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Effects of grains on glucose and insulin responses

[1]  [2]  [3]  [4]  [5]

Responses of healthy and diabetic South Africans to refined maize, rice, and bread found the glucose response to maize to be higher than to bread and similar to glucose, but the insulin response to maize was about two-thirds the response to bread (Segal et al, 1991; Wolever et al, 1994). Some Native American corn products have much lower glycemic indices – e.g., tortillas (~38) and hominy (~40)– than white bread (~70 when glucose = 100) (Foster-Powell and Brand-Miller, 1995). Even boiled sweet corn and popcorn have glycemic indices (both ~55) lower than white bread (~70) compared to glucose (100). However, maize porridge, whether refined or unrefined and taco shells, have glycemic indices (~71, ~74, and ~68, respectively) comparable to that of white bread (Segal et al, 1991; Foster-Powell and Brand-Miller, 1995).

Rice products have a wide range of glycemic indices, and rice is considered to be either a high- or low-glycemic-index food based on cooking method, cultivar, form of food, and subject group studied (Brand-Miller et al, 1992). Baking rice after boiling, compared to boiling only, resulted in a significantly lower glucose area under the curve 60 min after the meal as well as differences in viscosity and in vitro hydrolysis rates (Gatti et al, 1987). Larsen et al (2000) fed diabetic subjects white bread or polished rice of the same variety that was either not parboiled, mildly parboiled, or severely pressure-parboiled. All three rice meals resulted in significantly lower postprandial glucose and insulin responses compared to those of the bread meal. The glycemic indices were 55, 48, and 39, respectively, for the rice not parboiled, mildly parboiled, or severely pressure-parboiled. However, the treatment of rice to produce “instant” rice results in a product with a glycemic index higher than that of the bread (~87 vs. 70) (Foster-Powell and Brand-Miller, 1995). The amylose content (0-40% depending on cultivar) and final gelatinization temperature have been reported to be negatively correlated with the rate of glucose found in the plasma after rice consumption and to decrease with processing, while resistant starch generally increases with processing such as parboiling and noodle extrusion (Juliano, 1992).

The results for rice are inconsistent, with some rice-based foods having higher glycemic indices than white bread while brown rice may have a lower glycemic index than white rice (Foster-Powell and Brand-Miller, 1995). Rice bran has been reported to have a glycemic index of 27 compared to that of bread (Foster-Powell and Brand-Miller, 1995). However, addition of 10 g of rice bran to a liquid meal did not lower glucose or insulin responses of six healthy males (Cara et al, 1992). Comparison of brown rice and barley responses in 10 healthy subjects resulted in 30% lower glucose responses to the barley meal than to the rice (Thorburn et al, 1993). The higher level of fermentation of the barley, as measured by hydrogen expiration, was thought to be the mechanism by which barley improved glucose response. The South African study found higher glucose and insulin responses to refined white rice than to white bread (Segal et al, 1991). Rasmussen et al (1992a) compared responses of seven diabetics to 25 and 50 g of carbohydrate from white rice and white bread and found significantly lower glucose and insulin responses after the 50 g of rice compared to the 50 g of white bread but no differences in the responses to 25 g of either carbohydrate, indicating that the amount of food consumed affects glucose and insulin responses. Similar reductions were found in responses of men and women to 100 g office compared to white bread (Rasmussen et al, 1992b). [page 276]

These results show that the assumption that all carbohydrates are equal (exemplified by the current concept of carbohydrate exchange) is no longer accurate. Many factors, including obesity and diabetes, affect the glucose and insulin responses resulting from consumption of grain-based carbohydrate-containing foods. Consumption of diets high in foods with high glycemic indices promote insulin resistance, obesity, and (in susceptible segments of the population) noninsulin-dependent diabetes mellitus (Brand-Miller, 1994). Obesity is associated with decreased ability of the body to control blood glucose with normal levels of insulin (Expert Committee, 1998). This may also be an early step in the development of noninsulin-dependent diabetes mellitus (Expert Committee, 1998). Consumption of grain sources that reduce the level of insulin required to maintain normal blood glucose is a means of improving insulin resistance or increasing insulin sensitivity (Gannon and Nuttall, 1987; Kiens and Richter, 1996). Other tests that measure changes in insulin resistance are more time-consuming or invasive; examples are glycemic clamps (Elahi, 1996) and Bergman's minimal model (Bergman et al, 1985).

Summary

Various grains and grain products can be beneficial in lowering glucose and insulin responses. Although this review is not by any means complete, several generalities can be expressed. 1) The greater the particle size, the lower the glucose and insulin response. Or, inversely, the greater the level of processing and refining, the higher the response. 2) Higher amylose content results in lower glucose and insulin responses. Corn and rice can have either high or low glycemic indices because their amylose and amylopectin contents vary. 3) Grains with high levels of soluble β-glucans such as oats, rye, and barley are generally more effective in improving insulin sensitivity than wheat, which contains predominantly insoluble dietary fiber. The high viscosity of these soluble fibers is partially responsible for these beneficial effects. 4) The type and characteristics of the subjects tested are important in determining the level of reduction that can be achieved. Therefore, older, less slim, more glucose-intolerant subjects have the capacity for greater improvement in glucose and insulin responses than do young, fit, slim subjects. The present American diet has great room for improvement if the amount of daily whole-grain servings would be increased from less than one serving to the recommended three serving per day. Replacing foods from low-fiber grains such as cornflakes or white bread with whole-grain products having higher fiber or higher amylose content will reduce the risk of developing insulin resistance and obesity and improve the health of the American population. [page 277]