1. IntroductionPectic polysaccharides, or pectins, form a family of polysaccharides in the cell walls of higher plants [1]. The major role of pectins in planta involves maintaining mechanical properties (for example, the rigidity of a stem) as a component of the cellulosic matrices which control the structure and properties of plant tissues [2]. The pectin contents in many fruits and vegetables range from 0.1% to 2.5% on a wet basis and a daily consumption of 30 g of pectins are recommended by the European Food Safety Authority for benefits such as reduction in post-prandial glycemic responses, maintenance of normal blood cholesterol concentrations and increase in satiety (the last leading to a reduction in energy intake) [3].Pectins have galacturonic acid (GalA) as their main component, and as many as 16 other monosaccharides as co-monomers [4]. A schematic representation of pectin structural components is shown in Figure 1. Complex pectins contain several constituent pectic blocks, namely homogalacturonan (HG), rhamnogalacturonan-I (RG-I), rhamnogalacturonan-II (RG-II) and xylogalacturonans (XGA); the last is present only in a few plants [1]. HG is a linear polysaccharide formed by (1→4)-α linked D-galactopyranosyluronic acid (GalpA) residues. Carboxyl groups in HG are largely methyl-esterified when originally synthesized by the Golgi apparatus, and are then gradually de-esterified in vivo by pectinesterase during the growth of the plant. Based on the degree of esterification (DE), pectins can be divided into low methoxyl (LM) pectins (DE 50%). The backbone of RG-I is the repeating disaccharide unit, [→4)- α -D-GalpA-(1→2)- α -L-Rhap-(1→]. The side chains of RG-I are oligosaccharides composed mainly of neutral α-L-arabinofuranosyl (Araf), β-D-galactopyranosyl (Galp) or acidic β-D-glucuronosyl (GlcpA) residues [5,6]; these side chains are attached to the RG-I backbone via the C-4 of Rhap. Both rhamnogalacturonan-II and xylogalacturonans can be classified as substituted galacturonans [6], because their backbones are HG. Pectins from different sources, locations and growth conditions of plants and processing methods can differ in monosaccharide composition, molecular size/weight (both average size or weight, and size or weight distribution), degree of methyl esterification (DE) and degree of acetylation (DA), thus making the molecular structure of pectins very complex. Although sharing the same name, pectic polysaccharides with diverse compositions or structures can differ significantly in physicochemical and physiological properties as well as functions. For example, LM-pectins form a gel in the presence of divalent ions (mostly Ca2+) via coordination bonds [7], while HM-pectins gel via a combination of hydrogen bonds and hydrophobic interactions [8]. Pectins with higher DE values and lower molecular weights generally have a higher water solubility [2,8]. These examples show that, when studying a pectin’s effects, one should pay special attention to the molecular structure of that pectin and the status of pectin’s chemical environment (pH values, ionic strength, etc.). Starch provides at least half of the average daily caloric intake for most people world-wide [9]. After ingestion, starch is mainly hydrolyzed by α-amylases and intestinal brush border enzymes (i.e., sucrase, maltase and isomaltase) to glucose, which is absorbed in the intestine, leading to an increase in postprandial blood glucose levels [10]. A positive relationship has been proven between postprandial glycaemia level and obesity [11] and type II diabetes [12,13,14,15,16]. These findings indicate that controlling starch digestion is a promising approach to the prevention and management of these diseases. The digestibility of starch in food is mainly affected by the characteristics of the starch (e.g., molecular structure), the physicochemical conditions in the food matrix, the food processing method and the presence of other food components such as dietary fibers [17,18]. Several reviews have summarized the effects and mechanisms whereby dietary fiber can regulate the metabolism of carbohydrates in food to result in a healthier status [10,19,20,21,22].

Pectin has been found to regulate starch digestion, compared to other dietary fibers, pectins have some advantages in regulating starch digestion, including (1) ubiquity in food and wide applications in food industry, (2) reliable safety, (3) abundant health benefits, and (4) versatile molecular structures with the potential for tailor-made properties. With these advantages, people have studied pectin’s effects on starch in vitro digestion and in vivo utilization for decades, and from human subjects to a range of animals.

However, there is no recent review summarizing pectin’s effects on starch digestion (i.e., passage of amylolysis products through the gastrointestinal (GI) tract, absorption of glucose and the resulting postprandial blood glucose level), making the findings in this field difficult to integrate. Additionally, many previous studies ignored the importance of pectin’s structure-property relationships, leading to ambiguous or misleading conclusions. To help solve these problems, in this review we discuss how pectin affects the in vivo utilization of starchy food.

Generally, pectins regulate the digestion of starch by any or all of (1) inducing physicochemical changes in digesta, (2) inhibiting enzyme activities of amylases, (3) interacting with starch substrates, (4) being structural components in cell walls and (5) causing a series of physiological responses. The review concludes with discussion of the characteristics of pectins as modulators of starch digestion, some concluding remarks and suggestions on future perspectives.

4. The Characteristics and Future Perspectives of Pectin’s Effects on Starch Digestion

From the above mentioned studies about pectins as nutritional additives in food targeted to regulate starch digestion, we can find that the addition of pectin to digesta can normally regulate the in vivo BGL of subjects from different species and health conditions. Pectins share some similarities to other DF nutritional additives: for example (1) causing an increase in viscosity is one of the major reasons to their changes and (2) the effects are normally concentration-dependent.

Nevertheless, pectins also exhibit unique characteristics: some functional properties are determined by their complex and specific molecular structures. Many publications have clearly confirmed that pectin substrates with various structural parameters (DE values, monosaccharide compositions and molecular sizes) exhibit different extents of retardation of starch digestion. In addition, it was also observed that starch-containing digesta with pectin could have ideal digestibility [29,40] and are less dependent on viscosity [39] compared to other DFs (for example, pullulan, xanthan gum, guar gum and konjac glucomannan), suggesting pectic fractions take effects via more than one approaches simultaneously. The structure-based multi-functionality makes pectin stand out from other nutritional additive fibers, because a high viscosity in food can easily cause some unfavorable effects, including vomiting [26], abdominal discomfort [26,32], flatus [24] and diarrhea [104], as well as low food palatability [25,32,49,105]. Increasing the viscosity, either differing in concentration or molecular structure, is pectin’s dominant effect on digesta and a major reason for its regulatory effects. However, compared to other dietary fibers, effects of pectins do not simply rely on increasing viscosity of food or chyme, thus showing the multiple-functional advantages of pectins. Any adverse effects can be avoided or reduced by using pectins with other significant functional properties in food. For example, pectins with certain structural characteristics show significant digestion regulation ability without causing a high viscosity, although more in-depth studies are necessary in their practical applications [80]. The multiple functionality of pectins also gives flexibility in food production: e.g., pectins can affect both starch gelatinization and retrogradation, indicating that pectins can be added to starchy food either before or after thermal treatment, while both processes are able to regulate the digestibility of starchy foods. The structure-based multi-functionality of pectins in affecting starch digestion showed great potentials in future applications. Pectic fractions with different molecular structure, although generically classified as pectins, could result in very different properties under the same conditions. One disadvantage in many previous studies has been the lack of structural characterization of the pectin samples used, meaning that the results obtained from these studies cannot be used to understand which property or molecular domain of pectin functions were determinant. Although pectin’s effects on starch digestion and regulation on BGL have often been studied, the absence of extensive structure-property relationships leaves much to be done. Comparisons on functions from structurally different pectins are needed. To achieve that goal, firstly, structural information of the selected pectin sample should be obtained. Some basic parameters, like DE, monosaccharide composition and molecular sizes are essential, while more detailed data including acetylation degree, sugar linkage composition and side-chain composition would be beneficial. The experimental methods for investigating pectin’s effects on starch digestion could be standardized. For in vitro experiments, impracticable conditions (e.g., very low enzyme quantity or unacceptably high food digesta viscosity) should be replaced by methods (both apparatus, e.g., [106] and protocols [107,108,109,110,111]) which better simulate in vivo conditions. After obtaining some important structure-property (amylase inhibitory and starch granule binding, etc.) relationships, pectins with ideal molecular structures from either isolation from natural sources or chemical/enzymatic modifications could be further used for in vivo tests and subsequent use in foods. 5. Conclusions

Regulation of the digestion process of starch in food is an effective and promising way to control postprandial blood-glucose levels, which are closely related to the prevention and treatment of many pandemic health issues, such as obesity, hyperglycemia and type II diabetes. The presence of dietary fiber in food can significantly affect the digestibility of starch. Among the DFs, pectin is of particular interest, because of (1) its ubiquity in plant foods, (2) its versatile functions as an artificial food additive, (3) safety and reliability, and (4) various health benefits. Pectins have considerable complexity in molecular structure. Many researchers have studied the effects of pectin on starch digestion and the changes in in vivo blood glucose level brought by pectin in food, and most of these researches demonstrated that the presence of pectin could retard the digestion process or flatten the BGL.

There are three major reasons for pectin’s decreasing effect on digestibility and BGL, as follows. (1) Pectin would normally increase the viscosity of digesta or food chyme, thus inducing a series of physicochemical and physiological changes; (2) pectin could interact with amylases and inhibit their activity; and (3) the ingestion and fermentation of pectin in intestines also regulate digestion-related in vivo physiological responses. In vivo, an increased viscosity of chyme or food bolus could prolong the transit time of food in the GI tract, which would delay gastric emptying [26,34,35,36]. Increased intraluminal viscosity also slows the absorption of nutrients in the intestine and affects the production of digestion-related hormones. Pectin gel that sticks on the surface of the GI tract can also lead to histological or morphological changes in the tract, affecting the intestinal absorption ability and amylase activities. Besides influencing amylase activity by differentiating the growth of enterocytes, pectins have also been found to inhibit the activities of starch-digestion related enzymes. Both pectin and amylase have abundant functional groups and can thus interact with each other via non-covalent intermolecular interactions. The fermentation of pectin in the intestine could also contribute to BGL regulation. Pectin, being an indigestible fiber, can only be fermented by gut microbiota, and thus the presence of pectin could (1) physically stimulate the distal intestines, (2) influence the growth and balance of microbiota and (3) promote the production of SCFAs during fermentation. The SCFAs partially influence the content of plasma digestive hormones, which are responsible for appetite, satiety and GE, and thus indirectly influence BGL fluctuations and in vivo digestion of food.