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Effects of grains on glucose and insulin responses

[1]  [2]  [3]  [4]  [5]

Starch Structure

Starch is composed of long, straight chains of glucose (amylose) and highly branched chains of glucose (amylopectin). These differences have been most frequently studied in corn products because the range of amylose varies from 30 to 70% of the starch, whereas most grains average 25-30% amylose. The difference in starch structure has been shown to have a profound effect on the glucose and insulin responses. A study in our laboratory (Behall et al, 1988), reported insulin responses of 12 women and 13 men to be significantly lower after consumption of corn crackers containing 70% amylose compared to crackers with 70% amylopectin. Peak glucose responses were also lower after high amylose. A controlled dietary study (Behall et al, 1989) found significantly lower glucose and insulin responses to corn crackers in 12 men after they consumed a variety of foods high in amylose or amylopectin for five weeks each. A long-term study (each starch consumed 14 weeks) found hyperinsulinemic men to be more responsive to the beneficial effects of the high-amylose diet than were controls (Behall and Howe, 1995). Although hyperinsulinemic individuals are not considered to be in a diseased state, abnormal insulin and glucose levels have been used as the primary indicators of carbohydrate sensitivity and type 2 diabetics (Beebe and Rubenstein, 1987; Kuczmarski et al, 1994). Noaks et al (1996) fed either low- or high-amylose starch for four weeks each to overweight hypertriglyceridemic men and women. Plasma glucose response was 7% lower at 45 min and insulin response 28% lower at 75 min after the high-amylose versus low-amylose muffin challenge.

Other researchers have reported beneficial effects of high-amylose corn products after acute tests. Postprandial glucose and insulin were significantly lower after meals containing retrograded high-amylose cornstarch than after meals with pregelatinized standard cornstarch (Achour et al, 1997). [page 271] Weststrate and van Amelsvoort (1993) found significantly reduced insulin responses after high-amylose breakfasts, and both glucose and insulin responses were significantly reduced after high-amylose lunches. Granfeldt et al (1995a) and Semprun-Fereira et al (1994) tested healthy subjects using arepas (flat cornbread made from precooked cornmeal) made with standard or high-amylose corn flour. Glucose and insulin responses after high-amylose arepas were significantly lower than after ordinary cornmeal arepas. Krezowski et al (1987) observed significant reduction in glucose in type 2 diabetic subjects after consumption of high-amylose muffins compared with standard (low-amylose) muffins, cornflakes, or glucose. Insulin response was higher after the standard muffins than after the high-amylose muffins. These results show that amylose content and the level of processing (which affects resistant starch development) have a great influence on the responses to corn-containing foods.

Rice can vary substantially in amylose content, although it does not have as wide a range in content as corn. Comparison of responses to three rice varieties with varying amylose content (served as brown rice, puffed rice cakes, rice pasta, and rice bran) found only high-amylose varieties to lower glucose and insulin responses (Brand-Miller et al, 1992). Holt and Brand-Miller (1995) compared standard and quick-cooking rice and high- and low-amylose rice puffs and found responses to quick-cooking rice to be 60% higher than to standard rice. Low-amylose rice resulted in a 50% higher glucose response than high-amylose rice. However, Panlasigui et al (1991) concluded after testing three high-amylose varieties of rice that amylose content alone cannot predict the glycemic response and that gelatinization is also a factor.

Viscosity and Solubility

Fiber of oats, barley, and rye is, on average, about one-third soluble fiber capable of forming a gel and two-thirds insoluble (non-gel-forming) fiber. Wheat contains less soluble fiber and rice virtually none. Refined grains are typically low in total dietary fiber; refining primarily decreases insoluble fiber, which increases the percentage of soluble fiber remaining. The gel-forming property of soluble fiber sources has been proposed as the mechanism by which oats and barley reduce both glucose and insulin responses and cholesterol. Jenkins and coworkers were among the first to investigate the beneficial effects of soluble fibers, starting with isolated fiber sources. Jenkins et al (1978) reported that oat gum reduced glucose and insulin responses of healthy adults when added to a glucose solution. The high viscosity of the solution containing oat gum was concluded to be the property that delays gastric emptying and/or intestinal absorption, resulting in these lower responses (Wood et al, 1989). Braaten et al (1991) also tested responses to glucose and glucose with oat gum and found reductions in glucose and insulin when the nine healthy subjects consumed solutions to which oat gum had been added. In diabetics and controls, Braaten et al (1994) found both oat gum and oat bran added to farina to reduce glucose and insulin responses below those of farina alone. An oat-based soup was used as a means of weight reduction in 31 subjects (Rytter et al, 1996). After 23 weeks, subjects lost an average of 6 kg, and both glucose and insulin responses declined; however, it is difficult to separate the effects of the soup from the weight loss caused by reduced energy intake. [page 272] In our own research, we have found two levels of highly viscous oat extracts to lower glucose and insulin responses of middle-aged men and women (Hallfrisch et al, 1995); both levels were lower than that used by Wood et al (1994). Lower glucose and insulin responses were observed whether the oat extract consumed was uncooked, boiled, or baked (van der Sluijs et al, 1999).

Granfeldt et al (1995b) tested responses of nine older men to raw rolled oats, boiled rolled oats, boiled intact oat kernels, and white bread. Boiled intact oat kernels resulted only in glucose and insulin response reductions below the response to white bread. Preliminary data using rolled oats or oat flour indicate reductions in glucose response of 15-30% in moderately overweight women (Behall et al, 1999b). Comparisons of glucose and insulin responses of 24 hypercholesterolemic men to wheat, rice, and oats found no significant differences (Kestin et al, 1990). A smaller study of six men found no differences among glucose and insulin responses to 10 g of fiber from oat bran, wheat bran, wheat fiber, or wheat germ (Cara et al, 1992). These sources of fiber, however, are predominantly insoluble and would not result in highly viscous intestinal contents. The test subjects in these studies were also relatively young, lean subjects.

Barley is also high in soluble fiber and has the potential to improve insulin sensitivity and glucose metabolism; however, little barley is consumed by Americans. Yokoyama et al (1997) compared responses of five subjects to pastas containing wheat or wheat and 12 g of β-glucans from barley. Consumption of barley-containing pasta resulted in lower glycemic and insulin indices. Bourdon et al (1999) compared glucose and insulin responses of 11 healthy men (28-42 years). Subjects consumed traditional wheat pasta or a pasta in which 40% of the wheat flour had been replaced with either a highly viscous barley cultivar (Prowashonupana), which naturally has about 15% soluble β-glucans, or Waxbar barley enriched in soluble fiber by repeated milling. Insulin responses were lower after the barley pastas than after the traditional wheat pasta during the first hour. Although the glucose areas under the curves did not differ, the decline in glucose after barley pastas was more gradual than after the wheat pasta. Substantial reductions in insulin and glucose responses were found in seven young subjects after consumption of varying amounts of boiled barley compared with white bread (Wolever and Bolognesi, 1996a). Breads made with 10% whole barley or 15% pearled barley lowered glucose responses of 15 diabetics compared with responses to white bread (Urooj et al, 1998). Another study of diabetics compared long-term (24 weeks) effects of barley bread to white bread in 11 men with type 2 diabetes (Pick et al, 1998). The glycemic index was reduced, but insulin responses were increased when the men consumed the barley bread. However, the insulin regimen was changed for some subjects during the study. Comparison of barley and oat foods with white bread in nine healthy subjects (Liljeberg et al, 1996) found no effect of porridges, but consumption of high-fiber barley breads resulted in glycemic indices of 57-72% of the white-bread index and insulin indices of 42-72% of the white bread index. Whole kernels of wheat, rye, and barley added to breads resulted in glycemic indices lower than those of white bread, but adding whole oat kernels to the bread had no effect on the responses of 10 healthy subjects (Liljeberg et al, 1992). However; insulin indices for all breads to which boiled kernels had been added were lower than those for white bread. Preliminary data from our laboratory indicate that both flakes and flour of the high-soluble-fiber barley (Prowashonupana) lower glucose and insulin in moderately overweight middle-aged women (Behall et al, 1999b). [page 273]

Rye also is relatively high in soluble fiber, but few human studies report responses to rye consumption. Normal-glycemic men and women had significantly lower insulin responses after eating whole-kernel rye bread compared to wheat bread, with no difference in glucose response (Leinonen et al, 1999). Consumption of whole-meal rye bread resulted in significantly higher glucose response and lower insulin response compared to responses from whole-meal rye crisp-bread (Leinonen et al, 1999). Whole-kernel rye has been tested in a few groups of subjects with diabetes (Jenkins et al, 1986; Brand et al, 1990; Liljeberg et al, 1992). Compared to white bread, the whole-kernel rye bread produced significantly lower glucose response in diabetics, both insulin and noninsulin-depend-ent diabetics, with a glycemic index of 42-56 compared to the bread. However, rye crispbreads had glucose responses very similar to those of white bread. No differences in the glucose responses of 14 young diabetics were reported after they consumed dark rye bread and white bread (Birnbacher et al, 1995).

Wood et al (1994) concluded that reductions in plasma glucose and insulin (79-96%) after consumption of oat gum were primarily the result of viscosity changes. Granfeldt et al (1995b), however, found neither degree of gelatinization nor viscosity to affect glucose levels. Grains that contain highly viscous fibers, and foods made with these grains, can be effective in lowering blood glucose and insulin responses. These effects are most likely to be found in subjects for which lowering glucose and insulin is an improvement, that is, older, overweight subjects and those with type 2 diabetes. The effects on glucose and insulin responses are less significant if subjects are young, fit, and have normal glucose and insulin responses. However, to be effective, whole kernels or the soluble-fiber components of the grains must be consumed, and currently the typical American appears to prefer highly refined flours and cereals or their brans.