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© 2004 The American Society for Nutritional Sciences J. Nutr. 134:831-835, April 2004

Nutrient Metabolism

A Novel Barley Cultivar (Himalaya 292) with a Specific Gene Mutation in Starch Synthase IIa Raises Large Bowel Starch and Short-Chain Fatty Acids in Rats1

Anthony R. Bird2, Corinna Flory, Debra A. Davies, Sylvia Usher and David L. Topping

CSIRO Health Sciences and Nutrition, Adelaide 5000, Australia

2To whom correspondence should be addressed. E-mail: tony.bird{at}csiro.au.


   ABSTRACT
TOP
 ABSTRACT
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 LITERATURE CITED
 
Himalaya 292 (Hordeum vulgare, var. himalaya 292) is a novel, hull-less barley cultivar with a single nucleotide change in the gene encoding starch synthase IIa (EC 2.4.1.21). This leads to loss of enzyme activity, resulting in a grain with less total starch and a higher proportion of amylose. These changes, plus higher total and soluble nonstarch polysaccharides (NSP), could increase its resistant starch (RS) content. Accordingly, rats were fed a diet containing stabilized whole-grain barley flours from Himalaya 292 or two commercial varieties (Namoi or Waxiro) or wheat or oat bran at equivalent NSP concentrations for 14 d. There were favorable significant changes in a number of bowel health-related indices. Fecal output by rats fed Himalaya 292 was higher than by those fed Namoi or oat bran, whereas total large bowel digesta mass was higher than in those fed Waxiro. Cecal starch concentrations and pools were higher in rats fed Himalaya 292 than in all other groups. Fecal and cecal digesta pH was lower in rats fed Himalaya 292 than in all other groups except that fed oat bran. Colonic digesta pH was lower in rats fed Himalaya 292 than in those fed wheat bran or Namoi. Fecal total SCFA excretion was higher in rats fed Himalaya 292 than in those fed Namoi or oat bran. Although cecal total SCFA pools did not differ among groups, colonic SCFA were higher in rats fed Himalaya 292 than in those fed Namoi or Waxiro. These data indicate that changes in Himalaya 292 grain composition result in greater RS with consequent alterations in large bowel SCFA and pH when fed to rats.

KEY WORDS: • barley • cereal • dietary fiber • resistant starch • starch

The concept that greater dietary fiber consumption is beneficial for human health is well established. It was postulated first by Burkitt (1) from observations with native East Africans who consumed a diet high in unrefined cereals and had lower rates of noninfectious diseases than Europeans living in the same environment. These illnesses included problems of laxation, coronary heart disease, and certain cancers (especially of the large bowel). Since then, a substantial body of work has accumulated, confirming many of these actions, especially in the large bowel. Nonstarch polysaccharides (NSP) are major components of dietary fiber, and consumption of NSP-rich foods or isolates promotes laxation, relieves constipation, and protects against diverticular disease in the long term [for reviews see (2,3)]. However, population [e.g., (4)] and intervention [e.g., (5)] studies have yet to show a convincing protective effect in colorectal cancer. Indeed, it appears that some groups (such as the native Africans) have lower intakes of dietary fiber than higher risk populations but consume more starch and less animal products (6). From these and other data, it is becoming apparent that starch, as resistant starch (RS), makes an important contribution to large bowel health (3). RS is that fraction of starch and the products of starch digestion that enter the large bowel of healthy humans (7) where it (plus a variable component of NSP) is fermented by the microflora to yield SCFA, which play a pivotal role in promoting the normal function of the large bowel (3). Butyrate has attracted particular attention because it appears to play a major role in promoting a normal cell phenotype in colonocytes. As yet, there is no definitive link between butyrate and human colorectal cancer risk but it could provide a mechanism for the protective effect of RS. Human studies suggest that fermentation of some forms of RS promotes large bowel butyrate formation (8,9).

Consumption of starch and RS is very low in countries such as Australia and the United Sates where colorectal cancer risk is high (10). Large bowel health could be improved through greater consumption of starchy foods. However, given that there are likely to be barriers to the substantial individual dietary change required, enrichment of foods with RS as an ingredient is an option. This approach is being used with a high-amylose starch in the manufacture of a number of food products, thus raising their RS content (11). Extension of the range of foods seems desirable; as part of a program of screening potential new barley cultivars, one was identified with a single point mutation in the gene encoding for starch synthase IIa (EC 2.4.1.21) (12). This defect was induced by chemical mutation of the Himalaya strain; the grain from the novel cultivar (Hordeum vulgare, var. Himalaya 292) has a lower starch content and a substantial increase in the relative proportion of amylose together with other, potentially favorable, compositional changes. In this paper, we report the effects of consumption of a diet containing Himalaya 292 compared with wheat, oat, or barley products available currently, on large bowel starch and SCFA in rats.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 LITERATURE CITED
 
    Care of animals. Young-adult male Sprague-Dawley rats were used. They were purchased from the University of Adelaide Animal Resource Facility and housed in groups in standard wire-bottomed cages at the Animal Services Unit of CSIRO Health Sciences and Nutrition in a room with controlled temperature (22 ± 1°C) and lighting (lights on at 0800–2000h). All procedures relating to use of these experimental animals were approved by the CSIRO Health Sciences and Nutrition Animal Experimentation Ethics Committee and conformed to published guidelines (13).

    Diets and feeding. After arrival, the rats were adapted to a nonpurified commercial diet for 13 d. They were then weighed and allocated to 5 treatment groups of 6 rats each of equal mean live weight and switched to a purified diet. The composition of the basal diet, which was based on AIN 93G specifications (14) and prepared from standard ingredients, is shown in Table 1. The diets were balanced for macronutrients and comprised 180 g of protein/kg, 630 g of carbohydrate/kg (as 530 g of starch and 100 g of sucrose), 70 g of fat/kg and 50 g of NSP/kg. The following heat stabilized cereals were used: Himalaya 292, one of two hull-less standard Australian barleys (var Namoi or Waxiro) or oat bran (Oat Gold, The Uncle Tobys Company). Wheat bran was an unprocessed product suitable for human consumption with a particle size of ~2–3 mm (Bartlett Grains). Macronutrient composition of the cereals was determined as described previously (15,16) using Official Methods of Analysis of AOAC International (17) (Table 2). For neutral NSP, a modified version of the GC method of Theander et al. (18) (AOAC method 994.13) was used; the method employed a scaled-down procedure using a 2-h hydrolysis with dilute (1 mol/L) sulfuric acid followed by centrifugation (2000 x g, 15 min) to obtain the insoluble neutral NSP, and a further hydrolysis using 2 mol/L trifluoroacetic acid for the soluble neutral NSP. The diets were formulated so that each cereal product provided the full complement of NSP with allowance being made for its macronutrient content in the addition of the other components. Each of the cereal products was milled and passed through a 2-mm sieve before incorporation into diets, which were prepared by blending the various ingredients with a small quantity of water using a planetary mixer. The mixture was then pelleted (to a diameter of 8 mm and a length of 1–2 cm) by passage through a mincing machine operated at room temperature and atmospheric pressure (Zerco Nolex E55), dried for 16 h at 40°C, and then placed in sealed containers and stored at 4°C.


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  		TABLE 1 Composition of the experimental diets1

 

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  		TABLE 2 Proximate composition of the main cereal ingredients1

 
Rats had unrestricted access to treatment diets and drinking water for 14 d. During the last 4 d, the rats were kept in individual metabolism cages to allow accurate estimation of feed and water intake and total collection of feces, which were retained for analysis.

    Sampling and analytical procedures. Sampling procedures were described in detail previously (19). Briefly, rats were anesthetized with halothane in O2; the cecum and colon were opened and their entire contents removed, weighed, and stored at -20°C until analysis. The moisture content of cecal digesta was determined by freeze-drying a portion to constant weight. Digesta and fecal samples were diluted with a specified volume of internal standard (heptanoic acid) for analysis of SCFA and mixed thoroughly for determination of pH using a glass electrode. The slurries were then stored frozen to await further analyses. For analysis of total and major individual SCFA, slurries were thawed, centrifuged (2000 x g, 10 min), and concentrated by low temperature vacuum microdistillation for quantification by GLC (19). A portion of cecal digesta was freeze-dried overnight, ground, and then analyzed for starch using a commercial kit (AA/AMG 11/01; Megazyme International) based on enzymatic ({alpha}-amylose/amyloglucosidase) digestion of starch and spectrophotometric quantitation of liberated glucose.

    Statistical methods. The data are presented as means and pooled SEM of 6 observations, unless stated otherwise. For biochemical determinations, means of duplicate determinations were used in statistical analyses. A 1-way ANOVA was performed using the General Linear Models feature of SAS software (version 8.02, Statistical Analytical Systems Institute). The primary aim of the study was to compare Himalaya 292 with current cereals; therefore, differences between this group and other treatment groups were assessed using the protected difference option of SAS. Differences were considered significant at P < 0.05.


   RESULTS
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 LITERATURE CITED
 
    Food intake and body weight gain. Daily voluntary food intake (~21 g) did not differ among groups. Initial and final body weights did not differ among the groups nor was there an overall effect of diet on body weight gain (data not shown).

    Large bowel digesta and fecal mass and cecal moisture. Fecal output during the 4-d collection period was higher in rats fed Himalaya 292 (3.8 ± 0.3 g/d) than in those fed Namoi (2.5 ± 0.2 g/d, P < 0.05) or oat bran (2.3 ± 0.3 g/d, P < 0.01). There were no group differences in cecal digesta mass and although colonic digesta mass was highest in rats fed Himalaya 292, it was significantly greater only compared with rats fed Waxiro (Table 3). The digesta mass was greater in rats fed Himalaya 292 compared with those fed wheat bran or Waxiro. Cecal digesta moisture did not differ among groups, ranging from 78 to 82% (data not shown).


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  		TABLE 3 Mass of wet digesta in the large bowel of rats fed diets containing either a novel (Himalaya 292) or established barley varieties (Namoi or Waxiro), or wheat or oat bran1, 2

 
    Cecal starch. Cecal starch concentration and pool size were greater in rats fed Himalaya 292 than in all other groups (Table 4). Concentrations in rats fed the novel barley were 160–220% of those in other groups, whereas the corresponding range was 180–290% for pools.


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  		TABLE 4 Concentrations and pools of starch in the cecum of rats fed diets containing either a novel (Himalaya 292) or established barley varieties (Namoi or Waxiro), or wheat or oat bran1, 2

 
    Large bowel and fecal pH. Cecal and fecal pH were lower in rats fed Himalaya 292 than in all other groups except those fed oat bran (Table 5). This was also the case for the colon except that rats fed Himalaya 292 did not differ from those fed Namoi.


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  		TABLE 5 Large bowel and fecal pH of rats fed diets containing either a novel (Himalaya 292) or established barley varieties (Namoi or Waxiro), or wheat or oat bran1, 2

 
    Large and fecal bowel SCFA pools. The diets did not affect total or individual SCFA in cecal digesta (data not shown). In contrast, both total SCFA and acetate in colonic digesta were higher in rats fed Himalaya 292 than in those fed the other two barley diets (Table 6). The colonic butyrate pool was higher in rats fed the novel barley than in those fed Namoi. Total fecal SCFA were significantly higher in rats fed Himalaya 292 than in those fed either Namoi or oat bran (Table 6). Acetate pools were higher in rats fed Himalaya 292 than in all other groups with the exception of the group fed wheat bran. Propionate pools were significantly higher in rats fed Himalaya 292 than in those fed wheat or oat bran, whereas the butyrate pool was higher in rats fed Himalaya 292 than in those fed wheat bran, Namoi, or oat bran.


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  		TABLE 6 Colonic and fecal total and individual SCFA pools of rats fed diets containing either a novel (Himalaya 292) or established barley varieties (Namoi or Waxiro), or wheat or oat bran1, 2

 

   DISCUSSION
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
LITERATURE CITED
 
In this study we compared the effects of a diet containing whole-grain flour milled from a novel barley cultivar, Himalaya 292, with the effects of established cereal products. Wheat bran is a source of insoluble NSP and its effects in raising fecal and/or large bowel SCFA have been reported in rats (20), pigs (15), and humans (21). Oats and barley are better sources of soluble NSP than wheat and have well-documented plasma cholesterol–lowering actions (22,23). Oats and oat products also modify large bowel SCFA in animals but appear to be a relatively poor source of RS (15,24). The genetic change in Himalaya 292 is in the pathway of amylopectin synthesis; it leads to a relative accumulation of amylose at the expense of total starch content. On the basis of our experience with a high-amylose maize starch (8,25), this was expected to lead to an increase in RS. However, we chose not to use starch as the basis for comparison because there is no standardized, physiologically relevant, analytical method for RS. Conversely, fiber is a generally accepted agent for improving large bowel health, and this can be measured through standardized procedures. In this study, diets were formulated to contain the same level of NSP (which are major contributors to fiber) rather than total dietary fiber because the latter may include a component of RS. This lessened any apparent contribution of RS to the effects of Himalaya 292 (due to its low starch content). Nevertheless, cecal starch levels in rats fed the Himalaya 292 diet were double those of all other groups, which is consistent with greater RS. However, these data are only indicative because the rat is not an optimal model for dietary studies on RS (3,26). Rats were used in this study due to the availability of only limited quantities of the grain because it was carried out at an early stage of the agronomic program. It is anticipated that a study in a more suitable model species, e.g., pigs, and, subsequently in humans will be possible shortly.

Starch was recovered in the cecal contents of rats fed all diets and we believe that this is likely to be physically inaccessible starch, i.e., RS1 of the 4 major types (11). Two of the other forms of RS (RS2, granular; RS3, retrograded) may also have made a contribution, whereas chemical modification (RS4) can be excluded in this case. The probability that the RS was RS1 is supported by the observation of Steinhart et al. (27) that the dietary fiber content of foods was a key determinant of nutrient loss (including starch) into the large bowel. In rats, wheat NSP appeared to inhibit small intestinal starch digestion (16), which agrees with the current data. However, it is possible that there may be other factors involved such as transit time (3). The effect of fat is also a potential contributor to RS. For example, Himalaya 292 contains substantial levels of a "V-form" starch-lipid complex (12), which slows small intestinal amylolysis.

The feces of humans and other nonrodent omnivores, such as pigs and dogs, comprise an unsegregated mixture of undigested components (especially NSP) and bacteria with the relative proportions depending on diet (28). In contrast, rats excrete two discrete types of stool. One is composed largely of indigestible matter and the other of bacteria formed through the cecal fermentation of undigested matter (NSP, RS, protein etc.). The latter stool is eaten selectively by rats to recover nutrients and differs from the former in SCFA content (29). This is known as coprophagy or fecal refection and is not carried out by humans or other large omnivores such as pigs (30). Nevertheless, despite this limitation (26), rats remain a useful primary model for large bowel fermentation especially when the quantities of experimental diet were limited, as in this case.

We chose to represent SCFA values in large bowel contents and feces as pools (i.e., concentrations x weight or fluid volume) and daily excretion, respectively. This is because they are an approximation of total production and, hence, of substrate supply for the microflora. The lack of difference between groups in cecal total and individual SCFA pools is not unexpected, given the relatively low level of NSP inclusion. However, the higher fecal and colonic total SCFA in rats fed Himalaya 292 are consistent with more fermentative substrate (as RS) in this group compared with the others. This is supported also by the lower cecal (and colonic and fecal) pH values in this group. Greater SCFA production leads to a lower pH of large bowel contents through acidification and consumption of NH4+ through bacterial proliferation (3). To some degree, the differences in pH between the cecum, and the colon and feces, may also represent differences in transit. Studies in pigs showed that the distribution of SCFA along the large bowel varies with diet (15,31), and human data indicate that transit is an important determinant of fecal SCFA, independent of the rate of fermentation (32). Govers et al. (33) suggested that dietary fiber is an important contributor in ensuring that the SCFA produced in the proximal hind gut bowel are transported to the distal colon, the site of most organic large bowel disease. It has been confirmed in pigs (33) and rats (20,34) that combinations of NSP and RS are effective in raising colonic SCFA, which may help to explain the current data.

There have been a number of genetic modifications in cereals to alter the carbohydrate composition of the grain for health and processing purposes. The amylose extender variant of maize has been exploited to raise the proportion of amylose in the grain and then to increase the RS content of processed foods into which it was incorporated (11). A high ß-glucan cultivar of barley has also been developed and has the potential to improve human health (35). Recently, Dongowski et al. (36) reported the beneficial effects of consumption of combinations of barley NSP and RS in rats produced by extrusion of barley and high-amylose starch. The present study shows that Himalaya 292 has similar positive attributes of high-amylose maize for large bowel health, and the presence of high soluble NSP offers further potential in terms of plasma cholesterol reduction. Work is in progress to develop a range of foods containing the cultivar, and further studies in pigs and humans are planned to determine whether these foods retain the expected health benefits.


   ACKNOWLEDGMENTS
 
We thank M. K. Morell, S. Rahman, Z. Li, L. Adler, and J. W. Peacock for advice and encouragement. We thank R. Tupper of the Uncle Tobys Company for preparing the milled cereals and the gift of oat bran and M. K. Morell for providing the Himalaya 292, Waxiro, and Namoi grain.


   FOOTNOTES
 
1 Presented in part in preliminary form at the 21st Annual Scientific Meeting of the Nutrition Society of Australia, Woollongong, New South Wales [Topping, D. L., Morell, M. K., King, R. A. & Bird, A. R. (2002) Resistant starch and health: from concept to products. Asia-Pacific J. Clin. Nutr. 11(suppl.): S296 (abs.)]. Back

Manuscript received 3 June 2003. Initial review completed 23 July 2003. Revision accepted 15 December 2003.


   LITERATURE CITED
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 LITERATURE CITED
 

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