A comparison between an in vitro method to determine carbohydrate digestion rate and the glycemic response in young men

OBJECTIVE: This study examines the relationship between the results of in vitro determinations of carbohydrate digestion rates and the glycemic index. SUBJECTS: Ten healthy non smoking men, aged between 21 and 24-y-old, were selected to participate in the study. METHODS: Six different meals with similar levels of carbohydrates were assayed at breakfast in ten subjects and blood samples were obtained at 0, 15, 30, 60 and 120 min to determine glucose levels. The Englyst's enzymatic method was used in the in vitro study. The six meals were based in cereals (rice or spaghetti); legumes (lentil soup and beans with spaghetti); and potato (potato stew with meat and vegetables). RESULTS: The meals showed different glycemic indices: rice and spaghetti based meals had lower values (mean value 31.4 and 42, respectively); the intermediate values corresponded to lentil soup and the bean dish (49.3, and 76.8 respectively) and the higher glycemic index was shown by the potato based meal (82). CONCLUSIONS: A significant correlation was observed when the ratio of rapid carbohydrate digestion rate and the lente carbohydrate digestion rate was correlated with the glycemic index of the meals, but not when only the rapid carbohydrate digestion rate was considered. These results demonstrate a useful, simple and inexpensive method to estimate the biological response of high carbohydrate meals.     

The effect of various thermal processes on the glycemic response to whole grain wheat products in humans and rats

The effects of different thermal processes used to produce ready-to-eat cereals on the glycemic response to whole grain wheat were investigated in rats. The metabolic response to drum dried flour, which constitutes the major component in instant gruel and porridge, was also studied in healthy human subjects. Boiled flour was used for comparison. The degree of starch gelatinization and rate of starch hydrolysis in vitro were also measured. Incompletely gelatinized steam flaked and dry autoclaved products were digested more slowly in vitro and elicited lower glucose responses in rats compared with completely gelatinized drum dried, extrusion cooked or boiled samples. The initial glycemic response in rats was closely related to the rate of starch hydrolysis in the pepsin/alpha-amylase assay (r = 0.91, P less than 0.04). When pepsin was omitted, no significant correlation was obtained. The peak glucose, insulin and C-peptide responses in humans after breakfast meals of porridge prepared from drum dried flour and from boiled flour were similar, whereas the rate of depression of the glucose curve was more rapid after consuming drum dried porridge. It is concluded that the glycemic response to wheat products is affected by the processing conditions used. The more severe the processing conditions, the more rapid the digestion of starch.     

An integrative in vitro approach to analyse digestion of wheat polysaccharides and the effect of enzyme supplementation

The digestion of polysaccharides from the wheat cultivars Caphorn and Isengrain was investigated, and the efficiency of an enzyme preparation was tested using the TNO gastrointestinal model (TIM-1). The apparent digestibility (AD) of carbohydrates was determined based on the measurement of organic matter (OM), total monosaccharides, reducing ends (RE) and end products (EP: glucose, maltose and xylobiose). The AD of the OM from Caphorn and Isengrain measured using caecectomised cockerels did not differ from that measured using TIM-1: 72.0 (SD 2.6) v. 70.6 (SD 0.6) % for Caphorn (P = 0.580) and 73.0 (SD 2.3) v. 71.1 (SD 1.9) % for Isengrain (P = 0.252). After the 6 h TIM-1 digestion, 41.4-58.9 % of the OM, RE and EP were recovered from the jejunal compartment and 18.3-27.1 % from the ileal compartment, while ileal deliveries and digestive residues constituted the remainder. A commercial enzyme cocktail tested at 0.2 μl/g of wheat improved TIM-1 digestibility of Caphorn and Isengrain polysaccharides: 3.9 % (P = 0.0203) and 3.4 % (P = 0.0058) based on the OM; 9.7 % (P < 0.0001) and 3.1 % (P = 0.031) based on the total glucose; 47.2 % (P < 0.0001) and 14.2 % (P = 0.0004) based on the RE, respectively. The enzyme cocktail improved the release of the EP for Caphorn (3.8 %, P = 0.008) but not for Isengrain ( ? 0.8 %, P = 0.561). The higher efficiency of the enzyme supplementation on the digestion of Caphorn polysaccharides compared with Isengrain seems to be linked to the higher soluble carbohydrate contents and/or less ramified arabinoxylan of Caphorn.     

The effect of three snack bars on glycemic response in healthy adults

Many consumers prefer convenient, portable, and preportioned snack foods. Foods with a lower glycemic response are associated with reduced risk for chronic disease. The glycemic index and glycemic load of three nationally available snack bars were determined. Ten subjects, with mean age (+/-standard deviation) of 29+/-7 years and mean body mass index (+/-standard deviation) of 25.3+/-3.2, were tested on four occasions on nonconsecutive days. After an overnight fast, subjects consumed 50 g of available carbohydrate as a glucose beverage or as a portion of one of three bars: SmartZone nutrition bar (The Hershey Co, Hershey, PA), ZonePerfect nutrition bar (Abbott Laboratories, Abbott Park, IL), or SlimFast meal bar (SlimFast Foods Co, West Palm Beach, FL). Blood glucose was tested at 0, 15, 30, 45, 60, 90, and 120 minutes after consumption. Incremental area under the glucose response curve was calculated for each test bar and compared with that of the glucose beverage to determine glycemic index. The glycemic index (+/-standard error of the mean) for SmartZone was 10.9+/-3.9 and was significantly less (P<0.05) than that of ZonePerfect (43.7+/-7.3) or SlimFast (63.8+/-13.0). The glycemic loads (+/-standard error of the mean) for the SmartZone (2.0+/-0.7) and ZonePerfect (8.3+/-1.4) bars were significantly less (P<0.05) than the glycemic load of the SlimFast bar (21.1+/-4.3). Although the long-term impact of snack foods with a lower glycemic load requires further research, the SmartZone and ZonePerfect bars provide a lower glycemic response for consumers.     

The effect of phytic acid on in vitro rate of starch digestibility and blood glucose response

The effect of phytic acid on starch digestibility was studied in vitro and related to the blood glucose response (glycemic index) in healthy volunteers. The glycemic index was found to correlate negatively with the phytic acid content of the food tested. In vitro digestion studies involving human saliva at physiological pH and temperature showed that in the presence of sodium phytate (equivalent to 2% phytic acid based on the starch portion), the rate of digestion of raw wheat starch was reduced significantly by 50%. This was reversed by the addition of calcium which is known to complex phytic acid. Similarly, when sodium phytate was added to unleavened bread, its digestibility was also decreased and in feeding trials high phytate bread produced a flattened blood glucose response.     

Degree of starch gelatinization, digestion rate of starch in vitro, and metabolic response in rats

Glycemic response after ingestion of starchy foods varies. Starch in many common ready-to-eat foods is only partly gelatinized. In view of this, the relationships among degree of starch gelatinization, in vitro digestion rate, and in vivo metabolic response in rats were studied. Wheat starch with different degrees of gelatinization was used in the experiments. Plasma glucose and insulin responses as well as the rate of in vitro hydrolysis with alpha-amylase were strongly correlated to the degree of starch gelatinization (r = 0.88, r = 0.90, and r = 0.96, respectively). Plasma glucose and insulin responses were also positively correlated to the rate of hydrolysis with alpha-amylase in vitro (r = 0.98 and r = 0.76, respectively). These results suggest that the degree of starch gelatinization is an important determinant both for the rate of starch hydrolysis in vitro and for the metabolic response in vivo.     

Phytic acid and calcium affect the in vitro rate of navy bean starch digestion and blood glucose response in humans

Carbohydrate foods that are slowly digested appear beneficial in the management of diabetes and hyperlipidemia. This study determined the effect of endogenous and added phytic acid as well as Ca on the in vitro rate of starch digestion and in vivo blood glucose response to navy bean flour, prepared as unleavened bread. Removal of phytic acid from and addition of Ca to navy bean flour increased the starch digestion in vitro and raised the glycemic response in vivo while readdition of phytic acid to dephytinized flour produced the opposite effect. Carbohydrate malabsorption assessed by breath H2 measurement related negatively to glycemic response but the changes observed were much lower than the changes in glycemic response. This study confirmed the role of both added and endogenous phytic acid in slowing the in vitro rate of starch digestibility and in vivo glycemic response to legumes and the ability of Ca to modify this effect.     

The glycemic response does not reflect the in vivo starch digestibility of fiber-rich wheat products in healthy men

Starchy food products differ in the rate of starch digestion, which can affect their metabolic impact. In this study, we examined how the in vivo starch digestibility is reflected by the glycemic response, because this response is often used to predict starch digestibility. Ten healthy male volunteers [age 21 ± 0.5 y, BMI 23 ± 0.6 kg/m2 (mean ± SEM)] participated in a cross-over study, receiving three different meals: pasta with normal wheat bran (PA) and bread with normal (CB) or purple wheat bran (PBB). Purple wheat bran was added in an attempt to decrease the rate of starch digestion. The meals were enriched in 13C and the dual isotope technique was applied to calculate the rate of appearance of exogenous glucose (RaE). The 13C-isotopic enrichment of glucose in plasma was measured with GC/combustion/isotope ratio MS (IRMS) and liquid chromatography/IRMS. Both IRMS techniques gave similar results. Plasma glucose concentrations [2-h incremental AUC (iAUC)] did not differ between the test meals. The RaE was similar after consumption of CB and PBB, showing that purple wheat bran in bread does not affect in vivo starch digestibility. However, the iAUC of RaE after men consumed PA was less than after they consumed CB (P < 0.0001) despite the similar glucose response. To conclude, the glycemic response does not always reflect the in vivo starch digestibility. This could have implications for intervention studies in which the glycemic response is used to characterize test products.     

In vitro starch digestion correlates well with rate and extent of starch digestion in broiler chickens

Current feed evaluation systems for poultry are based on digested components (fat, protein and nitrogen-free extracts). Digestible starch is the most important energy source in broiler chicken feeds and is part of the nitrogen-free extract fraction. Digestible starch may be predicted using an in vitro method that mimics digestive processes in the gastrointestinal tract of broiler chickens. An experiment was designed to use this method for predicting site, rate and extent of starch digestion in broiler chickens. In vitro starch digestion was studied in 12 experimental diets differing in starch sources. These diets were also used in a digestibility trial with broiler chickens. Correlations between in vitro and in vivo starch digestion were calculated. Starch digestion after 2 h incubation correlated well with in vivo starch digestion in the first half of the small intestine (r = 0.94). A 4-h incubation period resulted in a good correlation between in vitro starch digestion and ileal starch digestion (r = 0.96). In vitro starch digestion rate (h(-1)) correlated well with in vivo starch digestion rate (r = 0.87). In vitro starch digestion of individual starch sources was additive. It appeared that legume seeds and waxy corn contained two starch fractions, which were digested at different rates. We conclude that starch digestion rate in broiler chickens is well predicted by the in vitro method.     

The effect of wheat proteins on rat behaviour and the effect of naloxone hydrochloride on this response

When wheat proteins are fed to rats, a different form of behaviour pattern is observed by computer studies to that of unfed rats. The animals show a higher response output. After an injection of naloxone hydrochloride, this enhanced pattern is cancelled out suggesting that naloxone inhibits this behaviour pattern. It is suggested that a blocking effect takes place at either intestinal or brain receptor sites.     

The effect of beta-glucan on the glycemic and insulin index

OBJECTIVE: To determine the effects of oat products with increasing beta-glucan content on the glycemic (GI) and insulin indexes (II) of oat products, and to establish the effect of physical properties of beta-glucan on these physiological responses. DESIGN: Test group (n=10) randomly attended to three glucose tolerance tests and glycemic response tests for four oat bran products. SETTINGS: Functional Foods Forum and the Department of Food Chemistry, University of Turku, and the Department of Food Technology, University of Helsinki. SUBJECTS: One male and nine female volunteers were recruited from university students and staff, and all completed the study. INTERVENTIONS: GI and II of different products were calculated for each subject using the average of parallel glucose tolerance tests and the subsequent glycemic/insulinemic responses for each product. Average indexes for products were calculated according to the individual data. RESULTS: The glycemic responses to oat products with increasing amounts of beta-glucan had lower peak values than the reference glucose load. The amount of extractable beta-glucan had a high correlation between the glycemic and insulinemic response. CONCLUSION: In addition to the total amount of beta-glucan in oat products, the amount of extractable beta-glucan in oat products explains the magnitude of the decrease in glycemic responses to carbohydrate products.     

The polyphenol-rich baobab fruit (Adansonia digitata L.) reduces starch digestion and glycemic response in humans

The baobab fruit (Adansonia digitata L.) is found throughout regions of Africa and is becoming increasingly recognized for its high nutrient and polyphenol content. Polyphenols have been beneficial for their effects on reducing the glycemic response (GR) and for improving various other metabolic parameters. Based on previous research, it was hypothesized that the baobab fruit extract would reduce starch digestion in vitro and would show potential for reducing the GR and for increasing satiety and diet-induced thermogenesis in humans. Six extracts of baobab from 6 different locations in Africa were measured for their antioxidant and polyphenol content using the ferric ion–reducing antioxidant power and the Folin-Ciocalteu methods, respectively. Baobab extract was baked into white bread at different doses to determine the optimal dose for reducing starch breakdown and sugar release from white bread after an in vitro digestion procedure. In vivo, baobab extract was consumed in solution at both a low-dose (18.5 g) and a high-dose (37 g) aqueous drink in 250 mL of water along with white bread, and resulting GR, satiety, and postprandial energy expenditure were measured. All extracts in this study were shown to be good sources of polyphenols. Baobab fruit extract added to white bread at 1.88 % significantly (P < .05) reduced rapidly digestible starch from white bread samples. In vivo, the baobab fruit extract at both low and high doses significantly (P < .05) reduced GR, although there was no significant effect on satiety or on energy expenditure.     

Particle size of wheat, maize, and oat test meals: effects on plasma glucose and insulin responses and on the rate of starch digestion in vitro

When normal volunteers ate isocaloric wheat-based meals, their plasma insulin responses (peak concentration and area under curve) increased stepwise: whole grains less than cracked grains less than coarse flour less than fine flour. Insulin responses were also greater with fine maizemeal than with whole or cracked maize grains but were similar with whole groats, rolled oats, and fine oatmeal. The peak-to-nadir swing of plasma glucose was greater with wheat flour than with cracked or whole grains. In vitro starch hydrolysis by pancreatic amylase was faster with decreasing particle size with all three cereals. Correlation with the in vivo data was imperfect. Oat-based meals evoked smaller glucose and insulin responses than wheat- or maize-based meals. Particle size influences the digestion rate and consequent metabolic effects of wheat and maize but not oats. The increased insulin response to finely ground flour may be relevant to the etiology of diseases associated with hyperinsulinemia and to the management of diabetes.     

Wheat bran and wheat germ: effect on digestion and intestinal absorption of dietary lipids in the rat

We investigated the effects of fiber-rich wheat bran and wheat germ on dietary fat and cholesterol assimilation. Rats were given a test meal containing [14C]triolein and [3H]cholesterol. After various digestion periods, addition of the wheat fractions (10% of meal solids) did not modify the lipid gastric emptying rate. Gastric and intestinal triglyceride lipolysis was significantly reduced when wheat fractions were present. The mucosal uptakes of [14C]lipids and [3H]cholesterol were significantly modified by the wheat fractions after 1 and 2.5 h. No shift in the site of intestinal absorption and no change in the distribution of labeled lipid in the intestinal mucosa was observed. Plasma [14C]lipids and [3H]cholesterol were significantly decreased by both wheat fractions whereas these increased cecal accumulation of dietary lipids and cholesterol. Thus wheat bran and wheat germ alter fat and cholesterol processing in rats. A mechanism of action accounting for the data observed in proposed.     

Low glycemic response to traditionally processed wheat and rye products: bulgur and pumpernickel bread

To look at the effect of processing wheat and rye on blood glucose responses with special reference to bulgur and pumpernickel bread, groups of 9-12 Noninsulin-dependent (NIDDM) and 5-6 Insulin-dependent diabetic volunteers (IDDM) were fed test meals containing 50 g carbohydrate portions of four wheat and three rye products. Glycemic indices for IDDM and NIDDM combined, calculated as the incremental area under the blood glucose response curve, where white bread = 100, demonstrated values of 96 +/- 5 for wholemeal wheat bread, 89 +/- 6 for wholemeal rye bread, 78 +/- 3 for pumpernickel bread, 65 +/- 4 for bulgur, 63 +/- 6 for whole wheat kernels and 48 +/- 5 for whole rye kernels. Results for IDDM and NIDDM were similar (r = 0.96, p less than 0.01). It is concluded that traditional processing of cereals, such as parboiling (bulgur) or the use of wholegrains in bread (pumpernickel) may result in the low GI value associated with the unmilled cereal. Cereal foods processed in these ways may form a useful part of the diet where a reduction in postprandial glycemia is required.     

Rapidly available glucose in foods: an in vitro measurement that reflects the glycemic response

BACKGROUND: A chemically based classification of dietary carbohydrates that takes into account the likely site, rate, and extent of digestion is presented. The classification divides dietary carbohydrates into sugars, starch fractions, and nonstarch polysaccharides, and groups them into rapidly available glucose (RAG) and slowly available glucose (SAG) as to the amounts of glucose (from sugar and starch, including maltodextrins) likely to be available for rapid and slow absorption, respectively, in the human small intestine. OBJECTIVE: We hypothesize that RAG is an important food-related determinant of the glycemic response. DESIGN: The measurement of RAG, SAG, and starch fractions by an in vitro technique is described, based on the measurement by HPLC of the glucose released from a test food during timed incubation with digestive enzymes under standardized conditions. Eight healthy adult subjects consumed 8 separate test meals ranging in RAG content from 11 to 49 g. RESULTS: The correlation between glycemic response and RAG was highly significant (P < 0.0001) and a given percentage increase in RAG was associated with the same percentage increase in glycemic response. After subject variation was accounted for, RAG explained 70% of the remaining variance in glycemic response. CONCLUSIONS: We show the significance of in vitro measurements of RAG in relation to glycemic response in human studies. The simple in vitro measurement of RAG and SAG is of physiologic relevance and could serve as a tool for investigating the importance of the amount, type, and form of dietary carbohydrates for health.     

The effect of plant sterol-enriched turkey meat on cholesterol bio-accessibility during in vitro digestion and Caco-2 cell uptake

This study evaluated the effect of a plant sterol-enriched turkey product on cholesterol bio-accessibility during in vitro digestion and cholesterol uptake by Caco-2 monolayers. Turkey products, one plant sterol-enriched (PS) and one plant sterol-free (C), were produced in an industrial pilot plant. Before simulated digestion, matrices were spiked with cholesterol (1:5 weight ratio of cholesterol to plant sterol). Plant sterols were included at a concentration equivalent to the minimum daily intake recommended by the European Food Safety Authority (EFSA) for cholesterol lowering. After simulated digestion, the percentage of cholesterol micellarization and uptake by Caco-2 cells in the presence of PS meat were measured. Compared to C meat, PS meat significantly inhibited cholesterol micellarization on average by 24% and Caco-2 cell accumulation by 10%. This study suggests that plant sterols in meat can reduce cholesterol uptake by intestinal epithelia and it encourages efforts to make new PS-based functional foods.     

The inhibition of nutrient digestion by wheat pentosans.

Two pentosan-rich fractions (water-extractable, WEP, and alkali-extractable, AEP) were isolated from a wheat milling by-product. When both WEP and AEP were added to a commercial-type broiler diet the apparent metabolizable energy (AME), nitrogen retention, feed utilization and growth of broilers were significantly (P less than 0.001) depressed. These depressions were closely correlated (P less than 0.001) to the level of pentosans in the diets. At the highest level of inclusion of pentosans (equivalent to 40 g arabinoxylans as AEP/kg) the ileal digestibilities of starch, protein and lipid were decreased by 14.6, 18.7 and 25.8% respectively.