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Department of Nutritional Science, Faculty of Applied Bio-Science, Tokyo University of Agriculture, Setagaya-ku, Tokyo 156-8502, Japan;
*
Bioscience Laboratories, Meiji Seika Kaisha, Limited, Sakado-city, Saitama 350-0289, Japan; and
Institute for Preventive Medicine, Nutrition and Cancer, Folkhälsan Research Center, Division of Clinical Chemistry, University of Helsinki, PB 60 FIN-00014, Helsinki, Finland
1To whom correspondence should be addressed. E-mail: mariu{at}attglobal.net.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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KEY WORDS: • fructooligosaccharides • genistein • daidzein • rats • time-resolved fluoroimmunoassay
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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, Price and Fenwick 1985
).
Isoflavones (genistein and daidzein) in soybeans and lignans
(enterolactone and enterodiol) formed in the intestine from their
precursors in various grains, seeds, fruits, some vegetables and tea
are commonly known phytoestrogens (Adlercreutz and Mazur 1997
, Mazur et al. 1998
). Dietary phytoestrogens
may prevent sex hormone–related diseases, such as breast cancer and
prostate cancer, through mechanisms that are insufficiently documented
at present (Adlercreutz et al. 1982
, Adlercreutz 1984 and 1990
, Adlercreutz and Mazur 1997
,
Attalla et al. 1997
, Landstrom et al. 1998
). On the other hand, they may also prevent postmenopausal
osteoporosis through an estrogenic effect (Arjmandi et al. 1996
). For instance, intake of soy products (Arjamandi et al. 1998, Omi et al. 1994
), genistein
(Anderson et al. 1998
, Ishimi et al. 1999
), daidzein or other phytoestrogens (Draper et al. 1997
, Ishida et al. 1998
) has been found
to prevent postovariectomized bone loss in rats and mice. It is
important to clarify the levels of phytoestrogens in the blood attained
through daily intake. Japanese people consume soy products containing
30–50 mg of isoflavonoids daily, and this appears to play an
important role in the prevention of cancer (Arai et al. 2000
, Kimira et al. 1998
,
Watanabe et al. 1997
). However, few studies regarding
the kinetics of isoflavones in blood have been reported. Moreover, the
intestinal environment is likely to influence their absorption. Almost
all phytoestrogens in food exist as glycosides, such as genistin and
daidzin; thus, to obtain physiologic activity, it is necessary to
hydrolyze the glycosidic bonds for intestinal absorption to occur
(Adlercreutz and Mazur 1997
, Murkies et al. 1998
, Price and Fenwick 1985
). These glycosidic
bonds are hydrolyzed by glucosidases of intestinal bacteria, such as
Lactobacilli, Bacteroides and
Bifidobacteria (Hawksworth et al. 1971
,
Xu et al. 1995
). Fructooligosaccharides
(FOS),2
a mixture of indigestible and fermentable sugars, have beneficial
effects. FOS stimulate the growth of bifidobacteria in the intestine
(Hidaka et al. 1986
), increase Ca, Mg and Fe absorption
and enhance bone calcium stores in rats (Ohta et al. 1995a
, 1996
and 1998
). Many of the effects of FOS feeding take place in the
large intestine (Ohta et al. 1994
, 1995b
and 1997
).
Consequently, we postulate that dietary FOS may affect bioavailability
of phytoestrogen glycosides and therefore improve their absorption form
in the gut. In this study, the kinetics of isoflavones in rats fed a FOS-supplemented diet or a control diet were examined by measuring genistein and daidzein concentrations in blood collected from the portal, central and tail veins, and by measuring urinary excretion over a 48-h period after a single administration of soy isoflavones.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Male Sprague-Dawley rats (6 wk old; n = 22, Clea Japan, Tokyo, Japan) were housed in individual stainless steel wire-mesh cages in a room at 25°C and 55% relative humidity with a 12-h light:dark cycle; rats had free access to food and water. This study was approved by the Animal Studies Committee of Meiji Seika Bioscience Laboratories, and rats were maintained in accordance with the guidelines for the care and use of laboratory animals of Meiji Seika Bioscience Laboratories.
The rats were fed a pelleted diet (MF, oriental Yeast, Tokyo, Japan)
for a 1-wk adaptation period. The rats (n = 22)
were randomly assigned to two groups of 11 rats each, a purified
control diet (AIN-93G) group and the FOS diet (AIN-93G + 5% FOS) group
(Reeves et al. 1993
). The composition of each of the
diets is shown in Table 1
. The experimental period was 7 d. All rats were fed 15 g
diet/d on d 1 and 2 and 20 g/d on d 3–7, and were allowed free access
to deionized water throughout the experimental period. Five rats from
each group were used for sampling portal and central venous blood. The
remaining 6 rats in each group were used for sampling tail venous
blood.
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Isoflavone conjugates were administered to all rats in this experiment.
Fujiflavone P40 [isoflavone content: 40% (daidzin, malonyldaidzin,
acetyldaidzin and daidzein account for 20.4, 0.1, 1.1 and 0.3%,
respectively; genistin, acetylgenistin and genistein account for 4.6,
0.3 and 0.1%, respectively; and glycitin and glycitein together
account for 13%)] was obtained from Fujicco, Kobe, Japan. FOS
(Meioligo-P, Meiji Seika Kaisha, Tokyo, Japan) is a mixture of 42%
1-kestose, 46% nystose and 9% 1F-ß-fructofuranosylnystose.
Isoflavone (Fujiflavone P40) for administration to all rats was freshly prepared as a 1 mL suspension in water and a single dose [isoflavones: 100 mg (conjugates)/kg body (8.5 mg as genistein and 33 mg as daidzein)] was administered via a stomach tube. The rats were not restricted from eating throughout the experiment.
Two days after catheter implantation, blood was collected through the
silicone tube connected on the neck to the portal and central veins.
Blood (100 µL) was collected at 0, 1, 3, 6, 24 and 48 h after administration of isoflavones from the portal and central veins
of 5 rats in each diet group under unanesthetized and unrestrained
conditions; for the other rats (n = 6), blood was
collected from the tail vein at these same times by the method of
Hara et al. (1984)
.
Urine samples were collected during the periods 0–24 h and 24–48 h
postadministration. NaN3 (0.003 mol/L) and
ascorbic acid (1 g/L) were added to the urine storage bottles to
prevent oxidation of isoflavones. Each urine sample was stored
separately in a bottle at -80°C until the assay.
Reagents.
Bovine serum albumin (BSA) and sulfates (EC 3.1.6.1; catalog no.
S-9626) were purchased from Sigma Chemical (St Louis, MO).
ß-Glucuronidase (EC 3.2.1.31; catalog no. 1585665) was from
Boehringer (Mannheim, Germany). The assay buffer was 50 mmol/L
Tris-HCl buffer, pH 7.8, containing (per L) 8.78 g NaCl,
0.5 g sodium azide, 5 g BSA and 0.1 g Tween 40. For
enzymatic hydrolysis of the isoflavone conjugates in urine, 0.1 mol/L
acetate buffer, pH 5.0, was used. Standards of genistein and daidzein
were synthesized as previously described (Adlercreutz et al. 1986
, Wähälä and Hase 1991
)
Time-resolved fluoroimmunoassay (TR-FIA) for measuring of serum and urinary genistein and daidzein.
One milliliter of 0.1 mmol/L acetate buffer (pH 5.0) containing 200
U/L ß-glucuronidase and 2000 U/L sulfatase
(hydrolysis reagent) was added to plastic tubes containing 50
µL of urine. The samples were mixed and incubated
overnight at 37°C; 20 µL of the resulting solution
was used for TR-FIA. (Adlercreutz et al. 1998 and 1999
,
Uehara et al. 2000
, Wang et al. 2000
).
Immunogen synthesis, immunization and labeling of isoflavonoid
derivatives with europium chelate in the case of genistein and daidzein
were described previously (Uehara et al. 2000
,
Wang et al. 2000
).
The TR-FIA methods used for assay of isoflavones in plasma and
urine are shown in Figure 1
. Before the assay, microstrips coated with goat anti-rabbit
immunoglobulin G were prewashed using 1296–026 DELFIA platewash
(Wallac, Oy Turku, Finland). Of the standard, serum or hydrolyzed urine
samples, 20 µL was pipetted into the microstrips; then 100
µL of antiserum in 50 mmol/L Tris-HCl
buffer containing 5 g/L BSA (pH 7.8) for genistein or daidzein,
(antiserum diluted 1:50,000 and 1:40,000, respectively) and 100
µL of europium-labeled genistein or daidzein (diluted
1:400,000 and 1:40,000, respectively) were added per well. The strips
were placed on a 1296–003 DELFIA shaker (Wallac) and shaken slowly at
room temperature for 90 min, then washed with a DELFIA platewasher
using the no. 29-T3 program. DELFIA enhancement solution (200
µL) 1244–105 (Wallac) was added to each well and the
strips were shaken slowly for an additional 5 min. Fluorescence was
read using the DELFIA Victor 1420 multilabel counter and the
accompanying software, version 1.0, for data analysis. The final result
was calculated by means of the following formula:
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The TR-FIA method, including hydrolysis and extraction, gives the
most specific results for plasma or serum, but we used a modified
method that omitted extraction of the unhydrolyzed plasma because the
quantities of several blood samples were insufficient for extraction.
The direct method for assay of genistein and daidzein in serum or
plasma measures only the free aglycones, the 4'-monosulfates and the
4'-monoglucuronides, and gives lower (30% in rats) values than
those obtained by gas chromatography-mass spectrometry (GC-MS)
analysis.
Urinary isoflavones were analyzed by a method including a hydrolysis
step. The mean value of urinary (hydrolyzed) genistein was much higher
by the TR-FIA method than by the GC-MS method, but that of
daidzein was similar by both methods. However, there was a significant
correlation between the values of genistein for urine as determined by
TR-FIA and GC-MS (r = 0.880, P
< 0.001). The urinary genistein concentration was adjusted to
correspond to the GC-MS values by means of the formula:
y = 0.465x - 0.696 (Uehara et al. 2000
).
The areas under the curve (AUC) of change in the plasma isoflavones
were calculated by the following formula:
 |
where tn is number n sampling time (h) and Cn is the plasma isoflavone concentration at tn.
Analysis of cecal contents.
The pH of cecal contents was measured directly using a compact pH
electrode (B-112, Horiba, Kyoto, Japan). The concentration of organic
acids in the cecal contents was analyzed quantitatively by HPLC
according to a method reported previously (Kritchevsky and Bonfield 1997
). Cecal contents were adjusted to a volume of 20
mL with deionized water and homogenized with a teflon homogenizer.
After undergoing centrifugation for 10 min at 3000 x g, the supernatants were analyzed by HPLC.
Statistical analyses.
Values were expressed as means and SEM After examining the
equality by Levene’s test, if there was a significant difference
(P < 0.05), each value was converted to the
logarithmic value. Three-way ANOVA was performed to determine the
main effects of diet, the time course of changes, the type of veins
from which blood was collected and the interactions with respect to
isoflavone concentrations in the portal and central venous blood.
Two-way ANOVA was used to determine the main effects of diet and
the time course of changes, and the interactions with respect to
isoflavone concentrations in blood from the tail vein and hepatic
uptake. Tukey’s test (Dawson-Saunder and Trapp 1994
)
was used for comparison of means within a factor. The effect of
absorption of isoflavones calculated from the areas under the plasma
concentration time course curves in the case of blood from the portal,
central and tail veins, and the effect of FOS feeding on food intake,
body weight gain and cecal wet weight, pH and concentrations of organic
acids were examined by Student’s unpaired t test; the
equality of variance was determined using Levene’s test. Differences
were considered significant at P < 0.05. All
statistical analyses were performed using the SPSS package program
version 6.1 J (Chicago, IL) and Excel 97 with Microsoft Windows 95 or
98.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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There were no significant differences in food intake (130 g for all rats in 7 d) and body weight gain between the control (50.2 ± 1.5 g) and FOS-fed (49.3 ± 4.6 g) groups.
Cecal wet weight, pH and concentrations of organic acids.
After 1 wk of feeding, cecal contents wet weight was significantly
greater in rats fed the FOS diet than in those fed the control diet
(P < 0.01). Cecal pH was significantly lower in rats
fed FOS (P < 0.01) compared with the control. Lactate,
propionate and butyrate concentrations were significantly higher in
rats fed the FOS diet than in those fed the control diet (P
< 0.05 or 0.01). There was no significant difference between the
FOS-fed and control rats in concentrations of succinate and acetate
(Table 2
).
|
Genistein concentrations in portal and central venous plasma after
administration of a single dose of isoflavones are shown in
Figure 2A
and
B
, respectively. Three-way ANOVA showed that the three main effects
(diet, time and vein) and the interaction (time and vein) were
significant (P < 0.002) for genistein (Table 3
). In the control rats, the genistein concentrations in portal blood
reached a peak at 1 h after administration (3.5 ± 0.7
µmol/L) and decreased linearly thereafter. In the
FOS-fed group also, the genistein concentrations reached a peak at
1 h (3.5 ± 1.0 µmol/L) and declined rapidly
during the period up to 3 h after isoflavone administration (1.7
± 0.4 µmol/L); however, a small second peak appeared
at 6 h after administration (2.0 ± 0.5 µmol/L).
At 48 h after administration, the genistein concentration tended
to be higher in the FOS-fed group (38.8 ± 10.5 nmol/L) than
in the control group (7.2 ± 2.0 nmol/L) (P = 0.09). The genistein concentration in central venous plasma was
approximately half of the portal blood concentration at each time point
during the 1- to 12-h period after isoflavone administration in both
the control group and the FOS-fed group. There was a significant
difference in the genistein concentration between the portal vein and
the central vein plasma samples at 1 h after isoflavone
administration in both the control and FOS-fed rats (P
< 0.05).
|
|
. Daidzein concentrations in portal and
central venous plasma after administration of a single dose of
isoflavones are shown in Figure 3A
and
B
, respectively. The daidzein concentration in portal blood reached a
peak at 1 h (6.3 ± 1.6 µmol/L) and was slightly
lower at 3 and 6 h after administration (5.2 ± 1.4 and 5.0
± 1.4 µmol/L, respectively) in the control group. In
the FOS-fed rats, the daidzein concentration in portal blood
reached a peak at 6 h (5.0 ± 2.2 µmol/L) and
declined thereafter, but there were no significant differences among
the values at 1, 3 and 6 h after administration. There was a
significant difference in the daidzein concentration in portal blood
between the control and the FOS-fed rats at 1 h after
administration of isoflavones (P < 0.05). In the
control group, the daidzein concentration in the central venous blood
reached peaks at 3 and 6 h (2.5 ± 0.6 and 2.5 ± 0.7
µmol/L, respectively) and declined thereafter. In the
FOS-fed group, the daidzein concentration in central venous blood
reached a peak at 1 h (3.4 ± 1.3 µmol/L), was
slightly lower at 3 h but had increased again at 6 h after
administration.
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Tail venous plasma genistein and daidzein concentrations.
Genistein and daidzein concentrations were measured in separate rats in
tail venous plasma; the values are shown in Figure 4A
and
B
, respectively. Two-way ANOVA showed that the two main effects (diet
and time) and the interaction (diet and time) were significant
(P < 0.004) for genistein, and the main effect (time)
and the interaction (diet and time) for daidzein (Table 3)
. In the
control group, the genistein concentration was maximal (1.9 ± 0.4
µmol/L) at 1 h after administration and decreased
thereafter. In the FOS-fed group, the genistein concentration was
elevated at 1 h and did not change until after 6 h of
administration. The genistein concentrations were significantly higher
in the FOS-fed group than in the control group at 24 and 48 h
after administration (P < 0.05). The daidzein
concentration reached a maximum at 1 h in the control group (2.2
± 0.6 µmol/L) but the peak area was delayed until
3–6 h postadministration in the FOS-fed group.
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Absorption of genistein and daidzein.
Genistein absorption calculated from the areas under the plasma
concentration curves for portal, central and tail venous blood in rats
fed the control diet and the FOS diet during the periods 0–6 h, 6–48
h and 0–48 h after isoflavone administration is shown in Table 4
. On the basis of the concentrations in plasma from the three veins of
the FOS-fed group compared with the control group, no significant
differences in the absorption of genistein existed during the 0- to 6-h
period after administration. During the 6- to 48-h period after
administration, the genistein areas under the concentration curves of
the central and tail venous plasma concentrations were significantly
higher in the FOS-fed group than in the control group (P
< 0.05). During the 0- to 48-h period, the genistein areas under
the concentration curves in tail venous plasma were also higher in the
FOS-fed group than in the control group (P < 0.01). For daidzein, there was a significant difference between areas
under the concentration curves of the FOS-fed and control groups
during the 0- to 48-h period in the tail vein samples (P
< 0.05) (Table 4)
.
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We assessed the hepatic uptake of isoflavones into the liver by
comparing the concentrations in portal and central venous plasma
(Fig. 5A
and
B
). Two-way ANOVA showed that time was a significant effect
(P < 0.0001) for genistein and daidzein (Table 3)
. The
hepatic uptake of both genistein and daidzein was greater 1 h
after administration of a single dose of isoflavones compared with the
values at other time points. There were no significant differences
between the control and the FOS-fed groups at any time.
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) than that in the control group during rgw 24- to 48-h period after
administration (P < 0.01). The excretion of daidzein
did not differ between rats fed the control and FOS diets at any time.
The amount of genistein recovered in the urine was 11 and 14% in
the control and FOS-fed groups, respectively. The amount of
daidzein recovered in urine was 21–23% in both groups.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Phytoestrogens occur mainly as glucuronide or sulfate conjugates in
plasma and urine. We measured only some of the isoflavone conjugates
and free aglycones present in plasma by TR-FIA, and this assay was
carried out directly without hydrolysis and extraction. The conjugates
are mainly 4'-monosulfate and monoglucuronides, but some other
conjugates seem to have been included because the main isoflavone
conjugates in rat plasma and urine are 7-O-glucuronides
(Sfakianos et al. 1997
). We compared the values in
direct and extracted samples of rat plasma (n = 15),
and the mean values by the direct method were 70% of the values by the
extracted method (data not shown). Therefore, the direct values are
underestimates, but the difference in isoflavone concentrations between
the different venous plasmas and between rats fed the FOS and control
diets could be determined. Our results suggest, however, that the
4'-conjugates of genistein and daidzein occur in relatively large
amounts after an isoflavone load.
Isoflavone conjugates may be deglycosylated by the strong acid in the
stomach. Enzymes such as ß-glucosidase produced by the intestinal
microflora before absorption are considered to be responsible for the
hydrolysis. Recently it was shown in humans that genistein and daidzein
can be detected in blood within 15 min after consumption of soy protein
(textured vegetable protein) (Bowey et al. 1998
),
suggesting hydrolysis in the upper gastrointestinal tract. Day et al. (1998)
reported that genistein 7-glucoside and daidzein
7-glucoside are rapidly deglycosylated by cell-free extracts from
human small intestine and liver, and they suggested that
deglycosylation of isoflavones by human cytosolic ß-glucosidase could
be an important first step in the metabolism of these compounds,
independent of deglycosylation by the colonic microflora. Ioku et al. (1998)
also measured ß-glucosidase activity in the
small intestine of rats using flavonoid glucosides as substrates, and
their findings suggested that dietary flavonoid glucosides are
hydrolyzed primarily in the jejunum, thereby liberating aglycones. In
this study, the concentrations of both genistein and daidzein in portal
blood rapidly reached a peak (within 1 and 3 h, respectively)
after a single dose of isoflavones. This finding indicates that these
isoflavones are readily absorbed in the small intestine after
intragastric administration. Piskula (2000)
reported that in tail vein plasma, total daidzein and genistein reached
maximum concentrations in food-deprived (intragastric
administration) rats at 7 min after administration of their aglycones.
Twenty-four hours after administration, the concentrations of genistein
and daidzein in blood from all three veins were < 10% of the
peak levels observed. The intestinal transit time of the diet in rats
has been estimated to be 20–30 h (Sakaguchi et al. 1987
). The length of time until disappearance of both
isoflavones was similar to the intestinal transit time. These results
suggest that in rats, the peripheral blood concentration reflects
mainly intestinal absorption, and that genistein and daidzein do not
remain in the blood and probably do not remain in the body for a long
time.
In the case of both genistein and daidzein, good correlations between
the portal and central venous plasma concentrations were observed in
this study. Indeed, the central venous plasma isoflavone concentrations
were nearly half the portal venous concentration during the 1- to 12-h
period after administration. This result indicates that about half of
the genistein and/or daidzein absorbed was taken up into the
liver, and this metabolic pathway is similar to that of
endogenous estrogens (Adlercreutz and Martin 1980
).
Also, good correlations between the central and tail venous blood
concentrations were observed for both genistein and daidzein. According
to the results of this study, 15% of plasma genistein and 35% of
plasma daidzein disappeared from the peripheral circulation by urinary
excretion.
Tew et al. (1996)
suggested that genistein has lower
bioavailability than daidzein because of the low urinary output of
genistein. However, Sfakianos et al. (1997)
examined the
intestinal absorption, biliary excretion and metabolism of genistein
using adult female rats fitted with indwelling biliary cannulas and
found that genistein was absorbed from the intestines very well; it was
excreted mostly into the bile, and only a small portion appeared in the
urine. Both genistein and daidzein are converted to sulfate and
glucuronide conjugates in rats. Daidzein may be eliminated more rapidly
in the urine than genistein. Estrogens with three hydroxyl groups are
more abundantly excreted into the bile (Adlercreutz and Luukkainen 1967
); because of the three hydroxyl groups,
genistein is more likely than daidzein to be excreted rapidly in bile.
In other words, it seems that the isoflavonoid concentration in
peripheral blood is probably regulated through biliary excretion. A
likely reason for the greater excretion of compounds with three
hydroxyl groups compared with those with two is the formation of polar
double conjugates (Adlercreutz et al. 1973
).
The recovery of genistein in urine was 10–12% and 13–15% of the
amount administered in the case of the control and FOS-fed groups,
respectively. The recovery of daidzein in urine was 23% in both
groups. Landstrom et al. (1998)
reported the urinary
recovery during two metabolic periods was 1 and 3% for genistein and
11 and 28% for daidzein in rats after tumor transplantation and soy
intake. In the study of King (1998)
, the recovery of
genistein and daidzein in urine of rats was 11.9 and 17.4%,
respectively.
The data obtained in this study demonstrate for the first time that FOS
modify the bioavailability of isoflavones. Through observation of cecal
contents weight, pH and composition of organic acids, we concluded that
the rats fed the FOS diets for 7 d were almost fully adapted to
FOS. In this study, genistein and daidzein remained detectable for a
significantly longer period in the tail venous blood of rats fed the
FOS diet compared with the controls. The absorption of genistein, as
calculated on the basis of the concentrations in portal, central and
tail venous blood during the 6- to 48-h period after administration,
was significantly higher in rats fed the FOS diet than in controls, but
the absorption of daidzein did not differ. This indicates that FOS
enhanced the enterohepatic recirculation and/or large intestinal
absorption of genistein. Adlercreutz (1962)
suggested
that the proportions of estrogen metabolites in the enterohepatic
recirculation already excreted into bile increased transiently in the
6- to 10-h period after the start of oral administration of estradiol.
In this study, a significant difference between the control and
FOS-fed groups in the genistein concentration in the tail venous
blood became evident 6 h after administration of the isoflavones.
FOS are indigestible because they are resistant to hydrolysis by
mammalian enzymes, and they stimulate the growth of intestinal
bifidobacteria. In this manner, dietary FOS may increase
ß-glucosidase activity in the large intestine (Hidaka et al. 1986
). As indicated above, in FOS-fed rats, a portion of
the isoflavone-glycosides consumed was likely to have been
deglycosylated and absorbed as aglycones in the large intestine. Our
findings regarding urinary isoflavone excretion also support this
hypothesis because the extent of excretion of both genistein and
daidzein was higher in rats fed the FOS diet than in control rats
during the 24- to 48-h period after administration. The relative
absorption of genistein, calculated on the basis of the urinary
excretion, was 20% higher in FOS-fed rats than in control rats.
FOS may be useful for maintaining elevated blood levels of isoflavones,
especially in the case of genistein. Additionally, we speculate that
FOS may enhance the metabolism of isoflavones, particularly for
daidzein to metabolites such as equol and
O-desmethylangolensin. This could explain the difference
observed between genistein and daidzein concentrations in plasma when
the two dietary groups were compared. Isoflavonoids undergo extensive
metabolism by gut microflora (Heinonen et al. 1999
).
Considerable evidence exists showing extensive interindividual
variation in isoflavone metabolism in humans (Lampe et al. 1998
, Rowland et al. 2000
, Watanabe et al. 1997
, Xu et al. 1995
). A
carbohydrate-rich diet increases equol production (Lampe et al. 1998
, Rowland et al. 2000
). However,
rats are constitutive equol producers in contrast to humans
(King 1998
, Landstrom et al. 1998
). FOS
can change the composition of gut microflora, and may change the
production of daidzein metabolites in rats. However, further studies
should be conducted.
In conclusion, with a single administration of isoflavones, both genistein and daidzein were immediately absorbed; their concentrations in peripheral blood increased, but their clearance was also rapid. Our findings suggest that the main factor responsible for the disappearance of both isoflavones, genistein and daidzein, is hepatic uptake/biliary excretion. Dietary FOS may prolong the clearance of isoflavones, especially genistein, by enhancing the large intestinal absorption of these compounds.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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Manuscript received May 30, 2000. Initial review completed July 10, 2000. Revision accepted December 7, 2000.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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