Other constituents

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The rate of starch digestion is seen to be influenced by several minor plant constituents (usually referred to as anti-nutrients) such as phytates, phenolic compounds (tannins), saponins, lectins, and several enzyme inhibitors. These components interfere with the catalytic activity of the glucosidase enzymes through different mechanisms, thereby limiting their action (Thompson, 1988). Fish and Thompson (1991) showed that lectin and tannic acid (from red kidney bean) individually could inhibit the starch digestive enzymes, salivary and pancreatic a-amylases; however, a combination of the two anti-nutrients abolished their inhibitory activity. The authors concluded that the effect of individual anti-nutrients may not necessarily relate to the effects observed upon consumption of the mixture of anti-nutrients as normally observed in foods. Although a majority of these minor components could potentially influence glycemic response, their practical significance has usually been limited by way of being removed or destroyed at various stages of food preparation and consumption.

Recently, there has been a renewed interest in the use of inhibitors of the human a-glucosidases to moderate carbohydrate digestion and its associated insulin response for treatment of non-insulin dependent (type-2)

diabetes. Acarbose, an oligosaccharide formed by strains of the genus Actinoplanes, functions as an inhibitor of both a-amylase and the mucosal a-glucosidases (sucrase-isomaltase and maltase-glucoamylase) (Hiele et al, 1992). It has been effective in the treatment of diabetes as it slows down digestion of disaccharides and starch (Chiasson et al., 1994; Conniff et al., 1994) and is used in some countries to treat diabetes. However, it has frequently been shown to have gastrointestinal side effects due to malabsorption of disaccharides. Starch blockers, purified amylase inhibitors from Great Northern beans (phaseolamin), have also shown promise in glucose homeostatis (Boivin et al., 1988) and are marketed as dietary supplements (Phase 2 Starch Neutralizerâ„¢; Udani et al., 2004). Unlike these blockers that need to be ingested in large quantities (4-6 g per meal) to show significant effect, trestatin (a mixture of complex oligosaccharides produced microbiologically) has been proven to be a potent inhibitor (3-6 mg per 75 g starch) of pancreatic a-amylase in several in vitro and in vivo studies (Golay et al., 1991). Glycemic and insulinemic responses in healthy and diabetic volunteers were moderated after consumption of breads containing tres-tatin added during processing without serious gastrointestinal side effects. Although the use of enzyme inhibitors has shown promise, further studies are needed to evaluate the efficacy of these inhibitors after addition to starch-based processed foods, the dose-response relationship, and, more importantly, long-term tolerance and side effects of the use of such inhibitors.

The presence of organic acids or acid salts, such as those produced during sourdough fermentation or added during baked food preparation, has been seen to influence glycemic and insulinemic responses (Liljeberg & Bjorck, 1996, 1998; Liljeberg et al., 1995). For example, consumption of sourdough bread (with lactic acid produced during fermentation) or breads with added calcium lactate, or sodium propionate, significantly reduced glycemic and insulinemic indexes as compared with wholemeal bread in the absence of these acids (Liljeberg et al., 1995). Intake of bread with a high concentration of sodium propionate not only lowered postprandial blood glucose and insulin responses, but also significantly prolonged the duration of satiety compared with all other breads. In vitro digestion of these breads, however, showed a significant decrease in the rate of amylase digestion only in bread containing lactic acid. The authors concluded that the effect of acid salts such as sodium propionate on metabolic responses and satiety was due to effects other than starch hydrolysis, such as delayed gastric emptying (Liljeberg & Bjorck, 1996). Similar effects were observed upon addition of vinegar to a starchy meal (Liljeberg & Bjorck, 1998). The potential of fermentative processes or processes that incorporate organic acids to improve the nutritional features of carbohydrates need to be considered.

Recent studies in our laboratory (M. Venkatachalam, G. Zhang and B. R. Hamaker, unpublished data, 2005) show that entrapment of starch in an alginate-calcium ion biopolymer matrix effectively creates a barrier

(cooked in the entrapped form) to digestion by amylases and provides a slow glucose release profile. Scanning electron microscope images of the cooked starch microspheres showed that the gelatinized starch trapped in the biopolymer matrix represents a highly dense food form that is gradually digested by the amylases from the periphery towards the center of the sphere. Various factors - including biopolymer type (alginate or blend of alginate with other polymers, such as gellan gum, chitosan, or carrageenan), biopolymer concentration, microsphere size, and calcium ion concentration - could be used to obtain biopolymer-entrapped starches with desired slow-digestion profiles. Such microspheres not only lowered glycemic response (as indicated by initial clinical studies), but may also serve as novel starch ingredients providing extended release of glucose in food products.

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