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Institute of Nutrition Science, Technical University of Munich, Freising-Weihenstephan, Germany
2To whom correspondence should be addressed.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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-tocopherol. Six healthy young women received an antioxidant mixture
consisting of ß-carotene, lycopene, lutein, canthaxanthin and
-tocopherol together with a standard meal. The meal did not contain
additional dietary fiber or was enriched with pectin, guar, alginate,
cellulose or wheat bran (0.15 g · kg body weight-1). The
increases in plasma carotenoid and -tocopherol concentrations were
followed over 24 h, and the areas-under-curves
(AUC24h) were calculated. The mean AUC24h of
ß-carotene was significantly (P < 0.05) reduced
by the water-soluble fibers pectin, guar and alginate with a mean
decrease of 33–43%. All tested fibers significantly reduced the
AUC24h of lycopene and lutein by 40–74% (P
< 0.05). The dietary fiber effect on the AUC24h of
canthaxanthin was almost significant (P = 0.059)
and there was no effect on the AUC24h of -tocopherol. We
conclude that the bioavailability of ß-carotene, lycopene and lutein
given within a mixed supplement is markedly reduced by different kinds
of dietary fiber.
KEY WORDS: • absorption • dietary fiber • carotenoids • -tocopherol • humans
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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-tocopherol and carotenoids
have attracted much scientific attention. -Tocopherol has been shown
in vitro and in vivo to inhibit oxidative damage especially in LDL and
thus may lower or even prevent atherosclerotic processes induced by
oxidized LDL (Diaz et al. 1997
, Esterbauer et al. 1992
, Holvoet and Collen 1998
). According to
epidemiologic studies, a diet rich in carotenoid-containing foods
is associated with a number of health benefits, i.e., reduced risk of
cancer, cardiovascular disease, macular degeneration and cataract
formation (Biesalski et al. 1997
, Rock 1997
). The antioxidant activity of carotenoids shown in vitro
was studied in randomized intervention trials by administration of pure
ß-carotene. However, these studies gave no convincing or even
negative results regarding the risk of certain cancers and
cardiovascular disease (Albanes et al. 1995
,
Biesalski et al. 1997
, Rock 1997
). Other
carotenoids have not been tested. Possibly, ß-carotene intake has to
be considered as an indicator substance for a "healthy diet" with a
high intake of fruit and vegetables. However, the modes of actions of
carotenoids and -tocopherol are diverse and have not been entirely
clarified. They are also present intracellularly and may be involved in
the regulation of gene expression or affect cell functions like
inhibition of monocyte adhesion and platelet activation (Devaraj et al. 1996
, Parker 1997
, Rock 1997
). Although specific activities are known, whether they are
the explanatory mechanisms of action is unknown, and the role of
interactions is still under discussion (Biesalski et al. 1997
, Parker 1997
).
Plasma ß-carotene response to a test dose of ß-carotene was much
greater from a purified supplement than from a natural food source,
such as carrots (Rao and Rao 1970
). Also, ß-carotene
supplementation was demonstrated to impair the bioavailability of other
carotenoids, possibly followed by biological effects (Kostic et al. 1995
, Paetau et al. 1997
).
Besides fat intake and the efficiency of extraction from the food
matrix, the amount and type of dietary fiber in the diet seem to
determine carotenoid bioavailability (Parker 1997
,
Rock 1997
, Williams et al. 1998
).
However, detailed information on the effect of different kinds of
dietary fiber on carotenoid absorption in humans is lacking. Only one
study showed a distinct adverse effect of pectin on ß-carotene
absorption (Rock and Swendseid 1992
). Dietary vitamin E
may also affect carotenoid bioavailability (Mobarhan et al. 1994
, Rock 1997
, Xu et al. 1992
).
As stated for carotenoids, knowledge about dietary fiber effects on
-tocopherol bioavailability is scarce (Kayden and Traber 1993
).
Therefore, in the present investigation the effects of various fibers
on plasma response to supplemented carotenoids and -tocopherol as an
indicator of absorption were tested. The substances were administered
as a mixture of the fat-soluble antioxidants together with a test
meal enriched with dietary fiber.
|
METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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The investigation was conducted in six young, nonpregnant women (ages
26–29 y). They were free of acute or chronic disease and took no
nutrient supplements before the study. All of them showed normal body
weight with a body mass index of 19.3–21.7 kg/m2
(Table 1
), were nonsmokers and did not take oral contraceptives. All
participants gave informed consent prior to the beginning of the study.
The procedures followed were in accord with the Helsinki Declaration of
1975 as revised in 1983.
|
Three days before the study (prephase), the subjects consumed a diet low in tocopherol and carotenoids to exclude plasma enrichment with antioxidants originating from food consumed before the test day. On the test day, the six subjects were offered a standard meal and an antioxidant supplement. The standard meal contained either no additional dietary fiber or one of the five types of dietary fiber to be tested. Each subject (unaware of the type of dietary fiber) received a different kind of dietary fiber within a test day. Every test day was followed by a wash-out period of at least 2 wk. All subjects completed the study, thus each received each type of fiber.
Dietary regimen, dietary fiber and antioxidant supplement.
During the 3-d prephase, the subjects were advised to consume a diet
low in carotenoids and tocopherol according to detailed instructions of
a dietitian and a list of foods to be avoided. Calculations of dietary
intake were based on dietary protocols (estimated record) completed
during the study prephases using a German standard food composition
data base (DFL 1986) after addition of carotenoid data
from three more sources [most data from Mangels et al. 1993, and some
additional data from Heinonen et al. (1989)
and Ollilainen et al. (1988)
]. The subjects’ compliance was also monitored by means of
plasma analysis of -tocopherol and carotenoids before and after the
prephase periods.
On the test day, the fasting (for at least 10 h) subjects were
offered a standard meal consisting of chocolate rice pudding for
breakfast. In preliminary studies, chocolate rice pudding was found to
be suitable to disguise the use of fiber to the subjects. The
standard meal (including the antioxidant supplement) contained 27% of
its energy as fat, 59% as carbohydrates and 14% as protein.
The meal provided 20% of the estimated daily energy
requirement of each subject (REE · 1.5), with the other 80% of
energy given in six portions every 2 h in the form of a
fiber-free liquid diet (Nutricomp F; B. Braun Melsungen AG,
Melsungen/Germany: 24% of energy as fat, 59% as carbohydrates, 17%
as protein; 7.5 mg -tocopherol/L; no carotenoids).
The standard meal was given without added dietary fiber or enriched with one of the five different types of dietary fiber. Either pectin (GENU pectin, type X5114, citrus pectin with a degree of esterification of 70%; Copenhagen Pectin A/S, Lille Skensved/DK), guar (MEYPRO GUAR, type CSA 200/50, Welding GmbH & Co, Hamburg/Germany), alginate (Manucol DMF, Kelko International, Waterfield/UK), cellulose (Vitacel®, type L 600, J. Rettenmaier & Söhne, Ellwangen-Holzmühle/Germany), or wheat bran (Dr. Kousa Weizenkleie, Milupa AG, Friedrichsdorf/Germany) were added to the standard meal, amounting to 0.15 g · kg body weight (bw)-1. Table water was used to obtain comparable consistencies of the meals.
The subjects were advised to consume an antioxidant supplement
containing all-trans-ß-carotene (0.4 mg · kg
bw-1; Hoffmann-La Roche, Grenzach-Wyhlen/Germany),
lycopene (0.7 mg · kg bw-1; LycoRed Natural Products
Industries, Beer-Sheva/Israel), canthaxanthin (0.2 mg · kg
bw-1, Hoffmann-La Roche), lutein (0.4 mg · kg
bw-1; Kemin Industries,, Des Moines, IA), and
-tocopherol (1.4 mg · kg bw-1; Hoffmann-La
Roche). An ultrasonic bath (5 min, 40°C) was used to get
ß-carotene, canthaxanthin and -tocopherol homogenously distributed
in 3 mL of cream, while the other two carotenoids were distributed in
olive oil (2 g). The combined mixtures were filled into a small
consumable cup to ensure complete intake without any waste. The
subjects ate the standard meal and the antioxidant supplement within 10
min.
Plasma preparation and analysis.
Blood samples were drawn into tubes containing the antioxidants EDTA (1.6 g/L blood) and TROLOX (1 µmol/L blood; both from Sigma, Deisenhofen/Germany) before as well as 2, 4, 6, 7, 8, 9, 10 and 24 h after the test meal. The plasma samples were stored at -24°C for a maximum of 4 wk until analysis.
Plasma carotenoid and -tocopherol concentrations were analyzed by
HPLC according to a described method (Hess et al. 1991
).
Briefly, after protein precipitation hexane was used for extraction.
The hexane layer was evaporated under vacuum and redissolved in 200
µL of HPLC eluent. Twenty µL were injected to a reversed-phase
C-18 column (precolumn ultra-sphere ODS, 4.5 mm x 45 mm;
column ultrasphere ODS, 4.5 mm x 150 mm; Beckmann
München/Germany). Analysis of -tocopherol and carotenoids was
performed by detecting absorbance at 292 and 450 nm, respectively,
after elution with acetonitrile/dichloromethane/methanol (7:2:1,
vol/vol/vol; 1.2 mL/min, 18°C). Absorbance was analyzed with an
UV/VIS detector UVD 340 S from Gynkotek (Germering/Germany).
Calculations were made using the internal standard method and relative
response factors. ß-Apo-8‘-carotenoic acid ethyl ester (carotenoid
quantification) and DL--tocopheryl acetate
(-tocopherol quantification) were added as internal standards.
Recovery of the substances after enrichment of serum samples was within
94 and 107% with a coefficient of variation < 2.3%
(n = 5).
Plasma total cholesterol and triacylglycerol concentrations were determined enzymatically using test kits (Triglycerides GPO-PAP, Cholesterol CHOD-PAP, both Boehringer Mannheim GmbH, Mannheim/Germany).
Areas-under-curves (AUC)3 calculation and statistics.
As a measure of absorption efficiency, the areas under the plasma enrichment curves of the antioxidants were integrated by trapezoidal approximation over a time period of 24 h (AUC24h).
The results are given as means and (SEM). Statistical
analysis was performed with SPSS, Vers. 7.5 (SPSS, Chicago, IL. For
analysis of variance of the AUC24h-values, a
two-factorial model with the factors "person" and "dietary
fiber" was applied. Comparisons of means were made with the
Student-Newman-Keuls-test (SNK-test) at an -level of 5%. The paired
t-test was used for comparison of fasting plasma
concentrations of antioxidants before vs. after a diet low in
carotenoids and -tocopherol with a significance level of
P < 0.05.
|
RESULTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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-tocopherol and the sum of
carotenoids of 4.7 ± 0.2 mg/d and 427 ± 16 µg/d,
respectively (Table 2
). On average, energy intake amounted to 8.23 ± 0.15 MJ/d (1967
± 35 kcal/d); fat contributed 31%, carbohydrates 56%, and
protein 14% of total energy intake. The diet provoked a significant
reduction of the carotenoid and -tocopherol plasma concentrations
(Table 2)
, indicating good diet compliance of all subjects.
|
.
Within test days, mean plasma cholesterol concentrations ranged between
4.06 and 4.26 mmol/L estimated at nine time points over a period of
24 h; mean plasma triacylglycerol concentrations ranged between
0.64 and 0.80 mmol/L. None of the subjects responded markedly
differently in plasma cholesterol and triacylglycerol. Analysis of
variance (multifactorial) revealed no significant effect of plasma
total cholesterol and triacylglycerol concentrations on plasma
carotenoid and -tocopherol levels. Accordingly, the plasma
concentrations of carotenoids and -tocopherol presented are not
corrected for the plasma lipid concentrations.
After intake of the combined antioxidant supplement and the standard
meal without added dietary fiber, the plasma concentrations of the
supplement components increased and reached maximum values 7–8 h,
later, however, the plasma ß-carotene increase continued throughout
the observation period of 24 h. Consistent with the results of
preliminary studies, plasma concentrations of ß-carotene showed their
maximum approximately 24 h after intake of the antioxidant
supplement (n = 2), and later measurements (30 h)
indicated no additional increase. Addition of any kind of dietary fiber
to the standard meal resulted in decreased mean plasma responses of
carotenoids and -tocopherol over 24 h. The AUC were calculated
to give a measure of the relative effect of the different kinds of
dietary fiber (Table 3
). Meal enrichment with the water-soluble fibers pectin, guar, and
alginate significantly decreased the AUC24h for
ß-carotene by 42, 43 and 33%, respectively (Fig. 1
). All kinds of dietary fiber significantly reduced the plasma response
curves of lycopene and lutein (Table 3)
. Expressed as the percentage of
the AUC24h of the test conducted without added
dietary fiber, addition of dietary fiber decreased the relative
absorption of lycopene and lutein by 40–74%. The dietary fiber effect
on the AUC24h of canthaxanthin failed to reach
statistical significance (P = 0.059, two-factorial
ANOVA, Table 3
). The plasma response curves of -tocopherol were not
affected by addition of dietary fiber.
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-tocopherol were
significantly affected by the study subjects (Table 4
). Given the lack of a dietary fiber effect on the AUC results for
canthaxanthin and -tocopherol, ß-carotene was the most interesting
compound affected by both factors "subject" and "dietary
fiber". The ß-carotene response of subject III especially was much
different from the others (Fig. 2
).When the results of subject III were excluded from the statistical
analysis, the effects of cellulose and wheat bran on the ß-carotene
response were significant.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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,
Kayden and Traber 1993
, Williams et al. 1998
). In the present investigation, fat intake on test days
was kept constant with fat contributing 27% of the energy content of
the test meal. During the 3-d study prephase, mean fat intake was 31%
of energy intake with a very low variation. Besides this rather low
contribution of fat, the high plasma concentrations of ß-carotene
(Table 2)
also indicate, that the six young women usually consumed a
rather "healthy diet" and apparently followed a "healthy
life-style" (normal and constant BMI, nonsmokers, sports activities,
no oral contraceptives). The increase in plasma levels after high
ß-carotene intake may continue up to several days (Furr and Clark 1997
). In accordance with Van Vliet et al. (1995)
, a
wash-out period of consuming low-carotenoid and
low-tocopherol diet was found to be sufficient to ensure that the
increase in plasma carotenoid and -tocopherol concentrations on the
test day are not a consequence of a high intake on the days before
(Table 2)
.
The literature describes a high intersubject variation for the plasma
response to carotenoid (ß-carotene) supplementation but a much lower
intrasubject variation (Micozzi et al. 1992
,
O’Neill and Thurnham 1998
). With the study design used,
the problem of intersubject variation was markedly reduced and the
SEM of the fasting plasma concentrations before test meal
intake (Table 2)
confirm a relatively low intrasubject variation
between test days for all carotenoids and for -tocopherol. However,
a significant effect of the subjects on the AUC values (with and
without dietary fiber addition) of ß-carotene, canthaxanthin and
-tocopherol was found (Table 4)
. Most strikingly, subject III
affected the mean plasma response for ß-carotene, resulting in a
nonsignificant effect of supplemental cellulose and wheat bran (Fig. 2)
.
The stage of menstrual cycle can affect plasma carotenoid and
-tocopherol concentrations in premenopausal women (Forman et al. 1998
, Lanza et al. 1998
). However, no data
clearly indicate that the plasma response to a carotenoid supplement
depends on the menstrual cycle phase or the fasting plasma
concentrations affected by the menstrual cycle phase. Due to the lack
of exact information on the menstrual cycle stage of the women in the
present study, the evaluation of this factor as a confounding variable
on plasma response values could not be performed.
Plasma response curves are widely used as an index of carotenoid
bioavailability (Parker 1997
). However, their use is
limited by several factors like the problem of first-pass-metabolism of
provitamin A carotenoids in the intestine or the recirculation of
carotenoids between tissues and plasma. Moreover, to reach a distinct
increase in plasma concentrations over baseline levels and to enable
the calculation of the AUC, carotenoid doses far beyond the typical
dietary intake are necessary and could have affected the results. As
found by others (Furr and Clark 1997
), ß-carotene
supplementation provoked a long-lasting increase in plasma
concentrations, giving a different plasma enrichment curve as those
found for the other antioxidants within 24 h. The
AUC24h results for ß-carotene can be criticized
for having included the plasma increase phase only and not having
clearly identified the plasma peak nor included the decrease phase.
However, Rock and Swendseid (1992)
who followed the plasma
concentration of ß-carotene over 192 h found very similar
results for the effect of pectin on the bioavailability of ß-carotene
as reported here (see below). In both studies the relative effect of
dietary fiber on ß-carotene bioavailability was of interest and there
is no evidence of a much different fiber effect during the decreasing
phase of ß-carotene plasma response compared to increasing phase.
Despite its limitations, the AUC method used should be suitable to
determine relative effects of dietary constituents such as dietary
fiber on the absorption of carotenoids and -tocopherol within a
repeated measurement study design where each subject serves as its own
control.
Most studies on carotenoid bioavailability were performed using
ß-carotene. Besides ß-carotene, -carotene, lutein, zeaxanthin,
lycopene and cryptoxanthin are most commonly consumed in a typical
Western diet and are most prevalent in human plasma (Rock and Swendseid 1992
, Yong et al. 1994
). Absorption
interactions between the different carotenoids and even between
ß-carotene and -tocopherol were evident in several studies,
although not all studies gave comparable results (Clark et al. 1998
, Fotouhi et al. 1996
, Johnson et al. 1997
, Kostic et al. 1995
, Nierenberg et al. 1994
, O’Neill and Thurnham 1998
,
Paetau et al. 1997
, Van den Berg and Van Vliet 1998
, Xu et al. 1992
). A mixture of different
carotenoids and -tocopherol was administered in the present
investigation. However, the amount of the single substances (mg/d) may
be about 10-fold higher than found in the diet of different populations
while the proportions between the supplemented substances may be
realistic, except for canthaxanthin (Riedl et al. 1997
,
Yong et al. 1994
). Due to the commercial availability of
a formulation suitable for administration in humans, canthaxanthin was
added to the mixture while -carotene and cryptoxanthin were missing.
In other studies on carotenoid absorption after a single oral dose,
comparable or even higher dosages were used (Brown et al. 1989
, O’Neill and Thurnham 1998
, Rock and Swendseid 1992
, Van Vliet et al. 1995
).
Five types of dietary fiber were given to the subjects at a dose of
0.15 g · kg bw-1. The rationale for the
selection of these fibers was to include members of the main types of
fibers that are found in the diet, either naturally occurring or as
ingredient used in the food industry. Pectin, guar (from the group of
gums) and alginate (from the group of mucialges) were chosen as
representatives of the group of water-soluble dietary fiber
components. The intention to use hemicelluloses and lignins besides
cellulose as the most important members of the group of
water-insoluble types could not be realized because suitable
preparations for administration to humans were not available. Instead,
wheat bran, containing both hemicellulose and lignin, was used. A
description of the properties and main dietary sources of the fibers
can be found elsewhere (Kritchevsky and Bonfield 1995
).
The actual fiber doses administered ranged between 7.8 and 10.4 g
per meal, representing about one-third of dietary intake
recommendations per day (30 g/d) and can be judged as a reasonable
amount of dietary fiber in a single meal. Consequently, the important
issue for the evaluation of the effect of dietary fiber on the plasma
response of supplemented antioxidants may be the interaction between
the high actual antioxidant doses and the "normal" fiber dose.
Results from studies in chickens suggested that various types of
dietary fiber (hemicellulose, lignin and pectin) reduce the
bioavailability of ß-carotene (Erdmann et al. 1986
).
In humans, Rock and Swendseid (1992)
demonstrated that adding pectin
(12 g) to a meal decreases plasma ß-carotene response to a single
dose of purified ß-carotene (25 mg) by more than one-half. This
finding was used to interpret the lower bioavailablity of ß-carotene
from different food sources compared to purified ß-carotene
supplements (Brown et al. 1989
, Rao and Rao 1970
). The results of the present study fit well with those
published by Rock and Swendseid (1992)
. When consumed with pectin, the
mean AUC24h of ß-carotene decreased by 42%
(Table 3)
. Similar to the findings for ß-carotene, pectin
supplementation decreased lycopene and lutein absorption by about 40%
(Fig. 1)
. In the case of ß-carotene, water-soluble types of
dietary fiber had a stronger effect on relative bioavailablity than did
water-insoluble fibers. This tended to be true also for
canthaxanthin. However, no distinction between the effects of
water-soluble and water-insoluble types of fiber can be made
for lycopene, lutein and -tocopherol.
None of the various types of dietary fibers significantly reduced the
-tocopherol AUC24h. However, the strongest
effects were achieved by adding alginate (27% reduction) and wheat
bran (24% reduction). One study in humans reported that addition of
hemicellulose (3.9 g) to the diet significantly reduced the plasma
response to a 500 mg -tocopherol supplement (Doi et al. 1983
). According to Mongeau et al. (1986)
, wheat bran
significantly decreased plasma response of -tocopherol in rats.
Pectin also was used as dietary fiber supplement in rats (de Lumen et al. 1982
, Schaus et al. 1985
). A decrease of -tocopherol absorption
was measured when pectin was >6 g/100 g of nonpurified diet while 3
g/100 g had no effect. In the present study with a dose of 0.15 g
pectin · kg bw-1, i.e., on average 8.85 g per test meal or about 3 g/100 g of the test meal, the effective
level might not have been reached.
As stated by Schneeman (1990)
, digestion and absorption of nutrients in
the presence of dietary fiber reflect the normal physiological
situation. The presence of dietary fiber in the gut may have an
important function in maintaining the gastrointestinal system by
regulating the rate and site of nutrient absorption. Various sources of
dietary fiber can slow down the process of digestion and absorption of
macronutrients, including fat (Eastwood and Morris 1992
,
Edwards 1990
, Schneeman 1990
). It seems
likely that the physical properties of fibers such as particle size,
viscosity, water-binding capacity, gel formation, and bile
acid-binding capacity are responsible for the observed effects and
might also affect carotenoid and -tocopherol absorption. First, the
absorption of the fat-soluble carotenoids is suspected to have an
absolute requirement for bile salt micelles (El-Gorab et al. 1975
, Hollander and Ruble 1978
). It was shown
for guar gum that this type of fiber effectively binds bile acids and
phospholipids (Vahouny and Cassidy 1985
) and reduces
phospholipid concentration in the intestine through an increase in
chyme volume (Gallaher and Schneeman 1986
), both of
which disturb micelle formation. Furthermore, dietary fiber may exert
effects on micelle formation by affecting the activity of pancreatic
enzymes (lipase) also involved in micelle formation (Schneeman 1990
). Second, soluble fibers increase viscosity and volume of
the intestinal contents, which can slow diffusion processes such as the
diffusion of micelles to the absorptive surface of enterocytes
(Phillips 1986
). And third, dietary fiber influences the
morphology of the small intestine, including alterations in intestinal
cell renewal (Sigleo et al. 1984
). It was suggested that
some carotenoid is held in the enterocytes and released in response to
subsequent meals (Furr and Clark 1997
). With an enhanced
enterocyte turnover, more carotenoids may leave the small intestine as
constituents of desquaminated mucosal cells. This effect may gain
importance with chronic feeding of dietary fiber but not with their
single supplementation.
Differences in molecular structure and polarity of the carotenoids and
-tocopherol might also help to explain the different effects of the
various kinds of dietary fiber. However, since dietary fiber presents
only few, if any, functional groups, the mechanism is probably an
indirect one, again dealing with disturbances in micelle formation. The
physical positioning of the various carotenoids within the mixed
micelles depends on their polarity. Hydrocarbon carotenes
(ß-carotene, lycopene), nonpolar compounds, may accumulate in the
hydrophobic core of the micelles, while the more polar xanthophylls
(lutein, canthaxanthin) may be located on the surface (Borel et al. 1996
). However, nearly equal mean decreases of the
AUC24h of lycopene and lutein by the tested kinds
of dietary fiber do not support these suggestions. On the other hand,
the absorption of canthaxanthin, a carotenoid with a keto group, and
-tocopherol, revealed a different response to fiber supplementation
compared to the other carotenoids. To some extent, the molecular
characteristics of the target antioxidant may be important. Further,
chemical and physical structures of the fibers specifically affect
interactions with the individual carotenoids providing hydrocarbon,
alcohol or ketone groups in the molecule. Possible binding mechanisms
such as the formation of hydrogen bonds or hydrophobic interactions
should be investigated.
In conclusion, it seems that the measured effects of various types of
dietary fiber on the relative absorption of different carotenoids and
-tocopherol given in a mixture with a standard meal cannot be
explained by a single criterion. Instead, they may be the result of
many complex interactions. However, the strong decrease in the relative
absorption of ß-carotene (especially by water-soluble dietary
fibers), lycopene and lutein (by all tested types of dietary fiber)
points out the important effect of fiber in the human diet on
carotenoid bioavailability.
|
ACKNOWLEDGMENTS |
|---|
-tocopherol
suitable for application in humans as well as analytical standards of
carotenoids were provided free of charge by Hoffmann-La
Roché, Grenzach Wyhlen/Germany. |
FOOTNOTES |
|---|
3 Abbreviation used: AUC24h, areas-under-curves calculated over 24 h1.
Manuscript received May 12, 1999. Initial review completed June 18, 1999. Revision accepted August 6, 1999.
|
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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