Role of resistant starch in weight management 831 Weight management direct evidence

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RS is by its very nature indigestible and so does not contribute directly to plasma blood glucose levels. Therefore, replacing digestible starch with RS is a natural fit for low-glycemic foods and diets. In a 2003 report, the World Health Organization (WHO) reviewed the strength of evidence on various factors that might promote or protect against weight gain and obesity. They assessed the totality of evidence, including randomized controlled trials (highest ranking), associated evidence and expert opinions. This group advised that based on the available evidence there is a 'possible' decreased risk of weight gain and obesity with low-glycemic-index foods.

Few studies have looked at the impact of low-glycemic diets on weight loss or maintenance, by directly measuring body weight or body mass index

Table 8.6 Estimated consumption of RS worldwide


Estimated intake (g/day)



5 female, 5.3 male (36% cereals,

Baghurst et al., 1996

26% vegetables, 22% fruits)

3.4-9.4 range

Roberts et al., 2004

8 g/10 MJ

Muir et al., 1998

8.6 g/10 MJ

Walker et al, 1997



Dysseler & Hoffem, 1994


20 g/10 MJ

Muir et al., 1998



Dysseler and Hoffem, 1994



Dysseler and Hoffem, 1994

Europe, mean


Dysseler and Hoffem, 1994



Dysseler and Hoffem, 1994



Dysseler and Hoffem, 1994



Platel and Shurpalekar, 1994


8.5 (7.2 g north-west, 9.2 g

Brighenti et al., 1998




Dysseler and Hoffem, 1994

New Zealand


Baghurst et al, 1996



Dysseler and Hoffem, 1994



Dysseler and Hoffem, 1994



Dysseler and Hoffem, 1994

3.2 (1.3 g bread, 1.2 g potatoes)

Elmstahl, 2002



Dysseler and Hoffem, 1994



Tomlin and Read, 1990

(BMI). However, those that have generally indicate a positive role for low-glycemic diets. Ebbeling et al. (2003) compared low-glycemic-load dietary advice with low-fat dietary advice in a group of young people aged 13-21 years with a BMI greater than the 95th percentile. The authors observed a lower BMI with the low-glycemic group after 12 months (6 month intervention plus 6 month follow-up). This supports observations from a US cohort of adults, where glycemic index but not total carbohydrate intake was positively associated with BMI, indicating a role for carbohydrate type over amount (Ma et al., 2005). In shorter-term studies (6 weeks to 4 months), a comparison of low-glycemic-index with high-glycemic-index or low-fat diets showed lower weight and/or BMI with the low-glycemic-index diet (Slabber et al., 1994; Spieth et al., 2000; Jimenez-Cruz et al., 2003).

Studies such as those mentioned above support a role for low-glycemic foods in weight management. More long-term clinical trials in which RS-enriched foods are included are needed to establish this association with greater certainty, particularly for weight loss. However a wealth of associated supporting evidence exists that indicates a clear role for RS in weight management, particularly when digestible starch in foods is at least partly replaced by RS. This evidence will be discussed here.

184 Novel food ingredients for weight control 8.3.2 Weight management, supporting evidence

RS is associated with nutritional, metabolic and physiological changes that make it an attractive ingredient not only for weight management (Table 8.1), but also for other chronic diseases associated with the metabolic syndrome such as dyslipidemia, insulin resistance, type 2 diabetes, hypertension and coronary heart disease (Higgins, 2004). Previously a role for RS was attributed to reduced digestibility, and the impact of lower glucose absorption. More recent research, however, indicates a broader health impact of RS on metabolism, via fermentation of RS to SCFAs in the large bowel. Fermentable carbohydrates have their own unique fermentation profile, in terms of relative type and amount of SCFAs. Hence the metabolic impact of fermentation will differ between RS and other fiber types. The hypothesized interaction between fermentation by-products and target metabolic tissues will evolve as more mechanistic information becomes available.

Energy value

Foods containing RSs have a lower caloric density than similar foods with digestible starch because RS does not contribute available glucose to the body, and RS increases energy wastage (excretion). However, contrary to expectation, commercial RSs do contribute some metabolic energy for two reasons in particular.

• Commercial RS ingredients typically contain some digestible starch. That is, there are no commercial ingredients currently available that are 100% RS. The higher the RS contribution to the ingredient, the lower the caloric contribution.

• Most of the SCFAs generated by colonic bacterial fermentation of the RS will be absorbed and made available for further metabolism. All fibers that are fermented in the body will contribute some energy through SCFAs.

Typically the energy contribution of RS ingredients is approximately one-third lower than for digestible starches. Reported values will differ between methodologies and ingredients.

Digestible energy: Digestible energy is typically measured as dietary energy less fecal energy. An RS ingredient would be expected to have lower digestible energy because it is partially indigestible, contributes to fecal bulking and increases excretion of other nutrients. Behall and Howe (1996) from the US Department of Agriculture (USD A) reported that high-amylose corn RS2 had 67% of the partial digestible energy of regular cornstarch, at 11.7 kJ/g. This was supported in rats, with the digestible energy contribution from high-amylose corn RS3 being 62% of that from wheat starch (Aust et al., 2001). These values refer to both the digestible and resistant fractions of the RS ingredient. Although the energy value of the RS fraction alone was calculated to be 8.9-9.2 kJ/g (Mathers, 1992) and was measured as 15 kJ/g for corn or 12 kJ/g for potato (Livesey et al., 1990), this energy generated via fermentation does not contribute significantly to whole-body energy (Cummings, 1996).

RS is known to increase energy wastage (excretion) from the gastrointestinal tract. Rats fed high-amylose corn RS2 and RS3 had increased fecal energy (de Schrijver et al., 1999). The increased energy wastage would be partially contributed by increased excretion of lipids (de Deckere et al., 1995), which was measured when rats were fed high-amylose corn RS2.

Metabolic energy: Lower metabolic energy of raw potato RS2 was shown in humans by Tagliabue et al. (1995), with 60% reduction in thermic effect. Results are inconsistent in animal studies, possibly due to differences in physiology, particularly fermentative capacity between species (Andrieux and Sacquet, 1986; de Schrijver et al., 1999).

Body composition and lipid storage

Not only BMI, but also body composition and regional differences in lipid deposition are associated with increased risk of chronic disease. Incorporating RS into foods lowers body lipid composition and distribution, principally via reduced lipogenesis (lipid production) and increased lipid oxidation (lipid utilization).

Body composition: The ability of RS to lower body lipid composition has been shown consistently across animal studies, indicating that the value of RS in foods for weight management extends beyond lower digestibility into a metabolic role. RS2 from high-amylose corn and potato causes lower body lipid accumulation (Williamson et al., 1999; Pawlak et al., 2004), and more specifically lower epididymal pad weight in the abdomen following consumption of various sources of RS (de Deckere et al., 1993, 1995; Pawlak et al., 2001; Kishida et al., 2001; Zhou and Kaplan, 1997). Confirmatory studies in humans are underway. RS has also been shown to impact the cellular morphology of adipose tissue, with rats fed mung bean RS2 having smaller adipocyte size (Lerer-Metzger et al., 1996; Kabir et al., 1998b). It has also been demonstrated that the consumption of RS2 from high-amylose corn in rats could not only reduce the body's lipid composition but also increase its muscle mass (Kiriyama, 1996).

Lipid storage and associated metabolism: Selective changes in body lipid storage could be associated with the affect of RS on insulin and hence insulin-regulated pathways, such as glucose utilization and lipid metabolism in adipose tissue. Lipogenesis (lipid production) from glucose conversion was lower in adipocytes for rats fed RS from mung beans (Kabir et al., 1998b). This could be explained by lower expression of GLUT4, the protein responsible for insulin-stimulated glucose uptake, and lower fatty acid syn-thase activity and expression, the enzyme responsible for the rate-limiting step in lipid synthesis, in the adipose tissue of rats fed high-RS diets (Kabir et al., 1998a).

Morand et al. (1994) investigated a role for corn-based RS in hepatic lipid and carbohydrate metabolism, and showed that lipogenesis (lipid production) as well as glycolysis was also lower in hepatic tissue. Gluconeogenesis was favored, which is also antagonistic to lipogenesis. Reduced lipogenic enzyme activities included: glucose-6-P dehydrogenase and malic enzyme, which are known for their role in supplying NADPH; ATP citrate lyase, which provides acetyl coenzyme A (CoA); acetyl CoA carboxylase, the key enzyme of lipogenesis; and fatty acid synthetase. Reduced glycolytic enzyme activities included glucokinase and pyruvate kinase activity; with higher gluconeogenesis activity via the rate-controlling enzyme phosphoenol-pyruvate carboxykinase (PEPCK).

In the study by Morand et al. (1994), in which most lipogenic enzymes were inhibited, acetyl CoA synthetase activity was not affected by RS. This suggests a role for the SCFA acetate in liver metabolism. Higgins (2004) noted that acetate can inhibit glycogenolysis, further suggesting a mechanistic role for SCFA to spare carbohydrate oxidation, potentially promoting lipid oxidation, thereby reducing lipid storage in response to RS consumption.

Lipid oxidation: Evidence supporting increased lipid oxidation with high-RS diets will be described in detail in the following section.

Whole-body energy metabolism

RS has been shown to impact whole-body energy metabolism via at least three measurements:

• lower respiratory quotient (RQ);

• lower diet-induced thermogenesis (DIT);

• lower energy expenditure (EE).

Effects in the short term are probably due to the lower supply of available carbohydrate in RS-rich meals, and the direct effect on carbohydrate and lipid oxidation. In the longer term, effects could be due to a contributory role of SCFAs in metabolic substrate selection.

Respiratory quotient: RQ is a comparative measure of oxidative substrate selection, i.e. carbohydrate oxidation relative to lipid oxidation. When the RQ is lower there is relatively more lipid oxidation. Lower RQ or delta RQ up to 5-6 h after a meal was reported with high-amylose corn based and raw potato based RS2 in two human studies (Tagliabue et al1995; Higgins et al, 2004). Over 23-24 h this was supported in rats (Aust et al, 2001), but not in humans (Howe et al, 1996; Achour et al, 1997).

Diet-induced thermogenesis: The calculated difference between postprandial metabolic rate and resting metabolic rate can be used to represent DIT, a 'tax' on dietary metabolism. Tagliabue et al. (1995) demonstrated that when raw potato RS2 replaced pregelatinized potato starch, DIT was lower in the first 5 h after meal consumption. This is attributed to the lower availability of starch for digestion and metabolism. This was not supported over a longer 8-h time period in the human study by Achour et al. (1997), which also did not observe a change in RQ.

Energy expenditure: The impact of RS on EE is less conclusive than for DIT and RQ, with only one study available to report on each of the absorptive, postprandial and total 24 hours periods following a meal. Tagliabue et al. (1995) reported a lower EE in the postprandial period, when raw potato RS2 replaced pregelatinized potato starch in a meal. This is probably due to the lower available starch content of the meal.

Carbohydrate versus lipid oxidation: The choice of substrate for energy metabolism affects RQ and DIT. RQ is lower when the balance of lipid: carbohydrate oxidation shifts towards lipids. Studies with RS have demonstrated a shift towards increased lipid oxidation, which may be important for weight management because reduced rates of lipid oxidation have been linked with greater weight gain (Brand-Miller et al., 2002).

When potato RS2 was fed to humans, glucose oxidation was lower and lipid oxidation higher, associated with a lower RQ (Tagliabue et al., 1995). Increased lipid oxidation relates to both total and meal lipid, as demonstrated by Higgins et al. (2004) who observed a 23% increase in meal lipid oxidation with as little as 5.4% RS in a single meal. Studies in animals support an effect of RS on substrate oxidation - high-amylose RS3 lowered RQ by lower carbohydrate oxidation (Aust et al., 2001). Mechanistically these observations are probably related to the effects of RS on cellular metabolism described previously.

Insulin response and sensitivity

Insulin sensitivity: Insulin is an important regulatory hormone, contributing to carbohydrate, lipid and protein metabolism. Most importantly, insulin is key for glucose homeostasis, regulating glucose uptake by muscle and adipose tissue. Insulin resistance, defined as a sub-optimal biological response to insulin (Hulman and Falkner, 1994) is a characteristic feature of the metabolic syndrome, and is known to develop as people age. High-

amylose corn RS2 enhances insulin sensitivity, assisting the body to handle dietary carbohydrate better (Robertson et al., 2003, 2005). Furthermore, in rats, dietary starch type has an important affect on the development of insulin resistance. Rats fed high-amylose corn RS2 were protected against developing insulin resistance for more than three times as long as for rats fed low-RS starch or glucose - up to 26 weeks versus only 8 weeks (Byrnes et al., 1995; Higgins et al., 1996).

Glucose response: Postprandial glucose and insulin response influence insulin sensitivity (Higgins, 2004). One of the most consistent effects of RS is the ability to lower glucose response, with many groups reporting decreased postprandial response when RS replaces digestible starch in foods (Higgins, 2004). This has been observed in non-diabetics, type 2 diabetics and various animal models. The magnitude of this effect is particularly dependent upon two considerations, namely the RS amount relative to other carbohydrates present in the cooked foods as eaten, and the RS type with higher-amylose starches being more effective.

Behall and Hallfrisch (2002) fed breads made with corn starches varying in amylose content from 30-70%. The higher the amylose content, the greater the RS content, hence the lower the effect on glucose response. Overall, bread made with 70% amylose starch had the greatest impact on the reduction in the postprandial plasma glucose response. Furthermore, Brown et al. (1995) demonstrated that the amount of RS ingredient included in the bread relative to other carbohydrates already present is an important consideration for obtaining a measurable impact on glucose response. White bread with 5% replacement of flour with high-amylose corn RS2 had a relative glycemic response of 95 (versus 100 for a commercial bread). Bread with higher flour replacement levels had a more marked response, with 10% and 20% flour replacement reducing the relative glycemic response to 74 and 53 respectively.

Insulin response: Insulin is a regulatory hormone for glucose homeostasis, therefore the lower glucose response observed with high-RS foods, causes a lower blood insulin response. In studies where RS has replaced digestible starch, many have reported a decreased postprandial insulin response (Higgins, 2004). Insulin levels may be predictive of weight gain (Ludwig, 2000). Furthermore, hyperinsulinemia in association with hyperglycemia, could reduce insulin sensitivity via mechanisms such as down-regulation of insulin receptors in muscle tissue and increased plasma free fatty acids (Higgins, 2004). SCFAs may also assist in increasing insulin sensitivity, by lowering free fatty acids (Higgins, 2004).

In the assessment of breads made with starches with increasing amylose content described above (Behall and Hallfrisch, 2002), high-amylose corn RS2 (60-70% amylose) had the greatest impact on insulin response reduc tion. Furthermore, RS is known to be dependent upon food processing, with high-amylose corn RS2 being more tolerant to normal cooking conditions. Brown et al. (2003) compared uncooked and cooked rat diets containing corn starches with 0-85% amylose. While insulin response was lower for uncooked starches with 27-85% amylose, only the 60-85% amylose corn RS2 lowered insulin response following cooking.

Satiety and related observations

Short-term studies indicate that low-glycemic versus high-glycemic meals can increase satiety and/or reduce subsequent hunger, as determined by visual analog scales or measurement of subsequent energy intake. After controlling for energy intake, macronutrient content, energy density and palatability, people consumed on average 29% less energy after low-glycemic meals than after high-glycemic meals (Roberts, 2000).

When high-amylose RS2- and RS3-containing meals were fed in two studies, people reported increased satiety (Jenkins et al., 1998), particularly in the postabsorptive period (Achour et al., 1997). Effects of RS on other observations relevant to satiety - such as appetite, fullness and satisfaction - are less consistent. van Amelsvoort and Weststrate (1992) fed subjects meals with high-amylose corn and rice, and reported increased satisfaction and fullness, with reduced hunger and desire to eat. de Roos et al. (1995) fed high-amylose corn RS2 and reported reduced appetite for a meal or snack. However, other studies were less supportive of these and other descriptors (Weststrate and van Amelsvoort, 1993; Raben et al., 1994; de Roos et al., 1995).

The lack of consistency for subjective satiety questionnaires is not surprising as many food factors other than RS content contribute to satiety descriptors; these food factors are often not adequately controlled for, thereby making interpretation difficult. More research based on objective measurements, such as satiety hormones, is warranted.

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