SOLUBLE FIBER SOURCES AND MINERAL BIOAVAILABILITY

12.4 SOLUBLE FIBER SOURCES AND MINERAL BIOAVAILABILITY

12.4.1 I NULIN AND F RUCTOOLIGOSACCHARIDES

Inulin and fructooligosaccharide (FOS) represent soluble, nondigestible fibers that contribute to the production of fermentation products in the large intestine. Feeding

inulin to rats for 3 weeks, at four dietary concentrations ranging from 0 to 20%,

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TABLE 12.3 Insoluble Fiber Sources and Mineral Bioavailablity and Utilization

Substrate

Experimental Subject (Dietary Conc.)

Effect

Reference

Rat Cellulose (12%, 16 Reduced calcium absorption 5 days) Rat

Wheat bran (15% No effect on mineral 76 diet)

bioavailability

Rat Cellulose (20%) Lower duodenal Ca uptake, 74 reduced bone calcium content Human

Cellulose (16 g/day, Normal bowel function; 89

30 days)

increased fecal Ca, Mg excretion; negative Ca balance, n/c Mg balance

Human Carboxymethylcellu Negative Mg balance 67 lose (7.5 g/1000 kcal)

Karaya gum (19–27 No change in bowel transit g/days, 4 weeks)

time, increased fecal dry weight, positive effect on Mg, Ca

Human Wheat bran (18.5 No effect on mineral 77

g/day, 3 × in 24 h)

bioavailability

n/c = no change

increased cecal calcium absorption to an extent that was directly proportional to the amount of inulin consumed up to a 20% dietary level. 40 Inulin, which consists of one α-glucopyranosyl unit linked to β-fructosyl units, has a positive effect on calcium bioavailabilty regardless of the degree of polymerization (DP), which can vary between 3 and 50 DP, depending on the source. For example, feeding inulin products such as Raftiline ® (average DP = 10) and Raftilose ® (average DP = 4.8) at 10% diet for 24 days will produce similar rates of fecal calcium excretion, which corresponds

to a significant increase in apparent calcium absorption. 41 It is important to note that in all of these studies, the level of dietary calcium was an important factor that may

have influenced the action of inulin to improve calcium bioavailability. For example, despite feeding equal amounts of inulin to animals that also received 0.8% dietary

calcium, significantly greater responses to the presence of inulin occurred in animals that were fed 0.3% calcium. 42

Similar findings have been reported in rodents fed FOS. The administration of

a 50 g of FOS/kg diet for 7 days exhibited a 28% increase in apparent calcium absorption over control animals. 43 This was confirmed by other workers who fed only 5% FOS, but for a relatively longer time (e.g., 12 days) and reported both greater true and apparent calcium absorption from calcium balance measurements

and bioavailability estimates using 45 Ca tracers. 44 Enhanced calcium absorption in gastrectomized and sham-operated rats fed FOS for 4 weeks reduced postgastrec-

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tomy osteopenia to a significant extent, but did not completely prevent loss in femoral mineral density compared to sham-operated rates fed the control diet. 45 Extending

the duration of FOS feeding to 6 weeks in the gastrectomized rats was successful at preventing postgastrectomy osteopenia. 46

Studies conducted in human feeding trials with both inulin and FOS have produced less consistent results. For example, feeding healthy young men inulin- containing diets (e.g., 40 g/day) for 4 weeks produced significant increases in calcium

absorption and apparent calcium balance. 9 This result was not confirmed by other workers who fed less inulin (e.g., 15 g/day) to healthy men and used dual-stable- isotope techniques to evaluate calcium absorption, 36 rather than the balance study protocol reported previously. Comparing the results and methodologies of both studies can explain the reason for these discrepancies, with marked differences in experimental design that included the amounts of inulin and FOS fed, as well as methods of assessing calcium bioavailability being particularly different. As shown above, measuring calcium absorption using radio-isotope methodology requires suf- ficient duration for urinary collection to account for calcium absorption from both the small and large intestines, respectively. As reported with earlier TOS findings, feeding 15 g of FOS for 9 days using a randomized, double-blind crossover design produced significant increases in true calcium absorption in response to feeding FOS only when urinary collections were extended to 36 h, from the original 24 h after

isotope administration. 37 The importance of the amount of FOS made available in the diet to produce a positive effect was observed with a study conducted in 59 prepubertal girls fed only 8 g of FOS/day for 3 weeks. 47 Although these workers employed a similar randomized crossover design and measured true calcium absorp- tion by the dual-isotope method, which included an extended urinary collection period to account for colonic calcium uptake, no significant effects of feeding FOS on enhancing calcium bioavailability were observed.

One explanation for a reduced effect of fermentable fibers on calcium bioav- alability over longer experimental durations may involve downregulation of tran-

scellular absorption in the proximate section of the small intestine in response to the long-term exposure to the fermentable carbohydrate-induced calcium absorp- tion in the large intestine. This phenomona was shown by Chonan and Watanuki 48 with galactooligosaccharide feeding to ovariectomized rats that produced a positive effect on calcium absorption after days of short-term feeding, but no effect after

28 days of feeding. Similar results have been reported in both adolescent and postmenopausal women fed FOS, who exhibited an increased calcium bioavail- ability at 9 days, 39,49 but no effect when fed for 3 weeks. 38 A 5-week feeding study with short-chain FOS has been shown to enhance magnesium absorption in post- menopausal women. 50 The 11% increase in magnesium absorption attributed to feeding FOS diets was equivalent to 10 mg of Mg/day and paralleled a 10 mg of Mg/day urinary loss of magnesium, thereby indicating that the true net benefit of feeding FOS was negligible.

Rodent studies have shown that the daily intake of 5 g of FOS/100 g of diet for

40 days produced significant increases in both calcium and magnesium absorption, which corresponded to significant enhancement of trabecular bone volume at the neck of the femur. 51 More recently, the prebiotic action of FOS to enhance intestinal

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microorganism-derived deglycosylation of soy isoflavones, produces greater bioac- tivity for increase femoral bone mineral content in ovariectomized mice. This dem-

onstrates the potential benefit of FOS for bone health. 12

12.4.2 R ESISTANT S TARCH

Resistant starch is another example of an incompletely digested carbohydrate that escapes the small intestine, but is metabolized or fermented by microbes in the large intestine. Feeding potato starch as a source of resistant starch to rats for 3 weeks has been shown to increase the cecal calcium absorption rate by more than fivefold,

compared to control rats. 52 Reducing the dietary intake of potato starch by twofold (e.g., 150 g/kg) produced similar results with increased soluble calcium and signif- icant increases in cecal weight and a decreased pH; 53 the latter effect was directly attributed to the production of short-chain fatty acids (SCFAs). An increase in calcium absorption in rats fed 150 g of resistant starch/kg of diet was shown to be equivalent to the feeding of 50 g of inulin/kg of diet. Moreover, feeding resistant starch to rats for 3 weeks in the form of 50% amylomaize starch produced an increased SCFA concentration together with a decreased cecal pH, which corre-

sponded to increased soluble calcium and an enlarged cecum. 54 Similar results were obtained in adjacent studies with the feeding of 10% lactulose. The underlying mechanism for enhanced mineral bioavailability from soluble carbohydrate sources such as inulin, FOS, some resistant starches, and polyols is linked to the extent of fermentation and resultant products of fermentation (Figure 12.1). The low digestibility of these carbohydrates in the small intestine ensures that the substrate will be available for prebiotic activity for indigenous microflora in the large intestine. Fermentation of carbohydrates by gut bacteria will result in

the production of short-chain fatty acids, 55 which has been attributed to increased cecal weight, reduced pH, and increased soluble calcium and magnesium in this segment of the intestine. 42,56,57 The solubilization of divalent mineral ions in the cecum, due to acidic conditions of fermentation, will favor an increase in soluble ion concentrations that are required for enhanced paracellular absorption. Increases in cecal weight in response to these sources of nondigestible carbohydrates has been attributed to triggered cell proliferation — both cell number (hyperplasia) and cell size (hypertrophy) — by the generation of a butyrate fermentation product. Moreover, generation of butyrate has been associated with stimulation of calbin-

din–D9K expression and increased concentration of 1,25 (OH) 2 D 3 receptor activ- ity. 58 This activity may explain the observation that feeding fermentable carbohy- drate for an extended time period can produce feedback inhibition, which downregulates calcium transcellular absorption in the upper part of the intes-

tine, 9,36,59 albeit magnesium bioavailability is enhanced. 32 The generation of short- chain fatty acids with fermentation may have another distinctly different effect on enhancing calcium bioavailabity in the large intestine. For example, generation of protonated SCFAs that diffuse across the apical membrane eventually dissociates and increases intracellular hydrogen ion concentrations that are secreted from the cell in exchange for soluble calcium ion (Figure 12.2). It has been proposed that the flux of hydrogen ion outside the cell enables protonation of more SCFAs, with

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Soluble fermentable fibre

Fermentation in large intestine Protonated SCFA uptake by

enterocyte (e.g. acetate) Production of SCFAs

Build-up of H + (intracellular) Ca/H exchange

Butyrate

Increased -calbindin — production

Lowering cecal- pH

Substrate for microbial proliferation

Enhanced (1,25) 2 D 3 Increase mineral solubility

Polyamine production

receptor activity

Increased gut absorption area (hyperplasia & hypertrophy)

Enhanced -paracellular— absorption (cecum)

Enhanced cellular absorption (cecum)

Increased mineral balance

Increased bone mass

FIGURE 12.1 Proposed mechanisms of fiber-induced changes on mineral bioavailability.

Enterocyte Apical Membrane

Extracellular

Intracellular

SCFA − H

SCFA + H +

FIGURE 12.2 Schematic of SCFA-induced mineral influx to enterocyte.

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the result that a Ca/H exchange cycle occurs. 60,61 This has been demonstrated in vivo , where graded amounts of SCFAs were reported to increase serum calcium concentrations, in contrast to the decrease in serum acetate response in the presence

of graded amounts of calcium. 62 Thus, generation of SCFAs was active in stimu- lating calcium absorption in the cecum by forming a SCFA–Ca complex, which in turn facilitates the absorption of soluble calcium ions. Couple this effect with the proposed enhanced blood flow triggered by the presence of volatile fatty acids

in the cecum, 63 and it is expected that the net result would be an increased uptake of calcium or magnesium in this section of the intestine. Finally, it should also be considered that increased proliferation of certain micro- organisms in the large intestine from fermentation products of low digestible car- bohydrate could also result in increased phytase activity, thus reducing the potential for phytate-induced restriction of divalent minerals. 64

12.4.3 P ECTIN

Pectin is a heteropolysaccharide that consists mainly of D-galactouronic acids and is linked with other sugar units, such as fucose, xylose, and galactose. It is a soluble and fermentable fiber that tends to form a viscous gel, as well as bind with divalent cations such as calcium. In vitro studies that have examined the effect of pectin on intestinal calcium absorption were conducted in simulated human gastrointestinal digestion procedures that involved dialysis and measurements of calcium release. 60 Calcium released from pectin was shown to be significantly reduced in the simulated small intestine as a result of the tendency of pectin to form a viscous gel upon ingestion and a capacity to bind with calcium due to the presence of the carboxylic acid side chain associated with uronic acid. Since the mineral binding is pH depen- dent (e.g., binding of calcium occurs at basic condition and is released at acidic conditions), it is a logical conclusion that calcium binding in the small intestine occurs in the basic environment. However, entry of pectin into the large intestine will result in partial fermentation, thereby releasing the calcium associated with pectin. With fermentation, the generation of SCFAs decreases the pH of the large intestine, thus facilitating the solubilization of inorganic calcium that leads to enhanced paracellular absorption. Therefore, the sum of both recoveries of calcium may not be different between pectin-containing and control diets. The workers attributed the reduced release of calcium in the simulated small intestinal to the tendency of pectin to bind to calcium and form a viscous gel that entraps free calcium ions. The presence of pectin carboxylic acid side chains that originate mainly from the uronic acid component also contributes to a pH-dependent binding affinity for soluble calcium in this section of the intestine, where paracellular absorption of calcium is the predominant mode of mineral uptake. The passage of pectin into the large intestine results in subsequent fermentation, and thus an induced release of pectin-bound calcium due to the lowering of gastrointestinal pH from the SCFA fermentation products. The increased solubility of calcium as a result of pH lowering explains the significant increase in calcium bioavailability observed in the large intestine-simulated model. A similar mechanism has been proposed for inulin. 52

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Presently, in vivo studies have not confirmed this finding. Experiments conducted in human trials with five medical students fed 36 g of pectin/day for 6 weeks, with

3 weeks of control period prior to treatment, produced no significant increase in calcium balance. 61 There was also no significant increase in the fiber content in the

feces before and after pectin feeding, indicating that pectin was fully fermented. In addition, the fat, nitrogen, total dry matter, and bile acid contents of the excreted feces were increased by 80, 47, 28, and 35%, respectively, indicating an increase in bacterial mass in response to the fermentation of pectin.

12.4.4 G UMS

Gums are soluble polysaccharides that include, in part, glucans, galactomannas, carrageenan, agar, and alginates. These substances are often used as thickening, emulsifying, and stabilizing agents in food materials. Although the effect of gums on calcium bioavailability is controversial, the overall consensus is that there is no effect of gums on enhancing intestinal calcium absorption. One in vitro study showed that alginate (AA), guar gum (GG), and locust bean gum (LBG) reduced

calcium availability when used as a thickening agent in infant formula. 66 These workers used dialyzed whey-based infant formula treated with graded amounts of AA, GG, or LBG, in a preliminary intraluminal digestion step that was adapted to the conditions of infants younger than 6 months, using human milk as the reference. Calcium availability decreased in the presence of each fiber source, with the greatest effect associated with GG- and AA-added formula. Calcium availability from GG-added formula was significantly lower than that from LBG- added formula using a similar concentration of fiber (0.42 g/100 ml of formula). This significant decrease in calcium availability is possibly attributed to both the affinity of fiber to form nonabsorbable complexes with calcium and its capacity to create a viscous environment in the small intestine that would result in entrapped calcium being unavailable for absorption.

Again, the result of gums affecting calcium bioavailability in vitro may be quite different when examined in vivo. For example, refined fibers such as

sodium–carboxymethylcellulose, LBG, or karaya gum produced no effect on cal- cium balance when fed at 7.5 g/1000 calories to 11 human subjects for 4 weeks. 67,68 The apparent calcium balance in non-insulin-dependent diabetic subjects has also been shown to be unaffected after their consumption of guar gum for 6 months. 68

Hara et al. 69 fed nephrectomized rats 50 g of a low-viscosity guar gum hydrolysate (GGH) soluble fiber, obtained by hydrolyzing guar gum with β-1,4-mannanase for

21 days. These workers found that GGH consumption helped to compensate the otherwise reduced proximal calcium absorption caused by nephrectomy by increasing calcium absorption in the large intestine. Femur calcium content, how- ever, did not change. A significant difference in calcium absorption as well as femur calcium content between gastroectomized and sham-operated rats fed with

GGH and a control diet was reported in a later study. 70 GGH-enhanced calcium absorption in the large intestine following GGH fermentation in the cecum will result in acidification of luminal contents, which eventually increases ionized

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calcium concentration. Calcium absorption in the proximal intestine can be unaf- fected due to the fact that GGH has low viscosity.

Gums, such as LBG and GG, generally form viscous gels within the intestine upon ingestion, and therefore it is possible that the change in viscosity restricts calcium bioavailability in the small intestine. Alternatively, fermentation of these particular sources of digestible fiber can release trapped calcium in the large intestine, thus enabling paracellular calcium absorption to occur and a compensation for the reduced calcium absorption occurring in the small intestine. 66

In other studies, phosphorylated GGH (P-GGH) was shown to produce a positive effect on calcium solubilization that influenced calcium absorption based on inhibi-

tion of calcium precipitation by formation of an insoluble calcium phosphate salt. 71 Feeding rats a 50 g of P-GGH/kg diet significantly increased the apparent calcium

absorption as well as the calcium content in the femur.

12.4.5 P OLYDEXTROSE

Polydextrose (PD) is less fermentable and less viscous than GGH. Feeding 50 g of PD/kg to rats for 21 days produced a significant increase in the apparent calcium

absorption and femur calcium concentration in both normal and gastrectomized rats, whereas the same dose of GGH enhanced only the apparent calcium absorption in

gastrectomized rats only. 10 The fact that SCFA production was less in the cecum of the PD-fed rats than in the GGH-fed rats and the control animals, implies that fermentation in the cecum was not a major contributor to the observed enhanced calcium absorption. A similar increase in calcium absorption was obtained when 50 g/l of PD, but not GGH, was added to mucosal fluid of an inverted ileum sac system. These findings indicate that the different effect of PD on calcium absorption, com- pared to GGH, could be attributed to the fact that PD influenced calcium absorption in the small intestine rather than in the large intestine.

12.4.6 P SYLLIUM

Psyllium is an example of a soluble fiber that has been found to reduce calcium bioavailability. 41,72 The strong affinity to form gels and bind to minerals in the

small intestine, but relatively poor fermentation capacity in the colon, likely explains the negative effect of psyllium on calcium balance. In vitro studies have shown psyllium to release the least amount of calcium, compared to pectin and lactulose in a simulated small intestine, due to the viscous nature of the material in the gut and the fact that psyllium binds to calcium. Moreover, psyllium is not fermented to the same extent as pectin or lactulose, thereby causing more bound calcium to occur in the simulated large intestine condition. An in vivo study using rats confirmed the hypothesis that psyllium consumption reduces calcium bioavail-

ability. 72 Feeding rats on 5 or 10% of psyllium for 4 weeks from three different fiber sources — purified psyllium (PP), Metamucil ® (MET), and All-Bran ® Bran

Buds ® (AB) — resulted in a decreased apparent calcium absorption at the 10% level. The negative influence of psyllium on calcium bioavailability was further

evidenced by its effect on bone mineralization, where the decreased apparent

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calcium absorption was accompanied by a significant decrease in the calcium content in hind limb bones and mandibles of rats.