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Effects of grains on glucose and insulin responses

[1]  [2]  [3]  [4]  [5]

Whole grains, if consumed, could provide a substantial contribution to the improvement of the diets of Americans, since many whole-grain foods and grain-fiber sources have been shown to be beneficial in reducing insulin resistance and improving glucose tolerance. Dietary guidelines by the U.S. Department of Agriculture (USDA, 1995, 1996) recommend the consumption of 6-11 servings per day from the grains group (such as bread, cereals, rice, and pasta), with three servings per day from whole grains. However, in the 1994-1996 survey data (Cleveland et al, 2000), U.S. adults averaged 6.7 servings of grain products per day in their diet. Only 8% of Americans consume at least three servings of whole grains per day, with the average consumption in the United States being less than one serving per day (Cleveland et al, 2000). It seems clear that a large part of the U.S. population could benefit from eating more grain products, especially whole grains.

Whole grains, grain fractions, and grain extracts have been reported to control or improve glucose tolerance and reduce insulin resistance. The inability of the body to maintain normal glucose levels with normal concentrations of insulin production or to require excessive levels of insulin (hyperinsulinemia) to do so has been called “glucose intolerance”, “impaired glucose tolerance”, “insulin resistance” and “Syndrome X”, a disorder in which insulin resistance, hyperinsulinemia, or both are present. Insulin resistance has been reported to be a major factor in the development of type 2 diabetes mellitus and, for many people, is the first observed abnormality in the progression of the disease (Daly et al, 1997). In addition, increased insulin concentrations generally indicate insulin resistance in nondiabetic individuals. Obesity has been reported to be the most common condition associated with insulin resistance (Daly et al, 1997). Increasing whole-grain intake in the population could result in improved glucose metabolism and could delay or reduce the risk of developing type 2 diabetes mellitus.

There are several mechanisms by which grains may improve glucose metabolism and delay or prevent the progression of impaired glucose tolerance to insulin resistance and diabetes. The form, amount, and method of cooking of these foods, as well as the age, sex, and health characteristics of the group of subjects studied, are all important factors in the effectiveness of the foods in altering these responses. These mechanisms are related to the physical properties and structure of grains. The composition of the grain (including particle size, amount [page 269] and type of fiber, viscosity, and amylose and amylopectin content) all affect the metabolism of carbohydrates from grains.

Particle Size

Most grains consumed in developed countries are milled and processed before being used to manufacture food products. Milling whole grains significantly disrupts the grain structure and changes the nutrient content; i.e., most of the bran and germ are removed and the starch content increases (Weaver, 2001). For example, whole-wheat grains contain approximately 14% bran, while refined white wheat flour contains less than 0.1% bran (Table 1). Whole rice and corn consumed as whole grains without being chewed resulted in lower postprandial glucose levels than the same food after it had been thoroughly chewed (Read et al, 1986). Lower glucose and insulin responses were reported by Collier and O'Dea (1982) after whole brown rice was consumed compared to responses after ground brown rice or glucose were consumed by both normal and diabetic subjects. The authors concluded that the difference in glucose response was due to the lack of processing of the whole grains before or during consumption.

Grains are not commonly consumed whole but after varying degrees of processing before consumption. When compared with white bread, whole wheat bread that contained boiled kernels elicited lower blood glucose and insulin responses (Braaten et al, 1991). A comparison of four types of wheat (whole-grain, cracked-grain, and coarse and fine whole-meal flour) in 10 healthy subjects resulted in glucose responses to whole-grain flour of approximately one-third the response to the fine flour (Holt and Brand-Miller, 1994). Insulin responses were similar. The highest responses occurred after the fine-ground flour, followed by the coarse flour and cracked grain, with the lowest response after the whole-grain product. We also have compared breads made with different particle sizes (Behall et al, 1999a). Consumption of breads made with white flour, standard whole-wheat flour, and ultra-fine-ground whole-wheat flour by middle-aged men and women resulted in lower glycemic responses compared with glucose, but the responses to the whole-wheat breads, although lower than those for white bread, were not different. The standard whole-wheat bread had particles similar in size to the flour used by Holt and Brand-Miller (1994).

Table 1
Composition Differences Between Whole- and Refined-Wheat Component

a - Adapted from Weaver (2001).

[page 270] Holm and Bjorck (1992) also compared responses of healthy subjects to white wheat breads with or without intact kernels. Breads with intact kernels resulted in lower glucose responses and higher satiety scores. Significantly lower glucose and insulin responses were also reported after consumption of coarse bread products containing kernels from wheat, rye, or barley (but not after consumption of breads with oat kernels) compared with white bread (Holm and Bjorck, 1992). Heaton et al (1988) observed higher insulin, but not glucose, responses after meals containing fine-ground flour compared to responses after meals with coarse-ground flour, cracked grain, or whole grain. Similar to those observed with the wheat, insulin responses were greater with fine-ground corn-meal than with coarse or whole corn. However, no differences in insulin response were observed between fine oatmeal, rolled oats, and whole groats (Heaton et al, 1988). Obese subjects with ileostomies fed coarse or fine whole-meal flours in a test meal had higher glucose and insulin responses after the fine-ground flour (O'Donnell et al, 1989). Starch content in the effluent was significantly higher after the test meal containing the coarse-ground grain, indicating the effect of the grain's particle size on starch digestion. A high correlation was observed between the percentage of starch hydrolysis in vitro of diets fed to pigs and the mean insulin areas under the curve.

Boiled whole kernels and larger particle sizes are also associated with lower glucose and insulin responses for a variety of grain sources.