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American Journal of Clinical Nutrition, Vol. 71, No. 5, 1123-1128, May 2000
© 2000 American Society for Clinical Nutrition

Original Research Communications

Arabinoxylan fiber, a byproduct of wheat flour processing, reduces the postprandial glucose response in normoglycemic subjects1,2

Zhong X Lu, Karen Z Walker, Jane G Muir, Tom Mascara and Kerin O'Dea

1 From the Centre for Population Health and Nutrition, Monash University, Melbourne, and Weston Bioproducts, George Weston Foods Limited, Altona, Australia.

2 Supported by an Australian Food Industry Science Center (AFISC) Scholarship through Food Science Australia (ZXL).

3 Reprints not available. Address correspondence to K O'Dea, Centre for Population Health and Nutrition, Monash Institute of Public Health Research, Monash Medical Center, Level 5, Block E, 246 Clayton Road, Clayton, VIC 3168, Australia. E-mail: kerin.odea{at}med.monash.edu.au.


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 SUBJECTS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Background: Arabinoxylan (AX) is the major component of dietary fiber in the cereal grains that make up a large proportion of our diet. However, the physiologic effect of AX is unknown.

Objective: The objective of this study was to determine whether AX improves postprandial glucose and insulin responses in healthy humans.

Design: AX-rich fiber was extracted from the byproduct of wheat-flour processing. Three isoenergic breakfasts, comprising bread, margarine, and jam, had 75 g available carbohydrate, 10 g protein, and 14 g fat and contained 0, 6, and 12 g AX-rich fiber, respectively. Fourteen healthy subjects consumed the 3 breakfast meals in random order on 3 mornings >=3 d apart after an overnight fast. Blood was taken from the subjects at regular intervals over 2 h and was analyzed for glucose and insulin. The palatability of bread containing AX-rich fiber was compared with that of a control bread.

Results: Compared with the control meal containing 0 g AX-rich fiber, the peak postprandial glucose concentration after meals containing 6 and 12 g AX-rich fiber was significantly lower (6.3 ± 1.3 compared with 7.2 ± 1.0 mmol/L, P < 0.01; 5.9 ± 0.9 compared with 7.2 ± 1.0 mmol/L, P < 0.001, respectively). The incremental area under the curve (IAUC) for glucose was 20.2% (95% CI: 5.8%, 34.7%; P < 0.01) and 41.4% (25.9%, 56.8%; P < 0.001) lower, whereas IAUC for insulin was 17.0% (2.0%, 32.1%; P < 0.05) and 32.7% (18.8%, 46.6%; P < 0.001) lower, respectively. Bread containing AX-rich fiber was as pala as 50% whole-wheat bread when evaluated with sensory analysis by 30 volunteers.

Conclusions: Postprandial glucose and insulin responses were improved by ingestion of AX-rich fiber. Further research is required to determine whether AX-rich fiber is of benefit to people with type 2 diabetes.

Key Words: Arabinoxylan • hemicellulose • dietary fiber • byproduct of wheat-flour processing • meal tolerance test • bread • blood glucose • blood insulin • healthy volunteers


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
SUBJECTS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Arabinoxylan (AX) is a hemicellulose that has a xylose backbone with arabinose side chains (1). As a major component of dietary fiber, it is found in many cereal grains (2). For example, wheat grain, which is composed of an outer layer of bran and an inner layer of endosperm, is a rich source of AX. The nonstarch polysaccharides (NSPs) in wheat bran are {approx}64–69% AX and 15–31% cellulose (3, 4), whereas NSPs in wheat endosperm are {approx}88% AX (5). Because AX is the major dietary fiber component in the cereal grains that make up a large proportion of our diet, it is important to study its physiologic effects. Although the beneficial effects of wheat bran on health are now well documented (6, 7), little is known of the health effects of purified AX.

AX is difficult to extract from wheat bran but can be produced from wheat endosperm during the commercial processing of wheat flour. When wheat flour is processed to produce starch and gluten, the fiber component, which is mainly AX, is left in the byproduct (8). We recently extracted an AX-rich fiber from this byproduct. We have preliminary data in rats that directly compare AX-rich fiber with guar gum and wheat bran and that indicated that AX-rich fiber behaves like guar gum. Both were predominantly fermented in the cecum (Lu et al, unpublished observations, 1997). AX-rich fiber therefore appears to act as a rapidly fermentable, soluble fiber.

The beneficial effects of soluble fiber on carbohydrate metabolism are well documented. Guar gum was shown to improve the postprandial glucose response in healthy people (9–11) and in people with type 2 diabetes (12, 13). In people with type 2 diabetes, soluble fiber was shown to improve long-term glycemic control (14, 15). Other soluble fibers, such as ß-glucan (16), pectin (17), psyllium (18), and konjac-mannan (19), were also shown to be of benefit. Similar data, however, are not yet available for the physiologic effects of AX, although a recent study in humans showed that the addition of 10 g AX isolated from maize/d to the diet for 6 mo improved glucose tolerance and hemoglobin A1c concentrations in obese people with diabetes (20). That study, however, lacked a control group. Therefore, the aim of the present study was to compare the effect of meals containing 0, 6, and 12 g AX-rich fiber on postprandial glucose and insulin responses in healthy subjects. Thus, we hoped to determine whether AX-rich fiber has potential for incorporation into fiber-enriched foods suitable for use in the management of type 2 diabetes. The poor palatability of guar gum when incorporated into fiber-enriched food products has limited its practical use. Therefore, a second aim of this study was to examine sensory responses to a bread containing AX-rich fiber.


   SUBJECTS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 SUBJECTS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Subjects for the physiologic testing of meals and blood sampling
Eighteen university staff and students were recruited for this physiologic study. Blood lipid concentrations were assessed in all the volunteers during a first visit. Fasting total body weights and heights were obtained with a digital scale (model 708; Seca, Hamburg, Germany) and a wall-mounted stadiometer (model 26SM, Seca), respectively. The subjects were wearing light clothing and no shoes when the measurements were obtained. Body mass index (BMI; in kg/m2) was calculated. Fourteen healthy volunteers (5 men and 9 women) aged 32.0 ± 6.6 y, with a mean (±SD) BMI of 22.7 ± 4.3, serum cholesterol of 4.12 ± 0.67 mmol/L, and serum triacylglycerol of 1.16 ± 0.45 mmol/L completed the study. The subjects received written information about the study and all gave their informed consent. The study was approved by the Ethics Committee of Deakin University. Subjects consumed breakfast meals containing 0, 6, and 12 g AX-rich fiber in random order on 3 separate mornings >=3 d apart after an overnight fast. The subjects were blinded as to the test meals they received. An indwelling catheter, kept patent with sterile 0.9% saline solution, was inserted into a vein in the cubetal fossa and a fasting blood sample was collected. Subjects then consumed 1 of the 3 test breakfasts within a 15-min period and further blood samples were collected 15, 30, 45, 60, 75, 90, and 120 min after breakfast began.

Arabinoxylan-rich fiber and the test breakfast meals
AX-rich fiber was extracted from the byproducts of wheat-flour processing. Briefly, an AX-enriched residue remaining after wheat-flour processing was collected onto a sieve (75 µm), washed thoroughly with water, and spray-dried to a powder. Three test breads were made by adding 0%, 7%, or 14% AX-rich fiber on a dry-weight basis to a standard recipe for white bread. These breads were baked by a professional baker and were similar in flavor. The bread containing 14% AX-rich fiber was slightly darker in color and was more moist than was the bread containing 0% AX-rich fiber. The bread was stored frozen at -20°C and thawed before use. The AX-rich fiber and each test bread were analyzed 3 times in duplicate for starch, protein, and lipid contents, as described previously (21). Briefly, starch content was measured with the Megazyme Total Starch Kit (Megazyme, Dublin), nitrogen content was measured by using an automated Kjeldahl procedure (Gerhardt Kjeldatherm, Turbosog and Vapodest, Bonn, Germany) before protein content was calculated by using a conversion factor, and lipid content was measured gravimetrically after chloroform:methanol (2:1) extraction. The water contents of the breads were determined after freeze drying and the water content of the AX-rich fiber was assessed with a moisture analyzer (Mettler LJ16; Mettler-Toledo AG, Greifensee, Switzerland). In addition, AX-rich fiber was analyzed 3 times in duplicate for total dietary fiber by using the procedure of the Association of Official Analytical Chemists (22) and for NSP content by using a spectrophotometric method (23). The sugar composition of the NSP component of AX-rich fiber was analyzed in duplicate by using gas-liquid chromography (23). The AX content of the AX-rich fiber was calculated as the sum of the arabinose and xylose contents. The ingredients used in the test breakfast are listed in Table 1Go. Each breakfast meal comprised 3–4 slices of test bread, lightly toasted, and 33 g jam to provide 75 g available carbohydrate. The breakfast meal also included 15 g margarine and a cup of weak tea with milk. The macronutrient contents of the jam, milk, and margarine used in the test breakfast were determined by reference to Australian food-composition tables (24) and the macronutrient contents of the test breads were analyzed as described above.


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  		TABLE 1. Ingredients and macronutrient contents of test breakfasts
 
Measurement of blood glucose, insulin, and lipids
Blood was collected into 2-mL fluoride heparin–containing tubes. Plasma glucose was measured in triplicate with an automatic analyzer (model 705; Hitachi, Tokyo) and commercially available colorimetric reagents (Boehringer Mannheim GmbH Diagnostica, Mannheim, Germany). Plasma insulin was measured in duplicate by radioimmunoassay (Linco Research Inc, St Charles, MO). The intraassay and interassay CVs for glucose at 6.45 mmol/L were 0.7% and 0.9%, respectively. The corresponding CVs for insulin at 287 pmol/L were 1.6% and 4.5%, respectively. Serum total cholesterol and triacylglycerol concentrations were assayed in duplicate with an automatic analyzer (Hitachi) and commercially available enzymatic colorimetric reagents for cholesterol and triacylglycerol measurement, respectively (Boehringer Mannheim). The interassay CV was 4.1% at 4.19 mmol/L and 3.1% at 7.67 mmol/L for serum cholesterol and 3.7% at 1.69 mmol/L and 2.5% at 4.53 mmol/L for serum triacylglycerol.

Calculation of glycemic index and insulinemic index and dietary fiber content of breads containing arabinoxylan-rich fiber
The incremental areas under the plasma glucose and insulin response curves were measured as described by Wolever and Jenkins (25) whereas the glycemic index (GI) of the breads containing AX-rich fiber was calculated as previously described by Jenkins et al (26). Briefly, the GI of bread containing AX-rich fiber was calculated by expressing the incremental glucose area for the test meal that included bread containing AX-rich fiber as a percentage of the incremental glucose area for the meal that included the control (white) bread consumed by the same subject. The insulinemic index (II) of the bread containing AX-rich fiber was calculated in a similar manner.

The dietary fiber contents and GIs of the bread containing AX-rich fiber were compared with those of other breads. The dietary fiber contents of the breads containing AX-rich fiber were calculated as the amount of total dietary fiber in all the ingredients of the bread (white flour, whole-wheat flour, and AX-rich fiber) divided by the weight of ingredients plus added water. The total dietary fiber content of the white and whole-wheat flour and other breads was obtained from Australian food-composition tables (24) and the dietary fiber content of the AX-rich fiber was directly analyzed. The mean GIs of the other breads were obtained from an international GI table as reported by Foster-Powell and Miller (27). Briefly, for each type of bread, GIs from different studies listed were averaged and an SD was calculated. GIs were taken only from studies in which white bread was used as a reference food.

Sensory evaluation
A bread containing AX-rich fiber was compared with a control bread by 30 untrained assessors (7 men and 23 women) recruited from healthy university staff and students. None of these subjects had participated in the physiologic testing of the meals as outlined above. The assessors (mean age: 29 ± 9 y) had no previous experience in tasting the experimental bread samples and had no knowledge of the experimental protocol for the meal tolerance test. The control bread used for the sensory evaluation was made with 50% whole-wheat and 50% white flour. This was compared with a bread containing AX-rich fiber that was made with 50% whole-wheat flour, 36% white flour, and 14% AX-rich fiber. For evaluation, the control and AX-rich-fiber breads were cut into similar small triangles and randomly coded with a 3-digit number to blind the assessors. No spread was used. Assessors were given a separate evaluation sheet for each bread sample and were asked to grade the breads in terms of flavor, color, texture, and overall quality by using a hedonic scale of 9 scores ranging from "like extremely" to "dislike extremely" (28). The assessors were instructed to score flavor on the basis of the aroma and taste of the samples and to score texture on the basis of touch and mouth feel (29). Water was provided for rinsing the mouth before each taste.

Statistical analysis
Results are presented as means ± SDs. A repeated-measures general linear model (GLM) using time, meal type, and time and meal type interaction as within-subject factors was used to compare the effects of meals containing different amounts of AX-rich fiber at different time points over 2 h postprandially. Differences in IAUC and in the GI and II were also tested by repeated-measures GLM followed by contrast GLM. To determine whether there was a dose-dependent effect, one-tailed Pearson's correlation was used to correlate between the mean IAUC for glucose and insulin and the different fiber contents of the meals. Differences in hedonic score between the 2 types of bread were analyzed by the Mann-Whitney nonparametric U test. In all analyses, a value of P < 0.05 was considered significant. All statistical analyses were performed by using SPSS 9.0 for WINDOWS (SPSS Inc, Chicago).


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 SUBJECTS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Composition of the arabinoxylan-rich fiber and of test meals
The composition of the AX-rich fiber used in this study is shown in Table 2Go. The AX-rich fiber contained no fat, little protein, and some starch but was rich in NSP (69.9%); the ratio of soluble to insoluble NSP was 1.6. The NSP of the AX-rich fiber contained 35.8% and 54.1% arabinose and xylose (% by wt), respectively, plus small amounts of mannose, galactose, and glucose. Thus, 89.9% of the NSP in AX-rich fiber was made up of AX. The breads supplemented with AX-rich fiber had a higher moisture content than did the control white bread (37.0 ± 0.4%, 43.4 ± 0.4%, and 48.8 ± 0.4% for the breads supplemented with 0%, 7%, and 14% AX-rich fiber, respectively). All meals used in this study were isoenergic (1967 kJ) and of a similar total weight (420 g). The ingredients and macronutrient contents of the meals are shown in Table 1Go. In each meal, carbohydrate, protein, and fat contributed 65%, 9%, and 26%, respectively, of total energy. The meals that included breads containing 7% and 14% AX-rich fiber provided 6 and 12 g AX-rich fiber, respectively.


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  		TABLE 2. Composition of the arabinoxylan-rich fiber added to the test breads1
 
Plasma glucose response to test meals
The mean plasma glucose response to the test meals is shown in Figure 1AGo. Fasting plasma glucose concentrations did not differ before the 3 treatments. After the subjects consumed the test meals, plasma glucose concentrations peaked at 30 min and had returned to baseline concentrations by 60 min. The peak postprandial glucose concentrations after meals containing 6 and 12 g AX-rich fiber were significantly lower than after the control meal (P < 0.01 and P < 0.001, respectively). There was no significant difference in the peak glucose concentrations between the meals containing 6 and 12 g AX-rich fiber (P = 0.219). At 45 min, only the meal containing 12 g AX-rich fiber produced a significantly lower plasma glucose response than did the control meal (P < 0.01).



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  		FIGURE 1. A: Mean (±SEM) effect of arabinoxylan (AX)-rich fiber on the postprandial glucose response over 120 min (n = 14) after a control meal containing 0 g AX-rich fiber (•), a meal containing 6 g AX-rich fiber ({triangleup}), and a meal containing 12 g AX-rich fiber ({blacksquare}). *Significantly different from the control meal (P < 0.01). **Value at the same time point is significantly different from the control meal (P < 0.001) but not different from the meal containing 6 g AX-rich fiber. Data were analyzed by using a repeated-measures general linear model (GLM) followed by contrast GLM using time, meal type, and time and meal type interaction as within-subject factors. B: Incremental area under the curve (IAUC) for glucose from 0 to 120 min. The bars represent means ± SEMs (n = 14). *Significantly different from the control meal (P < 0.05). **Significantly different from the control meal (P < 0.01) and the meal containing 6 g AX-rich fiber (P < 0.05) on the basis of repeated-measures GLM followed by contrast GLM.

 
Plasma insulin response to test meals
The mean plasma insulin response (Figure 2AGo) to test meals containing different amounts of AX-rich fiber resembled those seen with plasma glucose. Fasting plasma insulin concentrations did not differ before the treatments and plasma insulin concentrations peaked at 30 min for each meal and fell slowly thereafter. By 120 min, although insulin concentrations were declining, they had not returned to baseline concentrations. Differences in plasma insulin concentrations between the 3 meals did not reach significance at each time point over 2 h except at 45 min, when only the meal containing 12 g AX-rich fiber resulted in a significantly lower concentration compared with the control meal (P < 0.001).



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  		FIGURE 2. A: Mean (±SEM) effect of arabinoxylan (AX)-rich fiber on the postprandial insulin response over 120 min (n = 14) after a control meal containing 0 g AX-rich fiber (•), a meal containing 6 g AX-rich fiber ({triangleup}), and a meal containing 12 g AX-rich fiber ({blacksquare}). *Value at the same time point is significantly different only from the control meal (P < 0.05) on the basis of repeated-measures general linear model (GLM) followed by contrast GLM. Panel B: Incremental area under the curve (IAUC) for insulin from 0 to 120 min. The bars represents means ± SEMs; n = 14. *Significantly different from the control meal (P < 0.05) but not significantly different from each other on the basis of a repeated-measures GLM followed by contrast GLM.

 
Incremental areas under the curves for glucose and insulin
The IAUC for glucose (Figure 1BGo) was 20.2% (95% CI: 5.8%, 34.7%; P < 0.01) and 41.4% (95% CI: 25.9%, 56.8%; P < 0.001) lower after breakfasts containing 6 and 12 g AX-rich fiber, respectively, than after the control meal. The difference in IAUC for glucose between the meals containing 6 and 12 g AX-rich fiber was also significant (P < 0.05). The corresponding reduction in IAUC for insulin (Figure 2BGo) was 17.0% (95% CI: 2.0%, 32.1%; P < 0.05) and 32.7% (95% CI: 18.8%, 46.6%; P < 0.001), respectively, compared with the control meal. The difference in IAUC for insulin between the meals containing 6 and 12 g AX-rich fiber was not significant. An inverse relation was obtained between the amount of AX-rich fiber in meals and the mean IAUC for glucose (r2 = 0.989, P = 0.033) and insulin (r2 = 0.999, P = 0.008).

Relative to the GI of the control bread (GI = 100), the GIs of the breads containing 7% and 14% of AX-rich fiber were 79.7 ± 25.0 and 58.6 ± 26.7, respectively, whereas corresponding IIs were 83.0 ± 26.0 and 67.3 ± 24.0, respectively.

Sensory evaluation
No significant differences in the mean scores between the control bread and the breads containing AX-rich fiber were observed for flavor and color, texture, or overall quality. The median scores for color and flavor, texture, and overall quality were 4 (range: 2–6), 3 (2–7), 4 (2–7), and 4 (2–7), respectively, for the control bread, whereas the median scores were 4 (2–6), 3 (1–7), 4 (2–7), and 3 (2–6), respectively, for the bread containing 14% AX-rich fiber.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 SUBJECTS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
The AX-rich fiber prepared from the byproducts of wheat-flour processing that was used in this study was highly enriched in AX. AX made up nearly 90% of the total NSP content of this AX-rich fiber, which is consistent with the proportion of AX in the NSP of wheat endosperm reported previously (5). The ratio of arabinose to xylose in the AX-rich fiber obtained in this study was 0.66, which is also consistent with values ranging from 0.55 (30) to 1.19 (2) previously reported for wheat.

Addition of as little as 6 g AX-rich fiber to bread eaten in a breakfast meal significantly lowered postprandial glucose and insulin responses in healthy persons. The response was dose dependent, as indicated by the strong inverse relation between the amount of AX-rich fiber in the meals and the mean IAUC from each group for glucose (r2 = 0.989, P = 0.033) and insulin (r2 = 0.999, P = 0.008). Moreover, bread containing AX-rich fiber proved palatable and acceptable to subjects. The effect of AX-rich fiber on glucose and insulin responses in this study resembled those reported previously in studies using other soluble fibers, such as guar gum (31) and psyllium (18).

The mechanisms by which AX-rich fiber flattens the postprandial glucose response are as yet unknown, but because AX is a soluble fiber (Lu et al, unpublished observations, 1997), it is likely that its high viscosity may slow the rate of gastric emptying (9, 32) and reduce small intestinal motility (33), which results in delayed glucose absorption and hence, a flattened blood glucose response.

Many studies showed the efficacy of guar gum in lowering blood glucose response both in healthy people (9–11) and in people with type 2 diabetes (12–14). However, the widespread use of guar gum has been hindered by its relatively poor palatability. In contrast, breads supplemented with AX-rich fiber were shown to have palatability equal to that of a 50% whole-wheat bread. In addition, the breads made with AX-rich fiber had a low GI, which compares very favorably with that reported in a review of studies of other breads (27). Only mixed-grain bread has a similar GI (64 ± 17; n = 4 studies) to that of bread containing 14% AX-rich fiber (59 ± 27) and only bread made with a high proportion (>75%) of barley kernels has a lower GI (49 ± 10; n = 3 studies). Inclusion of mixed-grain or whole-wheat breads in the diet is usually recommended for people with type 2 diabetes (34). Mixed-grain breads are high in fiber (5.1% by wt) (24) and have a lower GI than other breads but they can be difficult to chew, particularly for elderly people with poor dentition. Although whole-wheat bread is rich in fiber (6.5% by wt) (24) and is easy to chew, it has a high GI (99 ± 10; n = 10 studies). Bread containing AX-rich fiber avoids these problems: it has a low GI, it is rich in dietary fiber (4.8% and 6.7% by wt, respectively, for the breads containing 7% and 14% AX-rich fiber), and, because it does not contain kernels, it is also easy to chew. In addition, bread containing AX-rich-fiber retains moisture and has a pleasant mouth feel.

In the past 2 decades, the possibility that low-GI foods have a role in both prevention and treatment of type 2 diabetes has been examined. Diets of high glycemic load have been reported to increase the risk of type 2 diabetes in men (35) and women (36). Moreover, these studies showed an inverse relation between intake of cereal fiber and risk of diabetes. Low-GI diets followed for 5–12 wk were shown to improve fasting blood glucose and serum fructosamine concentrations in people with type 2 diabetes (37, 38). In a meta-analysis in which the use of low-GI food for the treatment of diabetes was reviewed, Miller (39) reported that, on average, low-GI diets reduced glycosylated hemoglobin by 9%, fructosamine by 8%, urinary C-peptide by 20%, day-long blood glucose by 16%, cholesterol by 6%, and triacylglycerol by 9%. More recently, a study showed that consumption of low-GI foods by people with type 2 diabetes not only improved peripheral insulin sensitivity but also lowered plasma plasminogen activator inhibitor-1 (PAI-1), thereby improving the capacity for fibrinolysis (40). In our study, responses to the breads containing AX-rich fiber were tested in healthy persons. However, if the results parallel those of studies in which guar gum was used, similar or more pronounced responses might be expected in people with type 2 diabetes (12, 18), a conclusion that will require confirmation in subsequent studies.

In conclusion, the present study showed beneficial effects of AX-rich fiber prepared as a byproduct of wheat-flour processing on postprandial glucose and insulin responses in healthy persons. The benefit of AX-rich fiber is 3-fold. AX-rich fiber is a natural dietary fiber that can be added as a supplement to a wide range of cereal products, including bread, muffins, and breakfast cereals. These foods could have utility in the diet of the general population as fiber-rich foods that retain palatability. In addition, foods containing AX-rich fiber may have utility in the diets of people with diabetes or impaired glucose tolerance. Further studies are required to determine this potential. Finally, because substantial amounts of white flour are used worldwide for the production of starch and gluten, the supplementation of foods with AX-rich fiber provides a potential commercial use for an otherwise discarded product.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 SUBJECTS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 

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Received for publication August 20, 1999. Accepted for publication October 25, 1999.




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