Enzyme kinetic approach for mechanistic insight and predictions of in vivo starch digestibility and the glycaemic index of foods

BACKGROUND

Starch is a principal dietary source of digestible carbohydrate and energy. Glycaemic and insulinaemic responses to foods containing starch vary considerably and glucose responses to starchy foods are often described by the glycaemic index (GI) and/or glycaemic load (GL). Low GI/GL foods are beneficial in the management of cardiometabolic disorders (e.g., type 2 diabetes, cardiovascular disease). Differences in rates and extents of digestion of starch-containing foods will affect postprandial glycaemia.

SCOPE AND APPROACH

Amylolysis kinetics are influenced by structural properties of the food matrix and of starch itself. Native (raw) semi-crystalline starch is digested slowly but hydrothermal processing (cooking) gelatinises the starch and greatly increases its digestibility. In plants, starch granules are contained within cells and intact cell walls can limit accessibility of water and digestive enzymes hindering gelatinisation and digestibility. In vitro studies of starch digestion by α-amylase model early stages in digestion and can suggest likely rates of digestion in vivo and expected glycaemic responses. Reports that metabolic responses to dietary starch are influenced by α-amylase gene copy number, heightens interest in amylolysis.

KEY FINDINGS AND CONCLUSIONS

This review shows how enzyme kinetic strategies can provide explanations for differences in digestion rate of different starchy foods. Michaelis-Menten and Log of Slope analyses provide kinetic parameters (e.g., K m and k cat /K m ) for evaluating catalytic efficiency and ease of digestibility of starch by α-amylase. Suitable kinetic methods maximise the information that can be obtained from in vitro work for predictions of starch digestion and glycaemic responses in vivo.

Butyrylated starch affects colorectal cancer markers beneficially and dose-dependently in genotoxin-treated rats

Population studies suggest that greater dietary fiber intake may lower colorectal cancer (CRC) risk, possibly through the colonic bacterial fermentative production of butyrate. Butyrylated starch delivers butyrate to the colon of humans with potential to reduce CRC risk but high doses may exacerbate risk through promoting epithelial proliferation. Here we report the effects of increasing dietary butyrylated high amylose maize starch (HAMSB) on azoxymethane (AOM) induced distal colonic DNA damage, cell proliferation, mucus layer thickness and apoptosis in rats. Five groups of 15 rats were fed AIN-93G based diets containing 0–40% HAMSB for 4 weeks then injected with (AOM) and killed 6 hours later. Large bowel total SCFA, acetate and butyrate pools and hepatic portal venous plasma total SCFA, acetate and butyrate concentrations were higher with greater HAMSB intake. Distal colonic epithelial apoptotic index and colonic mucus thickness increased, while DNA single strand breaks decreased dose-dependently with greater HAMSB intake. Colonocyte proliferation rates were unaffected by diet. These data suggest that increasing large bowel butyrate may reduce the risk of CRC in a dose dependent manner by enhancing apoptotic surveillance in the colonic epithelium for damaged cells without promoting the risk of tumorigenesis through increased cell proliferation.

Hydroxypropylation of high-amylose maize starch changes digestion and fermentation-dependent parameters in rats

It was examined whether the physiological effects of high-amylose maize starch (HAMS) are influenced by hydroxypropylation. Rats were fed one of the following three diets: an AIN-93-based diet with waxy maize starch (WMS) as a starch source, or this diet with 150 g of WMS replaced by either HAMS or hydroxypropylated HAMS (HP-HAMS). The activities of amylase in bile-pancreatic juice and sucrose, maltase and isomaltase of the jejunum and ileum were not affected by diet, but the digestibility of HAMS was decreased by hydroxypropylation. The amounts of SCFA in caecal content and H2 excreted in the breath and flatus for HAMS were decreased by hydroxypropylation. Plasma glucagon-like peptide-1 (GLP-1), glucose and insulin concentrations were not affected by diet. On the basis of PCR-denaturing gradient gel electrophoresis (DGGE) profiles, the similarity in caecal bacteria population of the HP-HAMS group and HAMS group was low, but that of the HP-HAMS and WMS groups was high. The amount of caecal IgA was not affected by hydroxypropylation, but those in the HAMS and HP-HAMS groups were greater than that in the WMS group. Plasma and liver concentrations of TAG and cholesterol for HAMS were not affected by hydroxypropylation. These results show that the small intestinal digestibility and fermentation-dependent parameters such as caecal SCFA and H2 productions and caecal bacterial profile of HAMS were affected by hydroxypropylation, but parameters of glucose metabolism such as GLP-1 and insulin, those of lipid metabolism such as plasma TAG and cholesterol and the amount of caecal IgA were not.

The effects of feeding resistant starch on apparent total tract macronutrient digestibility, faecal characteristics and faecal fermentative end-products in healthy adult dogs

The benefits of whole grain consumption have been studied in human subjects, but little research exists on their effects in dogs. The objective of the present study was to test the effects of resistant starch (RS) in the diet of healthy adult dogs. Twelve adult Miniature Schnauzer dogs (eight males, four females; mean age: 3·3 (1·6) years; mean body weight: 8·4 (1·2) kg; mean body condition score: D/ideal) were randomly allotted to one of three treatment groups, which consisted of different amounts of RS supplied in a biscuit format. Dogs received either 0, 10 or 20 g biscuits per d (estimated to be 0, 2·5 or 5 g RS per d) that were fed within their daily energetic allowance. A balanced Latin square design was used, with each treatment period lasting 21 d (days 0-17 adaptation; days 18-21 fresh and total faecal collection). All dogs were fed the same diet to maintain body weight throughout the study. Dogs fed 5 g RS per d had lower (P = 0·03) fat digestibility than dogs fed 0 gRS per d, but DM, organic matter and crude protein digestibilities were not affected. Faecal fermentative end-products, including SCFA and branched-chain fatty acids, ammonia, phenols and indoles, and microbial populations were not affected. The minor changes observed in the present study suggest the RS doses provided to the dogs were too low. Further work is required to assess the dose of RS required to affect gut health.