ReviewSlowly digestible starch – its structure and health implications: a review
Introduction
Starch is the main carbohydrate in human nutrition. It is a major component in plant foods, offering a range of desired technological properties, in particular related to its texturising ability.
The nutritional quality of starch strongly depends on processing and the state of the starch. The glucose release as a source of energy for the body and the timeline of digestion are the major physiological properties of starch. The digestibility in the human small intestine can be modified from a rapid digestion, in particular in starch hydrolysis products, to indigestibility, which is the case in resistant starch (Englyst, Kingman, & Cummings, 1992). One of the main factors affecting starch digestibility and its physiological response was attributed to its amylose:amylopectin ratio (Behall, Scholfield, & Canary, 1988). In general, starches with a high amount of amylose are used as a source of resistant starch (RS) whereas fully gelatinised waxy starches serve as a source of rapidly digestible starch (RDS). Tester, Karkalas, and Qi (2004) reviewed starch structure and digestibility with the target of optimising its digestion. Classification, development, physicochemical investigations and health effects of RS have been reviewed elsewhere (Champ, 2004, Sajilata et al., 2006). However, the structural properties of the intermediate fraction of starch with slow digestibility and its potential health benefits are not well understood. Slowly digestible starch (SDS), such as native maize starch, offers the advantage of a slow increase of postprandial blood glucose levels, and sustained blood glucose levels over time compared to RDS with its fast and high peak and fast decline, partially under baseline (see Fig. 1). In addition, hormonal and metabolic responses correspond to the postprandial glycemia and differ compared to RDS. This can have implications for physical and mental performance, satiety and diabetes management. Although well-designed clinical studies that investigate the link between starch structure, glucose absorption and physiological benefits are limited, there is good reason to believe that SDS offers a range of health benefits due to its stabilizing and sustaining effect of the blood glucose level. Another benefit of products rich in SDS is their moderate impact on the Glycemic Index (GI). Clinical data show that a low GI diet is linked with a reduced risk of diabetes and cardiovascular disease (Jenkins et al., 2002).
So far, there are no commercially available SDS products on the market. However, new slowly digestible carbohydrates (SDC) such as Isomaltulose/Palatinose®, which claim a slow and sustained blood glucose level after intake, have been commercialized. Compared to SDS, both ingredients target similar physiological benefits. However, the functional properties and potential applications of both SDS and SDC differ strongly. The present review highlights current knowledge of starch structures causing a slow digestibility and explores their health implications.
In humans, starch and its derivatives are digested in several stages. In the mouth, in contact with saliva α-amylase, the starch polymeric chains are cleaved into shorter oligosaccharides. Once entering the gut, the partially digested material is further hydrolyzed by human pancreatic α-amylase (1,4-α-d-glucan glucanohydrolase, EC 3.2.1.1). The initial reaction rate of this hydrolysis decreases with increasing degree of polysaccharide branching, mainly due to steric hindrance (Park & Rollings, 1994). The main resulting products, maltose and branched dextrins, are converted into glucose by the brush border enzymes maltose–glucoamylase and sucrase–isomaltase and enter the blood stream.
In vivo measurements of carbohydrate digestion target the glycemic index determination as an indicator of the influence on the blood glucose level (Jenkins et al., 2002) or, to a lesser extent, the distribution of carbohydrates labelled with stable isotopes (Vonk et al., 2000).
The in vitro determination of carbohydrate digestibility to predict the glycemic response of ingredients or complex foods is of great interest since in vivo evaluations are invasive, labor-intensive and costly. Schweizer, Reimann, and Würsch published in 1988 an enzymatic method for measuring the digestion rates of starchy foods in vitro, a new way of calculating digestion indices (DI) and its relationship with in vivo response. Similarly, one of the most widely used methods to analyse the starch digestion kinetic was published by Englyst et al. (1992). Several steps, which simulate the in vivo enzymatic digestion of starch in the stomach and the small intestine, are applied and the timeline of glucose release is measured.
The starch fractions (see Table 1) are defined as:
RDS: amount of glucose released after 20 min,
SDS: amount of glucose released between 20 and 120 min hydrolysis, and
RS: total starch minus amount of glucose released within 120 min hydrolysis (Englyst et al., 1992).
Most studies found good correlation between the results of the Englyst method and the in vivo response in healthy subjects and Ileostomy patients in relation to the GI, verifying its suitability as a screening tool to predict physiological response (Englyst, Vinoy, Englyst, & Lang, 2003). Some authors did not report a good correlation between the Englyst in vitro and rat in vivo data but confirmed a suitability of the method for a relative ranking of foods (Bauer, Murphy, Wolf, & Fahey, 2003). Alternative in vitro hydrolysis methods exist, which differ in the choice and concentration of enzyme, and the treatment and timeline of digestion (McCleary, McNally, & Rossiter, 2002). This can result in considerable differences in starch digestibility.
Section snippets
The impact of native granular starch structure on its digestibility
In general, starch is consumed after processing. An excess of water and high temperature during processing results in starch gelatinisation and destroys its granular structure. However, in several low moisture food products such as biscuits, the granular structure of starch can be retained (Englyst et al., 2003).
Starch granules have a complex and highly ordered semicrystalline structure. More details about starch composition and fine structure can be found elsewhere (Buléon et al., 1998,
Hydrothermal treatment with retention of the granular structure and its effect on SDS formation
Annealing and heat-moisture treatment (HMT) are two hydrothermal treatments, which modify the physicochemical properties of starch, while maintaining its granular structure and birefringence. Annealing involves incubation of native starch granules in excess of water (>60% w/w) or at intermediate water content (40–55% w/w), while HMT is carried out at low levels of water (generally below 35% water w/w). Both treatments are performed between the starch glass transition temperature and the
Generation of SDS in the food matrix
A slow release and absorption of glucose may be generated in a food matrix according to the processing conditions and surrounding ingredients (Würsch, Del Vedovo, & Koellreutter, 1986). In cereal products, the starch gelatinisation extent, which is mainly controlled by the moisture level and the cooking time and temperature influences the formation of SDS (Englyst et al., 2003). For instance, in bread dough, although formation of resistant starch (RS3) may occur in the higher water-containing
Physiological effects of slowly digestible starches
Studies to date on health benefits of SDS are limited. Furthermore, most studies do not make a precise distinction between the starch fractions. The potential health benefits of SDS are linked to a stable glucose metabolism, diabetes management, mental performance, and satiety.
Conclusion
Slowly digestible starch is the starch fraction with slow but complete hydrolysis in the small intestine. Its physiological advantage compared to rapidly digestible starch lies in its property as source of sustained glucose and its stabilizing effect on the blood glucose level, resulting in distinct hormonal and metabolic profile. Benefits of this condition might be linked to diabetes management and effects on satiety/food intake and mental performance. Further studies need to verify these
Acknowledgment
The authors thank Simon Livings, Baltasar Vallès-Pàmies, Jason Chou, Robert J. Redgwell and Pierre Würsch for review of the manuscript and helpful comments.
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The authors contributed equally to the review.