Resistant Starch Makes Better Carbs

Foods containing high levels of resistant starch yield fewer calories and lower glycaemic loads—important formulation considerations for diabetics as well as the weight-conscious. Guy A Crosby, PhD, processes.

Proponents of high protein, low carbohydrate diets argue that intake of carbohydrates—especially starch—should be restricted. Dietary starch is converted to glucose, which the body stores for relatively short periods as glycogen, a high molecular-weight polymer of glucose. The body is capable of storing approximately 200 to 500 grams of glycogen.1 Any excess glucose that is not rapidly burned as fuel or stored as glycogen is converted to fat and stored in adipose tissue. Protein proponents therefore argue that to lose weight we should eat less starch.

In fact, not all starch is rapidly converted to glucose, as was commonly believed as recently as in the 1980s. We now know that a significant portion of dietary starch escapes digestion and absorption in the small intestine and reaches the large intestine essentially intact.2 This portion of starch is called resistant starch (RS) because it is resistant to stomach acid and digestive enzymes.3 Thus, RS behaves as dietary fibre, providing faecal bulk and fuel for the beneficial bacteria in the large intestine.4

Once in the large intestine, RS is extensively fermented by the microflora to short chain fatty acids (SCFA), primarily acetate, propionate and butyrate.5 The production of SCFA helps lower the pH of the gut and reduce levels of toxic ammonia in the gut and blood.6 Studies in both humans and rats inoculated with human microflora have shown that fermentation of RS produces significantly higher levels of butyrate in relation to acetate or propionate.7,8

Butyrate is readily metabolised by the cells lining the colon, which derive about 60 to 70 per cent of their energy from bacterial fermentation products, such as butyrate.9 Butyrate is therefore an important regulator of colonic cell growth and differentiation.10 This may explain the dramatic observations of RS pioneer Dr John Cummings, that the incidence of colon cancer is inversely related to the intake of starch, in particular RS, and that diets high in RS appear to provide protection against colon cancer.11 Notably, dietary intake of RS is two- to fourfold lower in the US, Europe and Australia compared with populations consuming high-starch diets, such as in India and China.12

Because resistant starch is not digested in the small intestine, the formation and absorption of glucose are significantly less compared with starch that is rapidly metabolised. In one small, but well-designed, Danish study, ten healthy, normal-weight males consumed test meals containing either 50g starch free of RS (0% RS), or 50g starch containing a high level of RS (54% RS). Postprandial concentrations of glucose, insulin, glucagon-like peptide 1 (GLP-1) and epinephrine were significantly lower following the high RS meal.13

These findings have important implications for diabetics as well as healthy individuals. Foods containing starch composed of high levels of RS, such as energy bars, have been shown to dramatically decrease postprandial blood glucose and insulin levels and improve blood glucose control in subjects with type 2 diabetes.14 In healthy individuals, studies have shown that RS provides only about 30 to 70 per cent of the energy of rapidly metabolised starch.15,16 The wide range of values may be explained by the use of different forms of RS and by the lack of a standardised method for analysing the RS content of foods at the time the studies were conducted. Nevertheless, these results indicate that foods containing high levels of RS yield fewer calories and lower glycaemic loads and clearly should be part of a healthy diet.17

Engineering RS
So, what is resistant starch, and how can we consume more of it in our diets? Much as glycogen is the storage form of glucose in mammals, starch is the storage form of glucose in plants. Starch is formed within plants as microscopic granules, with the starch molecules deposited in organised amorphous and crystalline regions within the granule.18 Starch molecules occur in two forms: amylose, a nearly linear polymer composed of several thousand glucose molecules linked end-to-end, and amylopectin, a much larger, highly branched polymer containing perhaps a million molecules of glucose linked in ways that form numerous short- and long-chain branches similar to glycogen.

The ratio of amylose to amylopectin, as well as the molecular size of the molecules, varies widely in different crops, such as corn, wheat, rice and beans. Granules of common cornstarch are composed of approximately 75 per cent amylopectin and 25 per cent amylose. Linear amylose molecules exhibit a natural tendency to form double helixes, which aggregate into tightly packed, highly stable crystallites by a process known as retrogradation.19 The terminal branches of amylopectin can also form short helixes, but the resulting crystallites are much less stable and are easily disrupted.20

The amorphous regions of starch, as well as some of the less-stable amylopectin crystallites, are readily digested in the small intestine, while the more-stable crystallites formed from amylose are highly resistant to penetration and digestion by mammalian enzymes, as well as hydrolysis by stomach acid. Therefore, only a portion of the starch is rapidly digested, while the remainder is slowly digested or resistant. Studies have shown the proportion of starch that is resistant is directly correlated with the amylose content.21

Processing Issues
George Fahey, PhD, and his collaborators at the Department of Animal Science, University of Illinois, showed that unprocessed legumes, such as black beans, kidney beans, lentils and peas are rich sources of RS, containing 17 to 28 per cent RS on a dry basis.22 Other good sources include unprocessed cereal grains, such as corn, sorghum and barley.

Not surprisingly, highly processed cereal flours and foods made from the flours, such as pasta, contain much lower levels of RS, averaging only about 1.5 to 8 per cent RS on a dry basis. Fahey pointed out that the crystalline structure of starch in legumes (C-type) is more stable compared with the crystal structure in cereal grains (A-type).23 This helps explain why processing cereal grains results in such a large decrease in RS content, while legumes are excellent sources of RS. As this example points out, processing conditions can have a profound effect on the levels of RS in processed food.24

Cooking under conditions of high moisture and temperature can significantly lower the RS content by disrupting crystalline structure. Increasing the levels of RS can be done in other conditions, such as extrusion followed by cooling to induce crystallisation.19 Levels of RS should therefore be determined in the foods as consumed.

Finally, one of the biggest obstacles to understanding the physiological role of RS has been the lack of a universal analytical method for quantifying RS under physiological conditions. Care must be exercised in comparing results from early studies due to the use of different analytical methods. Through the perseverance of Barry McCleary, PhD, Megazyme International Ireland, a method has been developed and subjected to blind testing by 37 laboratories and the results published in the International Journal of the Association of Official Analytical Chemists (AOAC method 2002.02).25 The method has also been accepted by the American Association of Cereal Chemists (AACC).

Resistant starch seems to have some health benefits on its own in the area of the gut. Because it resists digestion, it does not raise glucose, does not have the same caloric count as regular starch, and therefore leads to satiety but does not have the same 'heaviness' as a lot of fibres. So, it's a food form that has few calories and can be used in place of other starches as a low-glycaemic food.

Guy A Crosby, PhD, is a consultant, writer and lecturer on food and nutrition chemistry. He has more than 30 years of experience as a scientist and executive in academia and industry.
[email protected]

References

1. Flatt JP. Use and storage of carbohydrate and fat. Am J Clin Nutr 1995;61(suppl):952S-959S.

2. Cummings JH, et al. Digestion and physiological properties of resistant starch in the human large bowel. Brit J Nutr 1996;75:733-47.

3. Champ M. Determination of resistant starch in foods and food products: interlaboratory study. Eur J Clin Nutr 1992;46(suppl 2):S51-S62.

4. Phillips J, et al. Effect of resistant starch on faecal bulk and fermentation—dependent events in humans. Am J Clin Nutr 1995;62:121-30.

5. Scheppach W, et al. Effect of starch malabsorption on faecal SCFA excretion in man. Scan J Gastroenterol 1988;23:755-59.

6. Silvester KR, et al. Ileal recovery of starch from whole diets containing resistant starch measured in vitro and fermentation of ileal effluent. Am J Clin Nutr 1995;62:403-11.

7. Silvi S, et al. Resistant starch modifies gut microflora and microbial metabolism in human flora-associated rats inoculated with faeces from Italian and UK donors. J Appl Microbiol 1999;86:521-30.

8. Ahmed R, et al. Fermentation of dietary starch in humans. Am J Gastroenterol 2000;95:1017-20.

9. Roediger R. The place of short-chain fatty acids in colonocyte metabolism in health and ulcerative colitis: the impaired colonocyte barrier. In Cummings J, et al., editors. Physiological and clinical aspects of short-chain fatty acids. Cambridge (UK): Cambridge University Press, 1995. p 337-51.

10. Velazquez OC, et al. Butryate and the colonocyte. In Kritchevsky D, Bonfield C, editors. Dietary fiber in health and disease. New York: Plenum Press, 1997. p 123-134.

11. Hylla S, et al. Effects of resistant starch on the colon in healthy volunteers: possible implications for cancer prevention. Am J Clin Nutr 1998;67:136-42.

12. Birkett AM, et al. Changes to the quality and processing of starchy foods in a Western diet can increase polysaccharides escaping digestion and improve in vitro fermentation variables. Brit J Nutr 2000;84:63-72.

13. Raban A, et al. Resistant starch: the effect on postprandial glycemia, hormonal response, and satiety. Am J Clin Nutr 1994;60:544-51.

14. Reader DM, et al. Glycaemic and insulinemic response of subjects with type 2 diabetes after consumption of three energy bars. J Am Diet Assoc 2002;102:1139-42.

15. Behall KM, Howe JC. Contribution of fiber and resistant starch to metabolizable energy. Am J Clin Nutr 1995;62(suppl):1158S-60S.

16. Behall KM, Howe JC. Resistant starch as energy. J Am Coll Nutr 1996;15:248-54.

17. Asp NG. Resistant starch—an update on its physiological effects. In Kritchevsky D, Bonfield C, editors. Dietary fiber in health and disease. New York: Plenum Press, 1997.

18. Waigh TA, et al. Analysis of native structure of starch granules with X-ray microfocus diffraction. Macromolecules 1997;30:3813-20.

19. Haralampu SG. Resistant starch—a review of the physical properties and biological impact of RS3. Carbohydrate Polymers 2000;41:285-92.

20. Gordon DT, et al. Resistant starch: physical and physiological properties. In Yalpani M, editor. New technologies for healthy foods and nutraceuticals. Shrewsbury (MA): ATL Press, 1997. p 157-178.

21. Brown IL, et al. Resistant starch: plant breeding, applications development and commercial use. In McCleary BV, Prosky L, editors. Advanced dietary fibre technology. Oxford (UK): Blackwell Science, 2001. p 401-12.

22. Bednar GE, et al. Starch and fiber fractions in selected food and feed ingredients affect their small intestinal digestibility and fermentability and their large bowel fermentability in vitro in a canine model. J Nutr 2001;131:276-86.

23. Ring SG, et al. Resistant starch: its chemical form in foodstuffs and effect on digestibility in vitro. Food Chem 1988;28:97-109.

24. Goni I, et al. Analysis of resistant starch: a method for foods and food products. Food Chem 1996;56:445-9.

25. McCleary BV, Monaghan DA. Measurement of resistant starch by enzymatic digestion in starch and selected plant materials: collaborative study. J AOAC Int 2002;85:1103-1111.

Marketing Resistant Starch

With the recognition that resistant starch (RS) is an important component of a healthy diet, food ingredients manufacturers have turned their attention to finding ways that enhance the crystallinity of starch, thus increasing resistance to digestion.1

The first commercial RS was introduced as Hi-maize in Australia in 1993 by Starch Australasia, now part of National Starch and Chemical Co. This product is a natural granular form of starch produced from a corn hybrid containing more than 80 per cent amylose. Hi-maize analyses as 42 per cent RS and has gained widespread use in Australia in breads and other baked goods.2

Opta Food Ingredients in the US patented the first process for producing a non-granular form of RS by enzymatically hydrolysing high amylose corn starch to small linear fragments (maltodextrins), followed by crystallisation (retrogradation) of the starch fragments.3 The ingredient was introduced in the early 1990s as CrystaLean and is now used in products for diabetics. CrystaLean contains 41 per cent RS.4

Shortly after the launch of CrystaLean, National Starch introduced a very similar product named Novelose 330. More recently, National has developed processes for manufacturing granular forms of concentrated RS containing 47 to 60 per cent RS by heating and cooling high amylose corn starch under conditions of carefully controlled moisture and temperature.5 These products are marketed as Novelose 240 and 260.

The latest player is Cerestar (a Cargill company), which has launched Actistar made by crystallising hydrolysed tapioca starch (maltodextrins).6 Actistar contains 58 per cent RS.4

Numerous applications have been developed for the concentrated RS ingredients, including breads, other baked goods, nutrition bars, beverages, breakfast cereals, pasta and low-glycaemic products for diabetics. Foods can now be produced in which half the carbohydrates are in the form of RS. In addition to the potential health benefits, the concentrated RS ingredients are more stable toward food processing conditions compared with regular starches.7

References

1. Thompson DB. Strategies for the manufacture of resistant starch. Trends Food Sci Tech 2000:1-9.

2. Brown IL, et al. Resistant starch: plant breeding, applications development and commercial use. In McCleary BV, Prosky L, editors. Advanced dietary fibre technology. Oxford (UK): Blackwell Science, 2001. p 401-12.

3. Iyengar R, et al. Starch-derived, food-grade, insoluble bulking agent. 1991, US Patent 5,051,271.

4. McCleary BV, Monaghan DA. Measurement of resistant starch. J AOAC Int 2002;85:665-75.

5. Shi YC, Trzasko PT. Process for producing amylase resistant granular starch. 1997, US Patent 5,593,503.

6. Kettlitz BW, et al. Process for preparing starchy products. 2000, US Patent 6,090,594.

7. Haralampu SG. Resistant starch—a review of the physical properties and biological impact of RS3. Carbohydrate Polymers 2000;41:285-92.


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