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Department of Nutritional Sciences, University of Connecticut, Storrs, CT 06269
2To whom correspondence should be addressed.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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-hydroxylase were higher in guinea pigs fed SF
(P < 0.05) than in controls. These results
indicate that gender plays an important role in the metabolic responses
to dietary soluble fiber and that estrogen deprivation leads to a
detrimental lipoprotein profile.
KEY WORDS: • guinea pigs • dietary soluble fiber • gender • menopause • LDL oxidation
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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). Studies have been conducted to determine whether
men and women respond differently to dietary interventions. It has been
shown that plasma triacylglycerol
(TAG)and HDL cholesterol responses to dietary fat are gender-specific
because females appear to be less responsive than males (Mensik and Katan 1987
). Results from the Framingham Heart Study
indicate that high TAG and low plasma HDL cholesterol concentrations
are highly associated with cardiovascular disease risk in women while
for men, high plasma LDL cholesterol concentrations are highly related
(Castelli 1988
). Cobb et al. (1993)
demonstrated that
plasma TAG, VLDL and HDL cholesterol concentrations were
gender-specific in males and females fed diets with high vs. low
polyunsaturated to saturated fat ratios. In addition, postmenopausal
women have been shown to be at higher risk for CHD than premenopausal
women since they have significantly higher risk factors such as higher
plasma LDL cholesterol, apo B and TAG levels (Bonithon-Kopp et al. 1990
).
Epidemiological studies have demonstrated the protective effect of
dietary fiber against cardiovascular disease risk (Brown et al. 1999
, Khaw and Barrett-Connor 1987
).
Consumption of dietary soluble fiber results in plasma LDL
cholesterol-lowering as has been demonstrated in both human
(Andersen 1987
, Andersen and Tietyen-Clark 1986
, Everson et al. 1992
, Miettinen and Tarpila 1989
, Olson et al. 1997
) and animal
studies (Fernandez et al. 1994
,1995
,1995b
,
1995d
, 1997
, Garcia-Diez et al. 1996
, Kelley and Tsai 1978
, Matheson and Story 1994
, Terpstra et al. 1998
, Tinker et al. 1994
, Turley et al. 1991
). Decreases in
plasma LDL-cholesterol are protective against CHD. Several primary
mechanisms have been suggested to explain the fiber-mediated plasma
cholesterol-lowering including interruption of bile acid
enterohepatic circulation (Turley et al. 1991
) and
decreases in cholesterol absorption in the small intestine
(Kelley and Tsai 1978
). Some of the secondary mechanisms
of plasma LDL-cholesterol lowering have been addressed in studies
using guinea pigs. In these studies soluble fiber (SF) has been shown
to reduce plasma and hepatic cholesterol concentrations. In addition,
upregulation of 3-hydroxy-3-methyl-glutaryl coenzyme A (HMG-CoA)
reductase activity associated with increased cholesterol synthesis and
upregulation of cholesterol 7-hydroxylase (C7H) activity associated
with increased bile acid secretion have been observed (Fernandez et al. 1994
, 1995
, 1995b
,
1995d
). Fernandez et al. (1997)
have also demonstrated
that the reduction in cholesterol concentration in the liver induced by
dietary SF is associated with reduction in apo B secretion rate,
decreased conversion of VLDL to LDL and upregulation of hepatic LDL
receptors, resulting in increased LDL turnover, which eventually leads
to increased clearing of LDL from plasma.
The present studies were undertaken to determine the distinctive
effects of gender on the mechanisms of plasma LDL-lowering by SF in
male, female and ovariectomized (to mimic menopause) guinea pigs. The
sources of SF in the study were psyllium, pectin and guar gum. Guinea
pigs were chosen as animal models for this study because they transport
plasma cholesterol mainly in LDL and have a plasma LDL/HDL ratio
comparable to humans (Fernandez et al. 1995d
). The
distribution of hepatic cholesterol pools with higher concentrations of
free vs. esterified cholesterol and hepatic activities of HMG-CoA
reductase (Reihner et al. 1990
), acyl-CoA/cholesterol
acyltransferase (ACAT) (Einarsson et al. 1989
) and C7H
(Reihner et al. 1991
) are also similar to humans. In
addition, similar to humans, guinea pigs respond to dietary fiber with
a lowering of plasma cholesterol concentrations (Shen et al. 1998
), thus making them an appropriate model to study effects
of SF on regulatory mechanisms of plasma LDL cholesterol-lowering.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Enzymatic cholesterol and TAG kits, cholesterol oxidase, cholesterol esterase and peroxidase were purchased from Boehringer-Mannheim (Indianapolis, IN). PL and free-cholesterol enzymatic kits were obtained from Wako Pure Chemical (Osaka, Japan). Quick-seal ultracentrifuge tubes were from Beckman (Palo Alto, CA) and halothane from Halocarbon (Hackensack, NJ). DL-hydroxy- [3-14C] methyl glutaryl CoA (1.81 GBq/mmol), DL- [5-3H] mevalonic acid (370 GBq/mmol), cholesteryl- [1,2,6,7-3H] oleate (370 GBq/mmol), Aquasol, Liquiflor (toluene concentrate) and [14C] cholesterol were purchased from DuPont NEN (Boston, MA). Oleoyl- [1-14C] CoA (1.8 GBq/mmol) and DL-HMG-CoA were bought from Amersham (Clearbrook, IL). Cholesteryl oleate, glucose-6-phosphate, glucose-6-phosphate dehydrogenase, NADP, EDTA, NaF, Triton, bovine albumin and sucrose were obtained from Sigma Chemical (St. Louis, MO). Aluminum and glass silica gel plates were purchased from EM Science (Gibbstown, NJ).
Diets.
Diets were prepared and pelleted by Research Diets (New Brunswick, NJ).
Isocaloric diets were designed to meet all the nutritional requirements
for guinea pigs. Both the control and SF diets had equal composition
except for the type of fiber as indicated in Table 1
. The control diet contained 10 g/100 g of cellulose, a source of
insoluble fiber and 2.5 g/100 g of guar gum, while the SF diet
contained 5 g/100 g of psyllium, 5 g/100 g of pectin and 2.5 g/100 g of
guar gum (sources of SF). The amount of cholesterol in the diets was
adjusted to be 0.04 g/100 g, an amount equivalent to 300 mg/d for a
human diet. The fat mix contained olive oil/palm kernel oil/safflower
oil (1:2:1.8), a mix high in lauric and myristic fatty acids, that
causes endogenous hypercholesterolemia in guinea pigs (Conde et al. 1996
).
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Guinea pigs [n = 54 male, female and ovariectomized (to mimic menopause)] (Harlan Sprague-Dawley, Indianapolis, IN), weighing 300–350 g, were randomly assigned to either the control (n = 8/group) or the SF diet (n = 10/group) for 4 wk. Two guinea pigs were kept per metal cage and housed in a light-cycle room (light from 0700–1900h) and had free access to diets and water. Nonfasted guinea pigs were killed by heart puncture after halothane anesthesia, and blood and livers were harvested for analysis. All animal experiments were conducted in accordance with U.S. Public Health Service/U.S. Department of Agriculture guidelines. Experimental protocols were approved by the University of Connecticut Institutional Care and Use Committee.
Lipoprotein isolation.
Plasma samples were collected from blood obtained by heart puncture from guinea pigs under halothane anesthesia. A preservation cocktail of aprotinin, phenyl methyl sulfonyl fluoride and sodium azide was added to plasma samples to minimize changes in lipoprotein composition during isolation. Plasma (1 mL) was separated for measurement of LDL susceptibility to oxidation; 500 µL of plasma from each sample was stored at 4°C for further plasma lipid analysis, and the rest was used for lipoprotein isolation.
Lipoprotein isolation was done by sequential ultracentrifugation
(Redgrave et al. 1975
) in an LE-80K ultracentrifuge
(Beckman Instruments, Palo Alto, CA). VLDL was isolated at d
= 1.006 kg/L at 125,000 x g at 15°C for 19 h in
a Ti-50 rotor. LDL was isolated at d = 1.019–1.09 kg/L
in quick-seal tubes at 15°C for 3 h at 200,x g in
a vertical Ti-65 rotor (Fernandez et al. 1999
). LDL
samples were dialyzed in 0.9 g/L of sodium chloride-0.1 g/L EDTA, pH
7.2, for 24 h and stored at 4°C for further analysis.
Plasma and hepatic lipids.
Plasma samples were analyzed for cholesterol and TAG by enzymatic
methods (Allain et al. 1974
). Hepatic total and free
cholesterol and TAG were determined according to the method by Carr et al. (1993)
following extraction of hepatic lipids with
chloroform/methanol 2:1. Cholesteryl ester (CE) concentrations were
calculated by subtracting free from total cholesterol. Apo B
concentrations in the plasma were calculated as described elsewhere
(Fernandez et al. 1992
).
Lipoprotein characterization.
VLDL and LDL composition was calculated by determining free and
esterified cholesterol (Allain et al. 1974
), protein by
a modified Lowry method (Markwell et al. 1978
), and TAG
PL by enzymatic kits. VLDL apo B was selectively precipitated with
isopropanol (Homsquit et al. 1987
). The number of
constituent molecules of LDL was calculated on the basis of one apo B
per particle with a molecular mass of 412000 kD (Chapman et al. 1975
). The molecular weights were 885.4, 386.6, 645 and 734 for
TAG, free and esterified cholesterol, and PL, respectively
(Conde et al. 1996
). LDL diameters were calculated
according to Van Heek and Zilversmit (1991)
. HDL cholesterol was also
determined according to Warnick et al., with a modification, which
consisted of using 2 mol/L of MgCl2 for
precipitation of apo-B containing lipoproteins (Fernandez et al. 1999
). LDL particle size was determined by fast protein
liquid chromatography separation of plasma on Superose 6 column using a
solution of EDTA, NaCl and sodium azide (ESA) as column eluant
(Krul et al. 1989
).
Hepatic microsome isolation.
Hepatic microsomes were isolated as described previously
(Fernandez et al. 1995c
). Liver tissues were pressed
through a tissue grinder into cold homogenization buffer (50 mmol/L of
KH2PO4, 0.1 mol/L of
sucrose, 50 mmol/L of KCl, 50 mmol/L of NaCl, 30 mmol/L of EDTA, 2
µmol/L of dithiothreitol, pH 7.2) and homogenized with a
Potter-Elvehjem homogenizer. A microsomal fraction was isolated by
two 25-min centrifugations at 10,000 x g (JA-20 rotor,
J2–21) followed by ultracentrifugation at 100,000 x g in a
Ti-50 rotor at 4°C. Microsomes were resuspended in the homogenization
buffer and centrifuged for an additional hour at 100,000 x
g. After centrifugation, microsomal pellets were homogenized
and stored at -70°C. The protein content in the microsomes was
measured by the method reported by Markwell et al. (1978)
. Hepatic
microsomes were used to measure HMG-CoA reductase, ACAT and C7H
activities.
Hepatic HMG-CoA reductase assay.
The activity of microsomal HMG-CoA reductase (EC 1.1.1.34) was
measured in hepatic microsomes as described by Shapiro et al. (1969)
.
HMG-CoA reductase activity was expressed as pmol of
[14C] mevalonate produced per min per mg
microsomal protein. Recoveries of [3H]
mevalonate ranged from 60 to 90%.
Hepatic ACAT activity.
Hepatic ACAT (EC 2.3.1.26) activity was measured by the incorporation
of [14C] oleoyl CoA in CE in hepatic microsomes
by preincubating 0.8–1 mg of microsomal protein per assay with 84 g/L
of albumin and buffer for microsomal isolation (Smith et al. 1986
). Recoveries of [3H] cholesteryl
oleate were between 70 and 90%.
Hepatic cholesterol 7-hydroxylase activity.
Cholesterol 7-hydroxylase (EC 1.14.13.7) activity was measured
according to the method modified by Jelinek et al. (1990)
.
[14C] cholesterol was used as a substrate and delivered
as cholesterol-phosphatidylcholine liposomes (1:8, by wt) prepared
by sonication. An NADPH-regenerating system (glucose-6-phosphate
dehydrogenase, NADP, and glucose-6-phosphate) was included in the assay
as a source of NADPH (Fernandez et al. 1995b
)
In vitro determination of LDL susceptibility to oxidation.
LDL isolated from individual samples was dialyzed in EDTA-PBS.
Copper-mediated oxidation of LDL was performed by adding 0.5 mmol/L of
CuCl2.2H2O solution to
0.2 g of protein/L LDL. The effect of the dietary treatments on
the extent of oxidation was measured by incubating samples for 3 h
at 37°C. The lipid peroxide content of oxidized LDL was determined by
measuring the formation of thiobarbituric acid-reactive substances
(TBARS) expressed as malonaldehyde equivalents (Puhl et al. 1994
). The TBARS assay was conducted by adding 2 mL of TBARS
reagent (26 mmol/L of thiobarbituric acid (TBA), 0.92 g/L of
trichloroacetic acid in 0.25 mol/L HCl) to 550 µL of incubation
mixture at 100°C for 15 min. Then 25 mL of n-butanol was
added. The phases were separated by centrifugation at 1500 x g for 15 min. The pink color was developed in the aqueous
layer and extracted by n-butanol. Absorbance was read at 532
nm in a spectrophotometer.
LDL -tocopherol concentrations.
-Tocopherol concentrations were determined in LDL as described
elsewhere (Vergara-Jimenez et al. 1999
). Briefly, 250
µL of plasma LDL was mixed thoroughly with equal amounts of methanol
and hexane to release the -tocopherol contained within the LDL and
to precipitate the protein contained in the sample. Phases were
separated by centrifugation, and the upper layer was transferred to
another tube. The hexane extraction was repeated twice; then the
combined hexane extracts of each sample were evaporated under nitrogen,
and the residue was redissolved in 250 µL of methanol/dichloroethane
4:1 for the HPLC injection. An aliquot of 50 µL was analyzed by HPLC
using a Rainin Microsorb 3 µm C18, 15-cm column, with 100% methanol
as the mobile phase and absorbance detection at 292 nm to quantitate
standards and samples. Linear regression between the peak area and the
amount of the sample injected was used for quantitation. The slope of
the standard curve was used to calculate the concentration of
-tocopherol in samples, which was expressed as mmol/mg apo B in LDL.
Statistical analysis.
Two-way ANOVA (GBSTAT, Silver Spring, MD) was used to test the
significant fiber effects, gender effects and their interaction on
plasma lipids, plasma apo B, hepatic lipids, composition of VLDL and
LDL, plasma TBARS, LDL -tocopherol concentrations and activities of
hepatic HMG-CoA reductase, cholesterol 7-hydroxylase and ACAT
activities. The Tukey’s post-hoc test was used to evaluate the
differences among means in the male, female and ovariectomized groups
due to intake of control or fiber diets. Differences were considered
significant at P < 0.05. Data are presented as the
mean ± SD, n = 8 (for control diet)
or n = 10 (for SF diet).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Fiber and gender effects on plasma lipid levels and lipoprotein composition.
All guinea pigs fed the SF diet had 44% lower plasma total
cholesterol concentrations (Table 2; P < 0.001). The SF groups also had 22% lower
TAG (P < 0.05) and 64% lower apo B concentrations in
plasma (P < 0.05). Ovariectomized guinea pigs in both
diet groups had 31–35% higher concentrations of plasma total
cholesterol (P < 0.001), 24–36% higher TAG
(P < 0.05) and 38–41% higher apo B levels
(P < 0.001) as compared to males and females (Table 2)
.
|
). Concentrations in female guinea pigs (normal and ovariectomized
pooled; 0.38 ± 0.16 mmol/L) were higher than those in males (0.28
± 0.14 mmol/L) (P < 0.05), similar to humans,
whether pre- or postmenopausal women have higher HDL-C than men
(Cobb et al. 1993
, Kannel et al. 1976
).
Differences in plasma LDL cholesterol levels paralleled those in plasma
total cholesterol. LDL-C was 51% lower in the groups fed SF
compared to controls (P < 0.001). In contrast, plasma
VLDL cholesterol concentrations (P < 0.05) were 36%
higher in guinea pigs fed SF (Table 3)
. Ovariectomized guinea pigs in
both diet groups had 33–40% higher levels of plasma LDL cholesterol
(P < 0.05) than the males and females. Male guinea
pigs had 34–42% higher concentrations of plasma VLDL cholesterol
(P < 0.05) as compared to the female and
ovariectomized animals (Table 3)
.
|
). As a result of these compositional changes, VLDL diameters were
larger in the SF groups. LDL composition was also modified by dietary
fiber intake. LDL particles isolated from guinea pigs fed the SF diet
were smaller in size, containing fewer CE and FC molecules, compared to
the LDL isolated from the guinea pigs fed the control diet, a
compositional change that resulted in smaller LDL diameters (Fig. 2
). LDL particles isolated from guinea pigs fed the SF diet were
significantly smaller in size (P < 0.05) than those
from guinea pigs fed the control diet (Fig. 3
).
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Guinea pigs fed SF had 25% lower levels of hepatic free cholesterol
(P < 0.05), while, the cholesterol concentration in
the esterified pool was 43% higher (P < 0.001). There
was no significant difference in hepatic total cholesterol
concentration due to dietary SF (Table 4). Guinea pigs fed SF had a 26% lower concentration of hepatic TAG
compared to control groups (P < 0.05). No significant
differences were observed among male, female and ovariectomized guinea
pigs in their response to dietary soluble fiber treatment.
|
). In contrast, soluble fiber upregulated the activity of HMG-CoA
reductase, the major regulatory enzyme of cholesterol synthesis
(P < 0.05). Similarly, the activity of the regulatory
enzyme of cholesterol catabolism, cholesterol 7-hydroxylase, was
45% higher in the SF groups (P < 0.05). No
significant differences were noted due to gender and hormonal status.
|
LDL susceptibility to oxidation measured by the formation of TBARS
following 3 h of incubation in the presence of
Cu2+ was 88 and 78% lower in male and female
guinea pigs fed SF, respectively, compared to controls (Table 6
; P < 0.001). However, no differences were noted
between the ovariectomized guinea pigs fed control and SF diets
(interaction, P = 0.0001; Table 6
).
|
-tocopherol concentrations than the male guinea pigs (Table 6
; P < 0.05) which reflects the human situation.
Dietary SF did not affect LDL -tocopherol concentrations (Table 6)
. |
DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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, Mensik and Katan 1987
), but few studies have been conducted to determine how
dietary fiber may affect the potential secondary mechanisms of plasma
LDL cholesterol-lowering associated with gender and hormonal
status. In the present study, we have shown that the major secondary
effects of dietary SF were related to alterations in hepatic
cholesterol metabolism, which resulted in plasma LDL
cholesterol-lowering, major changes in LDL composition and
decreased LDL susceptibility to oxidation. Moreover, the ovariectomized
guinea pigs, which were a model for menopause, had the most detrimental
lipid profile even after treatment with dietary SF. SF effects on male, female and ovariectomized guinea pigs.
Dietary SF intake resulted in significant reductions of plasma total
and LDL cholesterol and TAG concentrations, which are among the major
risk factors associated with cardiovascular disease. It also reduced
plasma apo B concentrations, indicating that SF not only reduces the
amount of LDL cholesterol but also the number of LDL particles. Plasma
HDL cholesterol concentrations were unaffected by SF treatment, but in
general, female guinea pigs had a higher concentration of HDL-C in
plasma compared to male guinea pigs, which is similar to the human
situation (Cobb et al. 1993
). Our results are in
agreement with the findings from the Framingham Study, where HDL
cholesterol concentrations apparently do not vary with menopausal
status (Kannel et al. 1976
).
SF had an effect on lipoprotein composition. Intake of SF resulted in
secretion of larger VLDL particles with higher numbers of TAG, PL and
FC molecules and less CE molecules. These large VLDL may affect the
intravascular processing of lipoproteins and possibly contribute to the
hypocholesterolemic action of SF. VLDL particles containing a higher
proportion of CE are more easily converted to intermediate density
lipoprotein and then to LDL through the delipidation cascade
(Ginsberg 1990
). It has been postulated that larger
TAG-enriched VLDL is catabolized faster by the apo B/E receptor
(Nestel et al. 1983
). It is possible that the large
TAG-enriched, CE-deprived VLDL particles are related to decreased
conversion of VLDL to LDL and, in turn, are cleared faster by the
hepatic apo B/E receptor from circulation, thus contributing to the
lowering of plasma LDL cholesterol following SF intake. Another
variable that could contribute to the observed compositional changes is
altered plasma lipid transfer protein (LTP) activity. LTP, which
facilitates the transfer of CE, TAG and PL between plasma lipoproteins,
is one of the key mediators of normal lipid metabolism (Abbey et al. 1990
). Dietary SF also leads to the production of smaller
LDL particles with fewer CE and FC molecules. This compositional change
in LDL may have important metabolic consequences as reduction in free
and esterified cholesterol in LDL has been associated with faster
turnover of LDL in plasma (Fernandez et al. 1992
,
1993
).
The liver plays a central role in whole body cholesterol homeostasis as
the site for cholesterol catabolism through bile acid and neutral
sterol elimination and as the regulator of circulating LDL levels
through regulation of synthesis of VLDL and catabolism of LDL by the
apo B/E receptor (Spady 1992
). The decreases in hepatic
free cholesterol concentrations induced by dietary SF result in
upregulation of LDL receptors as indicated by the faster plasma total
and receptor-mediated LDL turnover (Fernandez 1995
).
The regulatory enzymes involved in hepatic cholesterol homeostasis were
significantly altered by SF intake. The significant upregulation of
C7H, the major regulatory enzyme of cholesterol catabolism, could be
the key step that triggers the response of HMG-CoA reductase, the
major regulatory enzyme of cholesterol synthesis. The decrease in the
hepatic free cholesterol pool induced by SF intake may be due to
mobilization of hepatic cholesterol for bile acid synthesis as
suggested by the increase in C7H activity. Horton et al. (1994)
have
reported that increases in C7H activity and parallel increases in C7H
mRNA levels could account for part of the hypocholesterolemic actions
of psyllium in hamsters. Increases in HMG-CoA reductase activity
with SF treatment are consistent with the explanation that the reduced
hepatic free cholesterol concentrations resulting from an increased
conversion to bile acids due to the increase in C7H, stimulate
cholesterol production and upregulate HMG-CoA reductase. The
interplay of these regulatory enzymes mediated by dietary SF may result
in an upregulation of hepatic apo B/E receptors and reduction of plasma
LDL cholesterol, presumably due to an increase in LDL catabolism
(Fernandez et al. 1994
).
Dietary SF reduced the susceptibility of LDL particles to oxidation in
male and female guinea pigs as indicated by the lower TBARS values, but
the LDL susceptibility to oxidation was unaffected in ovariectomized
guinea pigs, suggesting that hormonal status overcomes the beneficial
effects of fiber. Dietary SF causes a reduction in the amount of
cholesterol per particle as well as number of LDL particles in plasma.
It could be that a smaller number of LDL particles with less
cholesterol would be less susceptible to oxidation. Also, the length of
time that LDL remain in circulation may be another factor associated
with the susceptibility of LDL to oxidation. The longer LDL remain in
plasma, the higher the possibility of oxidation. CE depleted, smaller
LDL particles have been associated metabolically with faster LDL
turnover (Berglund et al. 1989
) and negatively
associated with increases in incidence of atherosclerosis in African
green monkeys (Carr et al. 1992
). Because SF treatment
increases the hepatic LDL receptors (Vergara-Jimenez et al. 1998
), clearance of LDL is possibly increased, and therefore
there are fewer particles in circulation, which results in lower
susceptibility to oxidation as measured by in vitro techniques. We
found that dietary fiber did not have a significant effect on the LDL
-tocopherol concentrations, contrary to the findings of
Vergara-Jimenez et al. (1999)
. The most likely explanation for the
difference is the amount of dietary cholesterol fed to the animals,
which in their case was 0.17 g/100 g, much higher than the 0.04 g/100 g
fed to the guinea pigs in this study. High dietary cholesterol intake
leads to larger LDL particles with higher CE and free cholesterol to
protein ratios (Fernandez et al. 1995e
). Such particles
may contain higher concentrations of lipid-soluble
micronutrients such as -tocopherol than the particles found in
guinea pigs fed low dietary cholesterol.
Gender differences in response to dietary SF.
Gender is a strong predictor of CHD susceptibility. One of the
important goals of this study was to determine whether gender plays a
role in the plasma LDL cholesterol-lowering properties of dietary
SF. We demonstrated that ovariectomized guinea pigs fed both control
and SF diets, had higher concentrations of plasma total and LDL
cholesterol, TAG and apo B. These findings are in agreement with those
of Bonithon-Kopp et al. (1990)
in a human study where they found
that postmenopausal women are at a higher risk for CHD because they
have significantly higher risk factors such as higher plasma LDL
cholesterol, TAG and apo B levels. We observed that LDL particles in
the ovariectomized guinea pigs did not increase resistance to oxidation
following SF intake, unlike those in males and females. One possible
explanation is the lack of estrogen in the ovariectomized animals. Sack et al. (1994)
showed that infusion of estradiol without the addition of
progesterone into postmenopausal women, mimicking the serum estradiol
concentrations in premenopausal women, caused a significant decrease in
the oxidative susceptibility of LDL. Another study by Wander et al. (1996)
demonstrated that hormone replacement therapy provided some
degree of protection from oxidation to the LDL particle by decreasing
the rate at which LDL were oxidized.
Female guinea pigs were more susceptible than males to a
hypercholesterolemic diet; however, the cholesterol-lowering
properties of dietary SF were as effective as with male guinea pigs. In
addition, females had higher levels of HDL cholesterol in plasma and
-tocopherol in LDL than the males, a finding in agreement with the
human situation (Cobb et al. 1993
, Kaplan et al. 1987
, Palli et al. 1999
).
We have demonstrated that intake of dietary SF effectively lowers plasma LDL cholesterol concentrations in male, female and ovariectomized guinea pigs fed a high saturated fat, low-cholesterol diet and that gender plays an important role in the metabolic responses to the intervention diet, in this case SF. The gender-associated responses to dietary fiber and the facts that i) ovariectomized guinea pigs displayed the most detrimental lipid profile, ii) female guinea pigs were more susceptible to the hypercholesterolemic diet, and, iii) females had higher HDL-C levels in plasma than males, further justify the appropriateness of the guinea pig model for these studies because they mimic the human situation. The findings from these studies expand our information about the relationships among gender, dietary fiber and plasma lipid levels, and their possible role in the pathogenesis of CHD.
|
ACKNOWLEDGMENTS |
|---|
-tocopherol values. |
FOOTNOTES |
|---|
3 Abbreviations used: ACAT, acyl-CoA/cholesterol acyltransferase; Apo B, apolipoprotein B; C7H, cholesterol
7-hydroxylase; CE, cholesteryl ester; CHD, coronary heart disease; HMG-CoA, 3-hydroxy-3-methyl-glutaryl coenzyme A; LTP, lipid transfer protein; PL, phospholipid; SF, soluble fiber; TAG, triacylglycerol; TBARS, thiobarbituric acid-reactive substances.
Manuscript received September 17, 1999. Initial review completed October 15, 1999. Revision accepted November 30, 1999.
|
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|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
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