Kamis, 17 Juni 2010

Slow Digestible Starch

A slow release and absortion 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 parts during cooling, a large portion of starch is gelatinised during cooking and induces a rapid digestibility of starch (Bravo, Englyst, & Hudson, 1998). In extruded cooked cereal products such as breakfast cereals, in addition to the thermal treatment, the high pressure and shear forces destroy the starch granular structure and increase its gelatinisation extent, making it more available to amylolitic enzymes (Le françois, 1989). In contrast, in pasta, a dense protein network is formed, which limits the accessibility of α-amylase to the starch and restricts the diffusion of water to the granules, which reduces to some extent the starch gelatinisation (Colonna et al., 1990; Englyst et al., 1992). Even though in pasta, the gluten network is not itself a total barrier for α-amylase to acces starch (Fardet et al., 1998), it creates a tortuous network of gelatinised and partially intact starch granules and may also interact with α-amylase. Furthermore, the treatment conditions such as the cooking temperature and time, modulate the nutritional properties of starch in the matrix (Quatrucci, Acquistucci, Bruschi, & Salvatorelli, 1997). In some biscuits with very low moisture levels during the treatment, the extent of gelatinisation is reduced and partially intact granules and gelatinised starch co-exist. The preservation of these partially intact granules results in a higher content of SDS compared to breakfast cereals and baked products (Englyst et al., 2003). In many plant sourced foods, such as legumes and minimallu processed cereal grains, starch granules are trapped within the plant cell walls (e.g. whole grains), which retard their degradation (Würsch et al., 1986). Disruption of the granule structure, e.g. by milling can increase the susceptibility to enzymatic degradation.

Management of blood glucose levels, which is the ultimate benefit of SDS, may be achieved through means other than influencing the susceptibility of starch digestion. Indeed, the other ingredients present within the food matrix may influence the glucose metabolism through regulation of the rate of gastric emptying, gut hormone profiles and glucose absorption (Berti, Patrizia, Monti, & Porrini, 2004). These effects can only be stuidied in vivo. For instances, the presence of protein seems to stimulate a higher insulin response, which results in a faster peripheral adsorption of glucose and lower postprandial blood glucose concentration (Berti et al., 2004; gannon, Nuttall, Neil, & Westphal, 1998; Juntunen et al., 2002). Fat (quality and quantity) may also reduce the postprandial glycemia by showing down gastric emptying (Cecil, Francis, & Read, 1999) and, if supplied in sufficient quantity, stimulate the secretion of insulin (Normand et al., 2001). Many studies have shown that soluble fibers can reduce the rate of gastric emptying by increasing the viscosity of the digestate in the upper part of the gastrointestinal tract (Juntunen et al., 2002). However, the structure of the fiber (e.g. whole grain) seems to be more important to regulate the glucose metabolism than the fiber quantity (Juntunen et al., 2002). In addition, the structure of the food matrix e.g. a liquid or solid matrix has an important effect on the physiological response. Antinutritive components such as α-amylase inhibitors in legume starches may inhibit the digestion (Tormo, Gil-Exojo, Romero de Tejada, & Campillo, 2004). In in vitro tests, maltose and maltotriose in high concentration showed an inhibitory effect on the α-amylase (Colonna et al., 1992).


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.


SDS and metabolic response

The metabolic effects of carbohydrates, in particular glucose, are related to the rate of carbohydrate absorption after a meal. A common measurement to address these effects is the Glycemic Index (GI). The glycemic index is defined as the incremental area under the blood glucose response curve after intake of a standard amount of carbohydrates from a test food relative to a control food (glucose or white bread) (Ludwig, 2002). Information about the glycemic response of typical portion sizes of different foods and thus total glycemic load (GL). It is defined as the product of the GI and the total dietary carbohydrates in a food or meal (Salmeron et al., 1997).

Although there is an ongoing debate on the clinical implication of the GI, it offers a tool to select and classify foods according to their fate during digestion (Ludwig, 2002). Jenkins et al. (2002) stated that low GI diets are associated with decresed rick of diabetes and cardivascular disease. Positive associations were found between dietary GI and risk of colon and breast cancer (Jenkins et al., 2002). Slowly digestible starch has a medium to low GI and thus reduces the glycemic load of a food product compared to rapidly digestible starch with a high GI (Ells, Seal, Kettlitz, Bal, & Mathers, 2005; Englyst et al., 2003).

A few studies investigated the postprandial physiological responses to the ingestion of RDS and SDS in healthy subjects and type 2 diabetics (Ells et al., 2005; Seal et al., 2003). Significantly greater and more rapid changes of blood glucose, insulin and nonesterified fatty acids (NEFA) concentrations were observed after consumption of RDS compared to SDS. A reduction of potential risk factors for the metabolic syndrome by exchange of RDS by SDS was proposed (Ells et al., 2005). In obese, insulin-resistant subjects, Harbis et al. (2004) showed that the intake of slowly available glucose resulted in an improved metabolic profile, particularly in lower postprandial insulinemia, lower levels of circulating triacylglycerols and apolipoproteins B-100 and B-48 in the triacylglycerol-rich lipoproteins. Also, rapidly and slowly digestible starches differ in their ability to stimulate secretion of gut incretin hormones. Glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) increased in the late postprandial phase (180-300 min) after SDS consumption. This could indicate beneficial effects of SDS in the late postprandial phase, e.g. related to glucose homeostasis and energy storage (Wachters-Hagedoorn et al., 2006).


SDS and diabetes

One of the goals in diabetes management is to reduce meal-associated hyperglycemia. Epidemiological studies suggest that reduced postprandial glucose peaks, reduced episodes of hypoglycemia, improved lipid response, lower concentration of glycosylated hemoglobin and fructosamine and greater insulin sensitivity are beneficial for diabetes management (Wolever, 2003). Slowly digestible starch intake results in a beneficial metabolic response for these conditions and was recommended for the prevention and management of diabetes (Axelsen et al., 1999; Ells et al., 2005; Seal et al., 2003). It was shown that SDS-containing foods at breakfast improved carbohydrates metabolism and reduced insulin requirement of insulin-treated type 2 diabetic patients (Golay, Koellreutrer, Bloise, Assal, & Würsch, 1992). Currently, due to the lack of suitable sources, uncooked corn starch as a source of SDS is recommended for patients suffering from diabetes. This can improve the glycemic response at the next meal and prevents evening hypoglycemia in diabetic patients with insulin treatment (Axelsen et al., 1999).


SDS and mental performance

Glucose is the primary fuel for the brain. It was shown that blood glucose levels can influence mental performance, particularly for higher demanding tasks like memory and later stages of a prolonged effort (Benton & Nabb, 2003a).

Whereas studies with glucose drinks compared to placebo drinks often showed a positive effect of glucose on cognition (Korol & Gold, 1998), the effects of meals are less consistent in evaluating specific effects of macronutrients on performance. A limited amount of data is available about the effects of the rate of carbohydrate absorption on cognitive performance. A breakfast high in SDC counteracted a decline in performance over the morning compared to rapidly available carbohydrates. Positive effects were shown with 7.9 g SDS in healthy volunteers (Benton et al., 2003b). in contrast, other authors could not observe a strong effect of the blood glucose level on cognition in response to carbohydrates source such as glucose, mashed potatoes or barley, differing in their rate of absorption and the blood glucose response (Kaplan, Greenwood, Winocur, & Wolever, 2000). They suggested that the individual glucose tolerance and beta cell functioning might be additional determinants of the effects of glucose on cognition (kaplan et al., 2000).

Further studies are necessary to investigate the minimal efficacious dose for these effects as well as the underlying mechanism. Metabolic responses such as insulin and neurotransmitter levels play a crucial role in the observed effects (Benton & Nabb, 2003a).


SDS and satiety

The concept that the blood glucose level derived from carbohydrate intake is the central regulator of satiety is based on the glucostatic theory of food intake regulation (mayer, 1953). This theory proposes that low blood glucose concentrations trigger the onset of feeding and high blood glucose levels signal satiety. Campfield and Smith (2003) reveiwed current knowledge on the complex regulatory mechanism between blood glucose dynamics and meal initiation, supporting the fact that transient declines in blood glucose promote hunger. Additionally, a stable and low insulin response after meal intake seems to be important for satiety regulation. This would support the hypothesis of beneficial effects od SDS on satiety. However, studies showing a positive effect of SDS on satiety are limited. Leathwood and Pollet (1998) reported a delayed return of hunger after intake of 25-40 g of slow release carbohydrates in the form of bean purée compared to rapidly digestible potato purée.

It can be concluded that SDS can have an impact on satiety-influencing factors such as postprandial blood glucose and insulin levels and the resulting metabolic response. It can also affect viscosity within the gastrointestinal tract. However, satiety is aslo influenced by further mechanisms such as effects on gastric emptying, gut hormones, contact with the small intestine, absorption characteristics and meal consumption.



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 menagement and effects on satiety/food intake and mental performance. Further studies need to verify these benefits and the minimal dosages to achieve them.

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