CARBOHYDRATES AND MINERAL BIOAVAILABILITY AND UTILIZATION

12.3 CARBOHYDRATES AND MINERAL BIOAVAILABILITY AND UTILIZATION

12.3.1 N ONFIBER C ARBOHYDRATE S OURCES

Polyols, namely, lactitol, xylitol, maltitol, and isomalt, are carbohydrates com- monly used in the low-sugar or sugar-free confectionary that produces products with a low glycemic index, reduced energy, and noncarcinogenicity. These sources of carbohydrate possess low digestibility in the small intestine, but are greatly degraded by fermentation in the large intestine. Malitol produces a beneficial effect

on intestinal calcium bioavailabity in the rat, 30 as does xylitol. 31 Human studies have indicated that a 100 g/day intake of a polyol-containing diet for 1 month effectively improved magnesium absorption in young healthy males by 25%, but had no effect on calcium balance. 32

Lactulose, analogous to lactose but synthesized through alkaline isomerization of lactose, is a good example of an osmotically active agent that has no effect on enhancing calcium bioavailability in vitro. 33 In vivo studies conducted in rats, how- ever, have reported an increase in fractional calcium absorption that was maximum at 10% lactulose concentration. This effect was specific to lactulose, since feeding the component sugars (e.g., galactose and fructose) at equimolar concentrations had

no effect on fractional calcium absorption. 31 A second example of a carbohydrate- based facilitated enhancement of calcium absorption occurs with transgalactooli- gosaccharides (TOSs), a mixture of glucose and galactose (or galactosyllactoses), which are obtained by a transgalactosylation reaction catalyzed by β-D-galactosi- dases derived from Aspergillus oryzae. 34,35 TOS, found in human and bovine milk and commercial yogurt, can improve apparent calcium absorption and retention in

Carbohydrates and Mineral Metabolism

rats in a manner similar to that of lactose at dietary intake levels of 5 and 10%. 34 For both lactulose and TOS, the increase in calcium absorption is accompanied by

a significant increase in both cecal wall and digesta weight, as well as a significant decrease in cecal pH. These responses of osmotically active carbohydrate are also

related to an increased fluid uptake within the small intestine lumen, which occurs in response to maintain isotonicity. The resultant increased distension and perme-

ability of the intracellular junction between enterocytes as a result of the solvent drag defines the mechanism for increased passive absorption of calcium in response

to TOS feeding. In human studies, TOS feeding for 3 weeks at a level of 10 g/day resulted in a significant increase in fecal bifidobacteria. 35 Although studies that have

focused on demonstrating the prebiotic potential of TOSs have not extended them to actually measure calcium bioavailability, other workers have attempted to link TOS feeding with increased calcium absorption. For example, feeding 15 g of TOS/day for 3 weeks to healthy young men failed to show enhanced uptake of

calcium from 24-h urinary excretion data. 36 Extending the urinary collection period to 3 days to capture the impact of late colonic absorption was successful in showing that TOS feeding could enhance calcium absorption. 37

12.3.2 D IETARY F IBER C ARBOHYDRATE S OURCES

The characteristics of chemical composition and structural integrity of plant cell walls are critical factors for the different physicochemical properties of recovered fiber sources that influence gastrointestinal function. Due to the relative complexity of dietary fiber sources to otherwise simpler carbohydrates, an absolute extrapolation of mechanism of action explaining calcium bioavailability–fiber interactions is not completely warranted, albeit some similarities do exist. Dietary fiber, by definition, refers to the nondigestible carbohydrate and lignin plant materials that are unavail- able for direct use by the host. Insoluble fiber sources, such as cellulose and wheat bran, have a limited effect on passage time and digestion or absorption activities in the stomach or small intestine, whereas the affinity to form viscous mixtures from soluble fiber sources (e.g., β-glucans) will change the rheological behavior of the

stomach contents and result in a slower stomach emptying time. 39 Dietary fiber that eventually escapes the digestive processes of the small intestine becomes available for fermentation by microflora in the large intestine to produce a mixture of end products that include short-chain fatty acids (SCFAs) such as acetate, butyrate, and propionate. The intensity of the fermentation, and thus the rate of synthesis of the SCFAs, is influenced by the source of the fuel (e.g., prebiotic) for the proliferation

of microflora. 39 A further characterization of dietary fiber can be made on the basis of relative solubility, which differentiates the fiber source into soluble (e.g., pectin, gums, and mucilages) and insoluble (e.g., cellulose and mucilages) forms. Depend- ing on the definition of the dietary fiber source, it is important to recognize that the effect of fiber on calcium bioavailability should be regarded as fiber specific (Table

12.2 and Table 12.3). For example, soluble fibers contribute more to viscosity changes in the gastrointestinal tract and products of fermentation, while insoluble

fibers may influence calcium absorption by altering intestinal transit time or the actual sequestering of ion.

Functional Food Carbohydrates

TABLE 12.2

Soluble Fiber Sources That Enhance Mineral Bioavailablity and Utilization

Experimental Subject

Substrate (Dietary Conc.) Effect Reference

Rat Inulin (10%) Enhanced cecal weight, 40 generation of SCFA, decreased pH, increased soluble calcium, magnesium

(15%) Increased soluble cecal calcium 41, 42, 57 concentration, stimulated ODS activity, increased butyrate output

Human Inulin (40 g/day) Increased apparent magnesium 9

balance

Rat FOS (75 g/kg) Enhanced calcium absorption 45, 46, 54 Mice

FOS (5% w/w) Reduced fecal calcium loss 11 FOS (5%) + isoflavone

Prevented femoral bone loss 12 Rat

FOS (6%, 14 days) Generation of SCFA in bowel 57 Human

FOS (5 g/day, 10 days) Enhanced magnesium absorption 50 Rat

GOS Enhanced calcium absorption 34 Human

TOS (10 g/days, 2 days) Enhanced intestinal bifidobacteria 35 TOS (15 g/days, 1–3 weeks)

Increased urinary calcium output 37 Human

Amylomaize RS (25–50%, 21 Increased SCFA production 9 days) Rat

Potato RS (35%, 21 days) Increased cecum absorption area, 32 lowered pH, increased calcium uptake

Human Pectin (36 g/days, 5 weeks) Increased bacterial population, 65 increased calcium absorption Rat

Pectin Enlargement of cecum, SCFA 66 production, drop in cecal pH Rat

Psyllium (5–10%) Increased fecal dry weight, lower 90 apparent Ca absorption, lower Ca content in femur/tibia

Monkey Increased hypertrophy in

jejunum/ileum

Rat Guar gum hydrolysate (50-g Increased calcium absorption, 69–71 diet/kg, 1–3 weeks)

lower cecal pH, higher SCFA