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
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
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.
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