DEVELOPMENT OF CARBOHYDRATE FOODS AND PREVENTION OF TYPE 2 DIABETES

11.9 DEVELOPMENT OF CARBOHYDRATE FOODS AND PREVENTION OF TYPE 2 DIABETES

As already demonstrated above, a number of food factors affect the rate of glucose delivery and uptake in the human body. Some are related to the characteristics of the raw materials, and others to the processing conditions. As starch is a major carbohydrate source in the diet, special emphasis should be on factors controlling starch digestibility. The monomeric composition of the carbohydrate moiety also plays a role, and the GI of pure, low molecular weight carbohydrates decreases in the following order: glucose > sucrose > lactose > fructose. However, recently a cautionary note was given to the excessive use of fructose, and it was suggested that

the primary dietary carbohydrates should be free glucose and starch. 12 The higher the levels of unavailable and complex carbohydrates in the diet, the better. Also, interactions with other food components, such as protein, lipids, and bioactive compounds, play a role in determining carbohydrate digestibility, and, e.g., arginine

has been shown to attenuate the rise of blood glucose. 18 This is an area where research hitherto is rather limited.

11.9.1 U SE OF D IETARY F IBER

As clearly shown on previous pages, dietary fiber is of utmost relevance when designing foods for improved glycemic control. It not only reduces the glycemic

load, but may also influence absorption of glucose through creating viscous condi- tions in the small intestine, or influence glucose metabolism through the formation of short-chain fatty acids in colonic fermentation. Dietary fiber also may change food structure in such a way that it retards carbohydrate digestibility and release in the gastrointestinal tract. Dietary fiber is a group name for various poly- and oli- gosaccharides, lignin, and associated substances sharing the property of not being

absorbed in the small intestine. 5 Dietary fiber content of a food may be increased

Functional Food Carbohydrates

either by choosing naturally dietary fiber-rich raw materials or by using commercial dietary fiber preparations as ingredients. Considering the large diversity of various dietary fiber carbohydrates available, it is obvious that care must be taken to achieve the desired physiological properties in the food produced.

Hydrolytic enzymatic reactions, catalyzed by endogenous and added enzymes, as well as mechanical and thermal energy, are the major causes for changed dietary fiber properties in food processing. 125–127 The major change in dietary fiber polysac- charides during processing is often depolymerization, leading to increased solubili- zation, but also reduction of viscosity. As discussed above, viscosity is an important determinant of soluble dietary fiber in retarding glycemic responses, 76,77,87 so careful processing is needed. Another challenge is to produce foods with a high enough dosage of soluble fiber to reach levels attenuating blood glucose levels. The dosages needed were discussed above.

11.9.2 P OTENTIAL OF B IOACTIVE C OMPONENTS

Bioactive substances include a range of secondary plant metabolites that can evoke physiological, behavioral, or immunological effects. They often occur in the same parts of plants as dietary fiber and are also sometimes referred to as co-passengers or associated compounds of dietary fiber. There are many possible ways for bioactive substances to affect the metabolism of glucose, and hence they could provide one means of modulating glycemic responses of foods in the future. Much more research is needed, however, before practical applications are expected. Inhibition of starch hydrolysis in the small intestine is one of the rate-limiting steps where these com- pounds could play a role. Some polyphenols have been shown to inhibit amylase, and hence may indirectly affect glucose and insulin levels. Diacetylated anthocyanin was shown to have α-glucosidase inhibitory activity suppressing postprandial glu- cose in rats. 128 In rats, Touchi extract was shown to have inhibitory activity against rat intestinal α-glucosidase, reducing the postprandial blood glucose and insulin levels after ingestion of cooked rice in four diabetic subjects. 129 Intestinal glucose uptake, mainly performed by the sodium-dependent glucose transporter (SGLT1), is inhibited by, e.g., green tea polyphenols, especially polyphenols with galloyl residues, which could possibly play a role in dietary glucose uptake in the intestinal tract. 130 Tea has also been shown to increase insulin activity by about 15-fold in vitro in an epidymal fat cell assay. 131 Insulin-potentiating food factors are another interesting opportunity for future food design.

11.9.3 U SE OF P REDICTIVE I N V ITRO M ETHODS

The rate of starch digestion can be evaluated in vitro using alimentary amylolytic enzymes. 132,133 An in vitro method based on chewing has been developed 134 to predict the metabolic responses to different starchy foods. Starch in the test foods is chewed in vivo prior to in vitro hydrolysis with pancreatic α-amylase in a dialysis tube for 3

h. The hydrolysis index (HI) is calculated as the area under the hydrolysis curve of the test food as a percentage of the reference (white wheat bread). A good correlation between the GI and HI (r = 0.877, p < 0.001) and between insulin index (II) and HI

The Role of Carbohydrates

(r = 0.647, p < 0.01) was obtained for 17 different foods, but it was noted that the method is not suitable for ranking foods having differences in the rate of gastric emptying. 134 Another similar in vitro starch hydrolysis method was also shown to have

a good correlation with the in vivo GI assay. 135 Starch hydrolysis in different foods at

90 min correlated well (r = 0.909, p < 0.05) with in vivo glycemic responses and was suggested as a simple way of predicting the GI. Starch has also been classified as rapidly available glucose (RAG) and slowly available glucose (SAG) by using specific analytical methods. 136,137 Gastrointestinal conditions are simulated by using both pepsin and a mixture of amylolytic enzymes, and adjusting pH, temperature, viscosity, and mechanical mix- ing. 136 SAG and fat content together accounted for 73% of the variance in GI, and RAG and protein content together 45% of the variance in II. 137

In vitro methods are an important tool in development of foods with a slower rate of carbohydrate digestion and absorption. Clinical research is always needed to document and understand human responses, but choosing raw materials and espe- cially development of food processing to produce a variety of foods with low GI, GL, and II requires screening methods to choose candidate products for the laborious, slow, and expensive human trials. These methods are especially useful in develop- ment of starchy foods, with large potential variation in rate and extent of starch digestibility and availability.

11.9.4 T AILORING OF F OOD S TRUCTURE

Food structure is one of the key factors affecting glucose and insulin responses. Food structure affects both enzymatic accessibility and the gastric emptying half- time. 6,7,138,139 In stomach, food pieces are subjected to the action of pepsin, acid conditions, and to the vigorous grinding action of gastric motility. Gastric emptying

is affected by particle size. 7 The only exit from the stomach to the small intestine is the poylorus, which allows food pieces of smaller than 2 mm in diameter to exit. The gastric emptying half-time is longer (75 min) for spaghetti than for mashed

potatoes (35 min). 6 Size reduction of foods that will leave the mouth as coherent and large particles will in the stomach take a longer time, and the blood sugar values will increase more gradually.

In whole-grain products and legumes, insoluble fiber in tissue particles retards the rate of starch hydrolysis. The less the tissue structures in cereal grains or

vegetables have broken down during processing, the slower is the rate of starch hydrolysis in the upper intestinal tract. A linear relationship has been reported between the proportion of barley kernels in bread and the glycemic response in humans. 140 When barley kernels were milled to flour, the corresponding whole-meal bread produced equally high glucose and insulin responses, as did the white bread reference product. Similar results have also been shown with wheat, rye, and oats. The kernels in breads and whole-meal rye flour contain integrated cell structures even after baking to bread. 141 It is probable that integrated tissue structures originat- ing from whole-meal flour in breads can decrease enzymatic hydrolysis due to limited accessibility, with no effects on gastric emptying times. The sizes of endosperm and aleurone tissue structures in whole-meal breads are in the range of

Functional Food Carbohydrates

250 μm to 1 mm. These sizes do not influence the gastric emptying times, since particles smaller than 2 mm are easily passed from the stomach without size reduc- tion.

In recent investigations, large differences were observed in the particle size of different breads after mastication and treatments that mimic the stomach phase. 142 Whole-meal rye bread residues remained as coherent, large particles (largest in the range of 2 to 20 mm in diameter), whereas white wheat breads broke down to very small particles (<3 mm in diameter). Also, pasta residues exist in large pieces after treatment mimicking stomach conditions. 142

Some foods, such as high-amylose rice 143,144 and different types of rye breads, 38,71,145 have been shown to decrease insulin responses of healthy subjects, but have a small or no effect on glucose responses. In healthy men, consumption of barley-containing pasta with β-glucan also led to a more blunted insulin response than consumption of wheat pasta, although the plasma glucose did not differ significantly. 146 This probably reflects the good glycemic control of healthy subjects, and suggests that more emphasis should

be put on studying effects on insulin responses instead of only glycemic responses.

Also, in the case of legumes, the tissue integrity and softness of the product seem to be important factors for starch digestibility and glycemic responses. Most legume products prepared by conventional cooking produce low glucose and insulin responses. 132,147,148 Great differences in GI (12 to 74) have been observed between different legume products due to variations in raw materials and the processing. Canning and mechanical disruption especially produce higher GI values. 149,150

The physical state of starch is in itself a key determinant of glycemic response of starchy foods. Native starch granules are digested slowly by α-amylase and, during gelatinization, the in vitro rate of amylolysis increases remarkably. 151 It has been shown with wheat starch gels that the starch hydrolysis rate decreases markedly when amylose leaches out of the granule. 152 It is possible that amylose, when located near the granule surface, also retards hydrolysis of amylopectin. Lower glycemic and insulin indices and a higher content of resistant starch have been reported for a barley bread baked from a high-amylose barley genotype by using long-time/low-temperature baking conditions. 153 Retrograded amylose is found in a cooled, cooked potato, bread, and cornflakes. Resistant starch content can be increased by prolonging the wet stage after cooking and heating or freezing the cooked food.

Enzymes can be used for modification of carbohydrate and protein structure. In the former case, the solubility of arabinoxylans in wheat or rye-based products can

be increased in situ by enzymic transformation of water-unextractable arabinoxylans to water-extractable arabinoxylans by using an endoxylanase with a strong selectivity for hydrolysis of water-unextractable arabinoxylans. 154 Bread-making processes for increased soluble fiber levels have been developed by replacement of 40% wheat flour by naked barley flour, using a Bacillus subtilis endoxylanase with a strong selectivity for solubilizing water-unextractable arabinoxylans to extractable ones. 155 This enzyme is not inhibited by wheat endogenous xylanase inhibitors.

Addition of acid in barley bread baking has reduced the rate of gastric emptying in reference to white wheat breads. 139 This has been shown to be due to the reduced starch digestion rate. 156 Acid in such concentrations will make the bread structure very firm and less porous. Porosity facilitates degradation of food in the mouth. 157

The Role of Carbohydrates

It has been suggested that the presence of lactic acid during heat treatment promotes interactions between starch and gluten, hence reducing starch bioavailability.

In a preliminary study (only five breads), a positive correlation was found between bread hardness and insulin index (unpublished data). Bread hardness can

be increased by cross-linking enzymes, such as transglutaminase. 158 Transglutami- nase increases gluten–fiber interactions and elasticity of dough and, as a result, a

harder and less porous crumb structure will be formed. 157