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4 Institute of Physiology and Biochemistry of Nutrition, Federal Research Centre for Nutrition and Food–Location Kiel, D-24103 Kiel, Germany and 5 Medical Physics, Department of Diagnostic Radiology and 6 Department of Oral and Maxillofacial Surgery, University Hospital Schleswig-Holstein, Campus Kiel, D-24105 Kiel, Germany
* To whom correspondence should be addressed. E-mail: katharina.scholz-ahrens{at}bfel.de.
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ABSTRACT |
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Synergy effects are also postulated for the combination of inulin and fructooligosaccharides (FOS) or FOS with various degrees of polymerization (DP). The mixture of such nondigestible carbohydrates of different chain lengths is assumed to be fermented sequentially and thus over a wider range of the large intestine and with improved efficacy.
The following review is restricted to reporting the current knowledge on the effects of prebiotics, probiotics, and synbiotics on mineral absorption, bone mineral accretion, and parameters of bone structure.
Prebiotics in animal studies: mineral absorption and bone mineral
Inulin, oligofructose, and galactooligosaccharides are the most intensively investigated prebiotics with regard to mineral absorption and retention. In addition, resistant starches and sugar alcohols have been shown to significantly increase mineral absorption and BMC (for reviews see 10–13). Recently it was shown that difructose anhydride prevented parameters of iron deficiency more effectively than FOS (14). Most of the studies on the effects of prebiotics, especially those with respect to bone development, have been performed in rats. It was shown that prebiotics stimulated the absorption of iron and of bone-relevant minerals such as calcium, magnesium, and zinc in short-term experiments and improved BMC in the long-term perspective (10–13). The stimulation of calcium and magnesium absorption by oligofructose was also confirmed for dogs (15). Although phosphorus accretion in the femur might be higher with FOS feeding under special conditions (16), phosphorus balance or retention was not affected in rats or dogs (15,17,18).
In rats results on synergy effects were equivocal (Table 1). In adult animals absorption of calcium and magnesium was stimulated in groups receiving inulin or resistant starch. However, the effect on calcium absorption was more significant if a combination of the 2 was given (19). In growing rats only inulin alone significantly increased BMC and density and decreased urinary excretion of collagen crosslinks, a marker of bone resorption, but not oligofructose alone or oligofructose combined with inulin (20). Several FOS with different DP stimulated magnesium absorption and retention, but magnesium retention was stimulated more significantly with oligofructose and calcium retention was stimulated only with a blend of oligofructose and an inulin with a high DP ("synergy1") than with 2 different types of inulin. The effect on calcium retention was less distinct: Only the synergy product stimulated calcium retention significantly (21). More studies are needed to elucidate the circumstances under which long-chain FOS, short-chain FOS, or a synergistic combination is best to improve calcium balance or bone mineral accretion.
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Oligofructose prevented ovariectomy-induced loss of BMC as indicated by lower bone volume/tissue volume (BV/TV). Beyond this observation bone trabecular structure was differently affected depending on the supplemented amount of oligofructose or the habitual calcium content or a combination of the 2 (11,18): In ovariectomized rats doubling the recommended level of dietary calcium for rats (10.0 vs. 5.0 g/kg diet) was associated with thicker but fewer trabeculae. In contrast, the addition of oligofructose at the recommended level of dietary calcium mostly prevented the loss of trabecular area without reducing the trabecular number. In a different manner oligofructose (50 g/kg diet) given at a high level of dietary calcium (10.0 g/kg diet) preserved bone trabecular area without reduction of trabecular number. Trabecular perimeter and cortical thickness were highest, and the loss of trabecular connectivity (trabecular bone pattern factor) was lowest compared with all other groups. Therefore, in ovariectomized rats a diet containing oligofructose in the presence of a high content of dietary calcium is regarded as most favorable with respect to bone strength and reduced fracture risk.
How could oligofructose affect the trabecular structure? Magnesium, boron, manganese, copper, and zinc are essential cofactors for enzymes involved in collagen synthesis and other bone matrix constituents that are required to build up the organic bone matrix (22–25), the precondition for mineral accretion. Thus, it is not surprising that low serum values of magnesium, copper, and zinc were associated with osteoporosis in postmenopausal women (26). An increase of dietary calcium alone impeded the absorption of magnesium (27) and may impede that of other minerals and trace elements that are important for bone matrix formation. Oligofructose not only stimulated calcium absorption but also that of zinc (28) and magnesium in rats with and without suppressed magnesium absorption as a result of high calcium supplementation (17,21). These observations can explain why calcium or oligofructose alone or the 2 in combination may generate different trabecular structures even at similar bone densities.
In growing rats, total and cortical but not trabecular bone volume was lower 4 wk after gastrectomy. Feeding FOS did not prevent these changes significantly but had a strong tendency to affect total and cortical bone volume of sham-operated rats (29). When intact growing male rats were fed 50 g/kg diet, trabecular bone volume was higher than that in control animals in the short term (30).
It has to be stressed that a short-term stimulating effect on mineral absorption does not necessarily allow us to draw conclusions on outcomes for bone mineral or even less for bone trabecular structure or bone quality. Studies in human volunteers on prebiotics and bone mineral accretion are scarce.
Prebiotics in human studies
Several studies have investigated the effect of prebiotics [inulin, oligofructose, or other nondigestible oligosaccharide (NDO)] on mineral absorption in humans, but with opposing outcomes. Some of the studies showed no significant effect (31–34), whereas others found that prebiotics stimulated the absorption of calcium (35–40) (Table 2) or magnesium (41). Ingestion of 40 g/d of inulin stimulated the calcium absorption in young men as assessed by metabolic balance (35). In other studies the stable isotope technique was applied: In adolescents 15 g/d of oligofructose (36) stimulated calcium absorption, as did 8 g/d of a combination of oligofructose and inulin in girls (39). When postmenopausal women received 10 g/d of lactulose or 20 g/d of transgalactooligosaccharides, the absorbed calcium was higher than that in controls (37,38).
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Tahiri et al. (33) applied the metabolic balance technique and the stable isotope technique in parallel and found no significant effect of short-chain oligofructose on calcium absorption in postmenopausal women. It might be that the 10 g given was too low. Another explanation for the lack of significance is presumably the menopausal stage. The calcium balance in the perimenopausal and early postmenopausal period is known to be mainly affected by hormonal changes, and dietary interventions are less obvious (43). When the women were stratified by time of postmenopause, a trend was observed (P < 0.1) for a beneficial effect of short-chain oligofructose on calcium balance in women >6 y postmenopause but not in women <6 y postmenopause (33). There was no significant effect in postmenopausal women when 5 g lactulose was consumed, obviously an insufficient amount, in contrast to the 10 g lactulose (37).
In young girls oligofructose was not effective despite a sufficient collection time of urine (39). The administered dose of 8 g given for 1 wk in the presence of 1500 mg of dietary calcium may have been insufficient. However, when the same amount of a mixture of oligofructose plus inulin was consumed, the effect was significant (39) (Table 2). This observation could be explained by a "synergy effect." The combination of different NDOs, or of NDO with different DP, e.g., oligofructose with a DP of 2–9 plus inulin with a DP > 23, may be called "synergy" in that it enables a serial fermentation of the NDO and thus leads to an extension of the sites of fermentation from the proximal to the distal colon. Generally the flora of an individual becomes more complex with age (44), and it might be that children possess a flora that is less skilled compared with that of adults to ferment FOS with a higher DP, i.e., inulin alone. From this point of view the positive effect of 8 g of synergy, but not of oligofructose alone, is somewhat unexpected, but inulin alone was not tested.
Recently, Abrams et al. (40) studied the effect of 8 g of a synergy product per day in 9- to 13-y-old girls for 1 y. Compared with the control group, the calcium absorption was higher after 8 wk and still higher after 1 y. The significance depended on a polymorphism of the Fok1 vitamin D receptor gene. Stimulation of absorption was not seen in girls with the ff polymorphism but was significant in those with the Ff and more pronounced with the FF polymorphism. Accretion of whole-body BMC and density was higher after 1 y in girls consuming the combination of short- and long-chain inulin-type fructans compared with the control group (40).
When oligofructose was given to infants in either the presence or absence of supplemental zinc, no effect was observed on plasma zinc concentrations. It has to be considered that no parameter of zinc balance or metabolism was reported because the study outcome was not focused on mineral absorption or status but was directed toward the prevalence of diarrhea (45).
Probiotics and synbiotics in animals and humans
Probiotics could have a potential effect on bone accretion independent of that of prebiotics. This could occur via microbial production of metabolites or enzymes or synthesis of vitamins (6,46,47) because several vitamins are also involved in calcium metabolism and are required for bone matrix formation and bone accretion as are vitamin D, C, or K (48) or folate (7). One may expect that the optimum combination of probiotics and prebiotics would gain the best results, and this optimum depends on the disease or risk that is aimed to be prevented or reduced. Few studies have investigated the effects of probiotics or synbiotics on the metabolism of minerals or trace elements or on bone health.
When infant formula containing Lactobacillus reuteri supplement (9 x 106 cfu) was given to infant rhesus monkeys, hematocrit was improved, but no effect on the retention of calcium, iron, and zinc was observed (49). This experiment, however, was not designed to investigate the effect of this microorganism on mineral balance. The primary outcomes of this experiment were nutritional status, gut colonization, and the ability to resist gastrointestinal infection.
Synbiotics stimulated the fecal bacterial counts of lactobacilli and bifidobacteria in human subjects, but little effect was seen when only the probiotic (Bifidobacterium lactis HN019) or the prebiotic (galactoologosaccharides) was given (50). Such observation may allow one to assume a more effective stimulation of mineral absorption by synbiotics compared with prebiotics or probiotics alone. The effect of an oral application of Bifidobacterium longum, either alone or in combination with lactulose, on the breaking force was tested in rats (51). Bifidobacterium longum alone induced a significant rise in bacterial counts but only tended to increase the breaking force. Only when Bifidobacterium longum was given together with lactulose was the breaking force significantly higher compared with that in the control group. In that experiment, the effect of lactulose alone was not tested.
It was reported that probiotic yogurt containing strains of Lactobacillus casei, Lactobacillus reuteri, and Lactobacillus gasseri increased apparent calcium absorption and BMC in growing rats (52). However, bone weight was 35% higher than that of the control group, and no information on food consumption and body weight was given.
Scholz-Ahrens et al. (53) and Marten et al. (54) performed an experiment in 5 groups of Fisher-344 rats that were either sham operated (G1, positive control) or ovariectomized (G2–G5) at the age of 6 mo. During the following feeding experiment over 16 wk, all rats received a purified diet providing all nutrients for rats on a restricted feeding regimen that guaranteed the same and complete feed consumption by all animals. In addition the animals received 1 g/d of acidified milk. In 2 groups probiotic lactobacilli (Lactobacillus acidophilus NCC90) were added (G3-probiotic and G5-synbiotic), and in 2 groups 2.5% prebiotics (a mixture of short-chain fructooligosaccharide and acacia gum) were added (G4 and G5). Rats that received prebiotics (G4 and G5) had significantly lower cecal pH and higher cecal contents compared with all other groups. The probiotic strain used in this experiment did not decrease cecal pH or raise cecal contents as an indicator for increased luminal fermentation (53). Compared with that in ovariectomized control rats, urinary phosphorus excretion was significantly lower when prebiotics or synbiotics were given, but phosphorus retention was not different (54). Calcium absorption as assessed by metabolic balance after 15 wk tended to be higher in the synbiotic group and was significantly higher in the prebiotic group compared with ovariectomized controls (54).
The absorption of calcium assessed by 7-d balance within a long-term experiment only allows the view through a narrow time window on available calcium. Therefore, we also analyzed BMC, a parameter of cumulative calcium retention after 16 wk. Femur ash content was lower (–5.8%, P < 0.05) in ovariectomized rats (231.2 ± 5.1 mg) compared with sham controls (245.4 ± 2.5 mg). There was a trend to prevent this ovariectomy-induced loss of ash in the femur by the supplements. The preventive effect by synbiotics was most pronounced (241.1 ± 2.4 mg, P < 0.10), that by prebiotics less pronounced (238.7 ± 3.2 mg, P < 0.22), and that by probiotics marginal (236.6 ± 2.4 mg, P < 0.35). Ash content in 2 lumbar vertebrae in ovariectomized rats (116.2 ± 3.3 mg) was lower (–6.6%, P = 0.05) compared with that in sham-operated controls (124.3 ± 2.1 mg). Compared with the ovariectomized control group, lumbar vertebrae ash content was slightly higher after probiotics with 120.3 ± 2.4 mg, more pronounced after prebiotics with 124.1 ± 2.2 mg, and significantly higher when synbiotics were fed with 127.2 ± 1.8 mg (53).
Bone structure may affect bone strength independent of BMC. Therefore, trabecular bone area of the proximal tibia was assessed, which was done by contact microradiography followed by computer-supported image analyses of digitized radiographs; for details see Scholz-Ahrens et al. (18). A significant loss of trabecular area of 20.1% was induced by ovariectomy (12.25 ± 0.7% vs. 15.44 ± 0.7% in sham-operated controls). This loss could not be prevented completely by any of the 3 treatments, but it was smallest in the 2 groups that received prebiotics (9.1% in rats on prebiotics, 12.8% in rats on synbiotics) compared with 16.9% in rats on probiotics (50). We conclude that the specific lactobacilli tested (L.acidophilus NCC90) alone are not promising to prevent ovariectomy-induced loss of bone mineral and trabecular area. Other strains (Lactobacillus helveticus) might be more effective (55). Furthermore, the tested mixture of short-chain FOS plus acacia gum appeared to have a lower potential to stimulate mineral absorption than oligofructose (18). With regard to BMC, the synbiotic combination tested was more effective in the axial than in the appendicular bone. Further studies are needed to confirm these findings.
Mechanisms
Several factors that contribute to the mechanism by which prebiotics stimulate mineral absorption and improve BMC and structure have been described and reviewed (10–13): The positive effects have been attributed to increased solubility of minerals because of increased bacterial production of short-chain fatty acids through increased supply with substrate; enlargement of the absorption surface by promoting proliferation of enterocytes mediated by bacterial fermentation products, predominantly lactate and butyrate; and increased expression of calcium binding proteins.
Besides these steps involved in prebiotics-induced facilitation of mineral absorption and bone mineral accretion, new findings suggest a more broadened mechanism on the action of prebiotics and, in addition, include direct effects of probiotic bacteria. Potential synbiotic effects are also possible. The following hypotheses are supported by experimental outcomes and contribute to the understanding of how bone health could be supported by prebiotics, probiotics, and synbiotics (see Table 3).
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Release of bone-modulating factors. It was shown that FOS increased the bone-preserving effect of phytoestrogens in ovariectomized mice (59) and rats (60). This effect was proposed to be based on the improvement of isoflavone bioavailability. Flavonoids are more rapidly absorbed in their free aglycone form than as the intact glycoside (61). Stimulation of bifidobacteria and lactobacilli by FOS increases the luminal bacterial ß-glycosidase activity, an enzyme that hydrolyzes the glycosidic bond of isoflavone conjugates. It converts daidzein to equol, an isoform with a higher bone-preserving potential (62).
Stabilization of the intestinal flora. When antibiotics (neomycin, metronidazol) were added to a control diet to study the effect of prebiotics in the absence of potential polyamine producers, rats tended to have less femur ash compared with sham-operated controls, ovariectomized controls, and oligofructose-fed rats. When oligofructose was added to the antibiotics-containing diet, femur ash was significantly higher than that of ovariectomized controls. At the same time rats treated with antibiotics had the highest pH in the cecal contents with 8.26 ± 0.04, which was significantly higher (P < 0.004) than that of sham-operated controls with 7.95 ± 0.03, ovariectomized controls with 7.98 ± 0.08, oligofructose-fed rats with 7.63 ± 0.04, and rats on oligofructose plus antibiotics with 7.62 ± 0.04 (11).
Ovariectomy induced a significant loss of bone trabecular area as a percentage of tissue area with 10.3 ± 0.6% compared with 13.3 ± 0.6% in sham-operated controls (P < 0.002) without affecting trabecular number or thickness. This loss tended to be smaller in rats on oligofructose with 12.1 ± 1.1%, and in rats on oligofructose in the presence of antibiotics with 12.5 ± 0.6%, compared with sham-operated controls. Antibiotics alone resulted in the lowest trabecular area (9.09 ± 0.76%), which was significantly lower than that in rats on oligofructose (P < 0.05) and even more pronounced compared with rats on oligofructose plus antibiotics (P < 0.002).
The low trabecular area was also reflected in the trabecular perimeter (Fig. 1). Compared with rats fed oligofructose, the lower trabecular area and perimeter in antibiotics-fed rats were mainly a result of significantly fewer trabecular endpoints, whereas if compared with rats on oligofructose plus antibiotics it was caused by a reduction in trabecular number and trabecular endpoints. Trabecular thickness was not significantly affected, although antibiotic treatment caused the thinnest trabecules (61.4 ± 1.6 µm) compared with the ovariectomized control (65.2 ± 1.5 µm) or rats on oligofructose alone (65.6 ± 2.5 µm).
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Impact of modulating growth factors. Because polyamines are known to act as luminal mucosal growth factors (65), and because some bacteria were shown to produce polyamines (66), Scholz-Ahrens and Schrezenmeir (11) tested whether the increase of BMC in oligofructose-fed rats might be mediated via the stimulation of polyamine-producing bacteria that benefited from oligofructose in the diet. An experiment in 6-mo-old ovariectomized rats was performed. After 16 wk no stimulatory effect of a supplemented mixture of spermine, spermidine, and putrescine on BMC, cecum weight, or pH in cecal contents was observed (11).
However, parameters of bone structure, which were assessed by microcomputed tomography followed by interactive steps on a computer-supported image analysis, were affected: In ovariectomized rats, animals on the medium dose of dietary polyamines (equivalent to 10 times the polyamine content of the semipurified control diet, which was 310 mg polyamines/kg diet) had significantly lower trabecular number (57.2 ± 3.1) compared with rats on the control diet (67.0 ± 3.2, P < 0.05), although the trabecular area as a percentage of tissue area of 10.08 ± 0.82% was not different from that of rats on control diets of 10.30 ± 0.57%, indicating that larger trabecules are preserved. On the other hand, in rats on the high-dose polyamines (equivalent to 10 times the polyamine content of the medium dose), the trabecular number (72.1 ± 5.0) was significantly higher, and trabecular area (11.19 ± 0.82%) tended to be higher compared with the group receiving the medium-dose polyamines. These results indicate that ovariectomy-induced changes of bone structure are modulated by polyamines in a dose-independent manner. An optimum amount of luminally or metabolically available polyamines may be required to preserve a high degree of tibia trabecular bone structure, particularly in estrogen deficiency.
Suppression of the bone resorption rate relative to the bone formation rate. A mixture of inulin and FOS was studied in ovariectomized rats, using stable istotope technique. Calcium absorption and bone balance were significantly increased, as were femoral calcium content and mineral density. The bone resorption rate relative to the bone formation rate was significantly depressed. No effect was observed on breaking strength (67).
Probiotic degradation of mineral-complexing phytic acid. In cereal based foods, whole-grain products in particular, minerals are complexed by phytate. In phytate-rich diets the availability of trace elements such as copper, zinc, and iron is significantly depressed in man and animals (68–70). Phytate did not depress the absorption of calcium in adolescent but in young and adult rats, an effect that could be counterbalanced by dietary casein depending on the age of the animals and calcium content of the diet (71). Monogastric animals and humans are lacking endogenous phytase. Therefore, adding phytase of either plant or microbial origin to the diet could help to overcome this problem, as it is practiced in farm animals (72). The addition of phytase-producing bacteria (Mitsuokella jalaludinii, a culture gained from rumens of cattle in Malaysia) was investigated in broiler chicken (73). Four different amounts of culture with increasing phytase activity were added to phytate-rich broiler feed, and these animals were compared with a control group without added culture (negative control) and a phytate-poor diet (positive control). The mean tibia ash of the chicken on phytate-rich diets was significantly depressed compared with phytate-poor diets. The depression was prevented with cultures equivalent to 250 U activity and more effectively with 500 U. With even higher doses, no further improvement was achieved.
Probiotic stimulation of calcium uptake by enterocytes. It has been shown that milk fermented with Lactobacillus helveticus had some effects on calcium metabolism in postmenopausal women (74). In the short term it reduced plasma parathyroid hormone and increased serum calcium but had no effect on carboxy-terminal telopeptide of type I collagen. This modulating effect on calcium metabolism may result from facilitated calcium uptake by enterocytes. An increase of calcium uptake was demonstrated in Caco-2 cells for some probiotic bacteria such as Lactobacillus salivarius (UCC 118) but not for such others as Bifidobacterium infantis (UCC 35624) (75).
Probiotic antiarthritic effect. In addition, other probiotic bacteria (Enterococcus faecium) are shown to have a bone-preserving effect through their antiarthritic potential. The loss of whole-body BMD in this model was reduced with methotrexate treatment. This effect was more pronounced when Enterococcus faecium was added to the treatment but not when it was given alone (76). This observation may be caused by the observed antiinflammatory effect of Enterococcus faecium in a rat model for adjuvant arthritis, leading to a healthier gut and thus mineral-absorbing surface and to less bone resorption.
It was observed that Lactobacillus helveticus reduced parathyroid hormone levels in postmenopausal women consuming Lactobacillus helveticus-fermented milk (74), and it was shown in vitro that Lactobacillus helveticus-fermented milk increased osteoblast bone formation in bone marrow-derived osteoblast precursor cells (77). These positive effects on parameters of bone health may not be a probiotic effect mediated by the microorganisms themselves but may be mediated by encrypted bioactive peptides that are released from milk proteins during the fermentation process (77).
Conclusion. Several studies in animals and humans have shown positive effects of NDO on mineral metabolism, bone composition, and bone architecture. These effects were not in all cases uniform. Certain experimental or dietary conditions and the physiological characteristics of the target group studied might favor a positive outcome of a study.
Dietary conditions include a high content of calcium up to a threshold level and optimum content of supplemented prebiotics, i.e., high as possible and low enough not to induce side effects such as bloating, abdominal pain, or diarrhea. Doses of 2.5% up to 15% of NDO in rats and of 8 g/d up to 40 g/d in humans were effective, depending on the potency of the NDO. The amount of habitual intake of NDO, which can easily be controlled in animal studies, may be a confounding factor in human studies and may in part explain the inconsistent results in humans. However, this remains to be investigated. The optimum combination of different NDOs should be used. In adult rats inulin combined with resistant starch was more effective than one or the other alone, whereas in growing rats the results are less conclusive.
The characteristics of an individual that affect bone metabolism include the following:
Physiological age. Subjects with a high demand for calcium, i.e., at puberty or after menopause, are more predestined to benefit from prebiotics than healthy adults. In young individuals synergy products may be favorably compared with inulin or long-chain NDO because of a less developed diversity of the flora.
Postmenopausal stage. Women who are more than 6 y postmenopausal may benefit more than women around postmenopause, whose bone loss is more affected by the decline in circulating estrogens.
Calcium absorption capacity. Individuals with a low absorption capacity because of filled calcium stores, i.e., at low circulating concentrations of calcitriol and low mucosal expression of calcium-binding proteins, may benefit less than individuals with chronically low calcium intakes and an up-regulated absorption capacity. Such individuals may benefit even more when they are supplemented simultaneously with oligofructose and calcium.
The potential for bone health of a synbiotic preparation is affected by the same aspects mentioned above for prebiotics. In addition, the prebiotic has to be combined with the best suitable probiotic. Moreover, a certain synbiotic may be effective in 1 species and not in another because species-dependent susceptibilities to different microorganisms can be assumed to be related to their diverging innate flora. However, little is known in this field, and future research is required, which should be directed toward the development of functional foods for defined target groups at specific risks.
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FOOTNOTES |
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2 Author disclosure: no relationships to disclose other than that mentioned in Footnote 3.
3 This work was supported in part by "Forschungsschwerpunkt Muskel- und Skelettsystem," Kiel, Germany, and by Nestle Research Centre, Lausanne, Swizerland.
7 Abbreviations used: BMC, bone mineral content; BMD, bone mineral density; DP, degrees of polymerization; FOS, fructooligosaccharides; NDO, nondigestible oligosaccharides.
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