.

For readers seeking additional background on glycemic impact, Witwer (2005) and Brand-Miller (2007) provide good overviews of GI, its benefits, criticisms and use in food labeling, while Jenkins (2007) reviews its history and development. Others debate the value of GI as a method of differentiating between foods on eating occasions, and DeVries (2007) offered these contrasting views.

 

References:

Brand-Miller, J. 2007. The glycemic index as a measure of health and nutritional quality: an Australian perspective. Cereal Foods World 52 (2): 41-44.

DeVries, J.W. 2007. Glycemic index: the analytical perspective. Cereal Foods World 52 (2): 45-49.

Jenkins, A.L. 2007. The glycemic index: looking back 25 years. Cereal Foods World 52 (2): 50-53.

Jenkins, D.J., Wolever, T.M., Taylor, R.H., Barker, H., Baldwin, J.M., Bowling, A.C., Newman, H.C., Jenkins, A.L., and Goff, D.V. 1981. Glycemic index of foods: a physiological basis for carbohydrate exchange. Am. J. Clin. Nutr. 34: 362-366.

Jones. J.M. 2007. Glycemic response definitions. Cereal Foods World 52 (2): 54-55.

Witwer, R. 2005. Understanding glycemic impact. Food Technology 59 (11): 22-29.

More on this topic can be found in “Baking Science & Technology, 4th ed., Vol. I,” Page 16, by E.J. Pyler and L.A. Gorton. Details are in our store.

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