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Effects of Grains on Glucose and Insulin Responses

Ph.D. Kay M. Behall and Ph.D. Judith Hallfrisch

Diet and Human Performance Laboratory
Beltsville Human Nutrition Research Center
Agricultural Research Service, USDA, Beltsville, MD 20705

 

Glycemic Index

The glycemic index is a calculation based on the postprandial blood glucose response (area under the curve) of a specific weight of carbohydrate-containing food compared to the response after the same weight of a reference food consumed by the same subject (Reaven and Miller, 1968; Wolever, 1991, Brand-Miller and Foster-Powell, 2000). The resulting glycemic index can be used to rank foods (Foster-Powell and Brand-Miller, 1995; Brand-Miller and Foster-Powell, 2000) (Table 2). Glucose and white wheat bread have both been used as the reference food. It is important to remember that white wheat bread is low in fiber content, uses finely ground flour, and (when compared to glucose) has a glycemic index of 70. This is substantially lower than many other foods, including cornflakes, some rice foods, potatoes, cereals, crackers, and sports drinks. Key factors that influence the observed glycemic index include cooking methods, physical form of the food, type of starch, fiber, and sugar. An insulin index has also been calculated utilizing the areas under the curve as described above for the glycemic index. A much more extensive database exists for the glycemic index of foods than for the insulin index (Foster-Powell and Brand-Miller, 1995; Brand-Miller and Foster-Powell, 2000). [page 274]


Glycemic Response

Most high-carbohydrate foods are based on grains (especially wheat, corn, and oats) or sugars. After processing to increase commercial palatability, most cereals have high glycemic indexes. Differences in responses observed after consumption of wheat and wheat products appear to be the result of processing and the particle size differences discussed previously. Wheat pastas are processed with minimal mechanical disruption of the starch granule during manufacture. Compared to bread, pasta meals have been reported to significantly reduce plasma glucose and insulin (35 and 39%, respectively) (Jarvi et al, 1995). The wide variety of pasta types exhibits a broad range in glycemic index, from 32 to 55 compared to glucose. Protein appears to interact with the starch to reduce gelatinization and slow stomach emptying. Wheat contains low to average amounts of amylose and relatively low levels of soluble fiber compared to oats.

Corn and corn products have a wide range in glucose and insulin responses depending on cultivar, form, processing, and levels of amylose and amylopectin. Cornflakes, by prediction and actual measurement, had higher glycemic and insulin indexes (139 and 149, respectively) than white bread when determined in eight nondiabetic university students (Wolever and Bolognesi, 1996b) and 14 diabetic children (Birnbacher et al, 1995). Wheeler et al (1996) compared responses of 16 young diabetics (14-25 years old) to cornflakes, sweetened cornflakes, glucose, and sucrose. Finding no differences between sweetened and unsweetened cornflakes and either cornflakes and glucose or cornflakes and sucrose, the authors concluded that cornflakes, whether sweetened or unsweetened, are not detrimental to these patients. While an occasional meal of cornflakes may be no more detrimental than glucose when consumed by a diabetic, cornflakes are not beneficial in reducing plasma glucose and insulin requirement, as is observed after consumption of cereals with lower indices. Golay et al (1992) fed 14 overweight type-2 diabetic patients cornflakes or muesli for breakfast for two weeks, each with all other dietary components the same. After a glucose tolerance test at the end of each period, plasma glucose responses were not different. However, insulin was significantly lower at fasting (17%) and 2 hr (21%) after the muesli compared to that after cornflakes.

Table 2
Glycemic Index (GI) of Selected Grains and Grain Products
Compared to the Response after a Glucose Tolerance Test *

Whole Grain GI Processed Grain GI
  Barley, including pearled 25   Rolled barley 66
  Corn, sweet kernel 55   Corn flakes 84
  Oatmeal, old-fashioned 49   Oatmeal, quick 66
  Rice, brown 55   Krispy rice 88
  Rice, sweet, low-amylose 88   Rice, high-amylose 59
  Rye, whole-kernel 34   Rye flour bread 65
  Wheat, whole-kernel 41   Puffed wheat 74
  Bulgur wheat 48   Wheat flakes 75

* Adapted from Foster-Powell and Brand-Miller (1995).

These authors concluded that the change of cereal type, from rapid- to slow-release starch, improved carbohydrate metabolism and probably insulin sensitivity in the diabetic patient. [page 275]