Much of the focus to date on resistant starch has been directed at lowering blood glucose and enhancing gut health. However, in 2007, its multifunctional attributes cited within the academic literature are expanding. Mark J Tallon, PhD, explores the next level of application for RS and what food-technology challenges lie ahead

Resistant starch (RS) has been defined according to how digestible it is on a scale of one to four. RS1 (ie, partially milled grain) is the least digestible (some say indigestible), while RS3 (ie, retrograded amylase) can be found in bread and cereal. RS4 (ie, acetylated, hydroxypropylated) is slightly different than these former classifications in that it has been chemically modified to decrease digestibility. The advantage is RS4 can be utilised across a wider range of products including breads, extruded snacks and soups.

The health benefits associated with RS include the elevation of short-chain fatty acids that increase colonic blood flow and lower luminal pH, which may help prevent gastric ulcers,¹ lower blood glucose,2 stimulate the growth of beneficial gut bacteria3 and, more recently discovered, reduce the incidence of colon cancers.4^,5 These weighty benefits aside, RS can also bring about additional health benefits that will lend itself to yet further growth of the 48.5 million tonnes utilised globally, according to the USDA and EU Commission.

Novel health benefits
Much of the evidence regarding feeding RS for health benefits is targeted at enhancing gut health and modulating blood glucose. However, evidence published in 2006 has given insights into the modulation of chemical signals that are integral to appetite control and also cardiovascular risk factors.

Obesity: Researchers from Louisiana State University recently assessed the impact of RS (amylase-resistant corn starch) on reducing abdominal fat and the mechanism behind it.6 This study delivered three different reduced-energy diets and then compared these diets' influence on weight reduction following the inclusion of RS, methylcellulose or no fortification at all. The results as expected showed a reduction in abdominal fat across all diets, although significant differences between diets existed regarding the enhancement of chemical and genetic factors related to energy balance.6 Inclusion of RS within the diet stimulated gut peptide YY and glucagon-like peptide expression, which study authors suggest makes RS a more effective natural approach to the treatment of obesity.

Cholesterol: Many risk factors have been implicated in the pathogenesis of cardiovascular disease (CVD), with none as publicly acknowledged as cholesterol. In a study from Obihiro University, Japan, researchers assessed the influence of fortifying (15 per cent of total daily intake) a diet for four weeks with either starch from the Benimaru (BP) or Hokkaikogane (HP) potato on total cholesterol and serum triglycerides.⁷ Both sources of RS significantly affected the synthesis of cholesterol and triglycerides. BP promoted the excretion of bile acids resulting in lower serum cholesterol, while HP inhibited the synthesis of fatty acids (FAS and SREBP-1c), which may lead to decreased triglyceride levels.⁷

Taken together, supplementation may lead to a significant decrease in risk factors associated with CVD. Further work directly related to outcome in humans following dietary intervention with RS is needed, especially regarding diets that mix RS sources to produce possible additive or synergistic health benefits.

Cancer: In late 2006, concerns were raised about the use of casein-based supplements that may promote oncogenesis (cellular changes resulting in tumour formation).4 However, data released in 2007 on chemically induced tumour formation may provide some new insights into RS and its applications as both a functional and medicinal food. Authors investigated the interaction of RS with digestion-resistant potato protein (PP) on colonic fermentation events and their relationship to intestinal tumour formation.⁵ Experimental diets included the following: no added RS or PP, 10 per cent high amylose maize starch (source of RS) replacing digestible starch, 15 per cent PP replacing casein, and 10 per cent high amylose maize starch+15 per cent PP.

The results of the study were very interesting and demonstrated that feeding RS significantly increased short-chain fatty acid (SCFA) levels in the caecum and colon. Importantly, butyrate concentration was significantly increased in the distal colon with RS. Feeding PP increased protein fermentation products, but this effect was reduced by adding RS to the diet. Intestinal neoplasms and colorectal adenocarcinomas were reduced by feeding RS regardless of whether PP was fed, whereas PP alone increased the incidence and number of small intestinal neoplasms including the adenocarcinomas.⁵ The authors concluded, "RS not only protected against intestinal tumour formation but also ameliorated the tumour-enhancing effects of feeding indigestible protein."

Much work remains to be carried out but this data raises some interesting insights into RS and the enhancement of health.

New sources of RS
One of the most exciting areas of change in the study of RS is emanating from work assessing the structural chemistry between different sources of starch. One of the big impacts from consuming RS is its slow rate of digestion, which is a primary factor in the control of blood glucose. RS can now be obtained from a variety of sources including milled grains; maize (native starch); banana flour; potato pulps; and normal, pop, sweet and waxy corns. Of interest is why RS displays its unique breakdown characteristics, as this will give a greater insight into the source of RS utilised as a functional food.

At present there are quite a few issues that can determine the release profile of starches and as such their functionality in relation to health benefits. These characteristics include shape, size, surface pores and channels, and degree of crystallinity of starch granules. However, due to the limits of this article I will focus on crystalline structure and the content of starch residues.^8,9

In a study from Southern Yangtze University, researchers identified that the high proportion of slow-digesting starch (SDS) in cereal starches, as compared to potato starch, was related to their A-type crystalline structure.⁸ The take-home message from this is that in selecting a source of RS, the slower the breakdown you need the greater concentration of the shorter A-type crystalline structures is required.

The slow digestive properties of native cereal starches represented by normal maize starch have also been investigated.⁹ Following testing for digestion (Englyst test) and structural analysis (scanning electron microscopy), maize SDS (typically 50 per cent slowly digestible starch content) was composed of primarily equal portions of amylopectin (AP) and amylose (AM) at all points during the digestion test. Therefore, it is likely that a change in the composition of the SDS composition may influence both the rate of release and also the total content of SDS. So how can we influence the composition of RS sources?

In a study released in 2007 from the Laiyang Agricultural University in China, researchers have begun to uncover some clues about what factors are responsible for the biosynthesis of starch.¹⁰ The relationships between the rates of starch synthesis are directly related to the activities of enzymes throughout the grain-filling period. The days from anthesis (beginning of change) to maturity in a crop's life are defined as the grain filling period or the grain-filling duration (GFD). In this study, results indicated that the rates of starch synthesis and the activities of sucrose synthase (SS), soluble starch synthase (SSS), granule-bound starch synthase (GBSS), starch-branching enzyme (SBE), and starch-debranching enzyme (DBE) each exhibited a single peak during GFD.¹⁰ Results showed that the activities of these enzymes were markedly different between corn types (normal, waxy, pop and sweet).

As the rates of starch synthesis are correlated with the activities of SS, SSS, GBSS and SBE during the grain-filling process, if it were possible to manipulate their activity, the levels of starch and type could theoretically be changed. Taking a closer look at the data, we see GBSS is responsible for amylose synthesis especially in the later period of GFD; also, SSS and SBE are associated with amylopectin biosynthesis. Therefore, the manipulation of the expression of these enzymes will influence branching and starch specific content all related to the digestion rate of RS.

Future food applications
The applications for starch are truly astounding and should keep advancing in line with advancement in food technology and our ability to influence composition. At present there is considerable debate regarding transgenic (GE) crops. However, naturally selecting crops that express enzymes that enhance certain compositional characteristics is still a viable method for manipulating RS produce. This is already occurring to some extent through selective breeding of a mutant strain (designated ae, amylase-extender), corn can be produced with 50 to 70 per cent amylose, and some strains can consist of 90 per cent or more amylase.

RS is breaking new ground in cancer, obesity and CVD. There are also possible openings within IP regarding specific RS forms and mineral absorption which to date have not been fully exploited.11 As such RS and its applications with regard to functional foods are likely to take root within our industry for some time, exciting consumer and manufacturer alike.

Mark J Tallon, PhD, is CSO of OxygeniX, a London-based consultancy firm specialising in claims substan-tiation, product development and technical writing. www.oxygenix.com. Dr Tallon is also co-founder of Cr-Technologies, a raw-ingredients supplier. www.cr-technologies.net. Respond: [email protected]

References
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