We gave our subjects 12 g per day of different fibers, a palatable amount representing the usual gap in dietary fiber intake in the USA. Despite this increase in fiber intake, few changes in laxative effect were found. There was no change in stool output, weight or consistency with treatment, but it should be noted that all subjects had normal stool weights at the start of the study and were not constipated. This modest amount of fiber was well tolerated by the subjects, and the study fibers could easily be added to low-fiber foods.
The present study was designed as a placebo-controlled, crossover study, and statistical analyses were planned according to this design. Each fiber treatment was compared to the placebo to determine a treatment effect. Subjects often behave differently while enrolled in a clinical trial. Using a placebo as opposed to baseline measurement takes into account any differences as a result of being a study participant, but without an increase in fiber consumption. As such, baseline data, aside from anthropometric measurements (data not shown), were not collected.
The type of fiber can influence fecal output and characteristics in humans. A randomized crossover experiment with practical doses of isolated fibers (14–15 g/day psyllium, guar gum or xanthan gum) showed that each fiber had a different effect on fecal output and the response was highly individualized [12
]. The effect of RS on fecal output has been mixed and may depend on the physicochemical properties of the specific RSs. High doses of wheat dextrin (up to 80 g/day) had only a minimal effect on fecal output [13
], while another study with lower doses of RS2 and RS3 (30 g/day) showed that both altered fecal weight in healthy humans [14
]. Retrograded maltodextrin (40–60 g/day) administered for 21 days produced an increase in total bowel movements and frequency of watery feces [15
]. Fecal output was increased by the addition of 17–30 g/day RS from a variety of sources (banana, wheat, potato and maize) [16
]. A low dose of konjac glucomannan (4.5 g/day) significantly increased the number of bowel movements per day as well as daily fecal wet weight [17
]. This illustrates that the laxative effect of fiber cannot be generalized across all types of fiber or even generalized within a specific class of fiber, and the fiber dose for increased laxation is dependent on the fiber type.
One mechanism by which dietary fiber is thought to alter stool weight and consistency is by fermentation of the fiber to SCFAs. SCFA concentrations after the treatments used in this study did not differ from control. Since the majority of SCFAs (>90%) are rapidly absorbed from the lumen [18
], excreted SCFA concentrations provide only an estimate of actual SCFA production. Mean stool pH did not reflect the total SCFA concentrations, which may be due to the presence of other anionic compounds (succinate, lactate, bicarbonate) in the stool. SCFA production shifts from butyrate to acetate and propionate as colonic pH increases (5.5–6.5) [19
]. The observed change in pH was not reflected in the percentages of acetate, propionate and butyrate recovered in the stool. The pH change may have been too small to produce a measurable difference in SCFA production. Other studies with RS have shown a change in SCFA and fecal pH. However, these studies administered doses ranging from 30 to 50 g/day [20
]. A larger sample size and more specific fermentation metabolite analysis may be required to see a relationship between pH and SCFA concentration.
The addition of 12 g/day fiber to the diet represented a significant increase in daily fiber intake for the subjects. Fiber intake in female subjects was slightly over the recommended levels, while male subjects, even with the supplementation, still consumed approximately 8 g/day fiber less than the Dietary Reference Intake (DRI) fiber recommendation.
GI symptoms ratings with fiber supplementation were mild to moderate in our study. RS is well tolerated in humans, even in high doses (45–60 g/day) [15
]. RS doses above 60 g/day produced greater flatulence, abdominal distension and abdominal cramping compared to control treatments in previous reports [13
]. Similar to the current study, administration of 10.33 g/day RS3 for 7 days did not affect flatulence [26
]. The high tolerance of RS in the present study was expected due to the low daily dose (12 g/day). Gas production (flatulence and distention) after pullulan administration has been reported previously and was significantly higher when subjects consumed 50 g of high-molecular weight pullulan (molecular weight 100,000) compared to control [27
Lipid values with all treatments fell near or within the recommended ranges for triglycerides (<150 mg/dl), total cholesterol (<200 mg/dl), LDL cholesterol (<100 mg/dl) and HDL cholesterol (>40 mg/dl) [28
]. RS, when incorporated into foods or consumed as a supplement, has inconsistent effects on cholesterol and triglyceride concentrations [14
]. All of the studies reporting decreased triglycerides required the subjects to consume controlled meals with RS, which may have improved compliance. Typically, viscous, soluble fibers are most likely to lower total serum cholesterol and LDL cholesterol.
Most studies with RS see no change in fasting glucose or fasting insulin [22
] after chronic RS intake; however, a recent study with obese women showed that 40 g of RS administered for 21 days decreased fasting glucose [33
]. Pullulan decreased the postprandial glucose response in humans compared to a digestible control (maltodextrin) [34
Ghrelin is a gut hormone that signals satiety. The majority of data on ghrelin and dietary fiber are postprandial data. The present study showed no effect of chronic fiber intake on fasting ghrelin concentrations. Previous studies have reported mixed effects of fiber on fasting ghrelin concentrations [35
]. Standard fasting ghrelin concentrations have not been established, so it is not possible to categorize subjects as ‘normal’ or ‘out of range’. In the present study, hunger before meals did not differ with the fiber treatments.