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School of Nutrition and Health Sciences, Taipei Medical College, 250, Wu-Hsing Street, Taipei 110, Taiwan ROC
2To whom correspondence should be addressed.
| ABSTRACT |
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KEY WORDS: rats rice starch resistant starch propionate cholesterol
| INTRODUCTION |
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In an epidemiologic study, the intake of carbohydrates in 12 different
countries was compared with the incidence of colorectal cancer. A
strong inverse relationship was found between starch consumption
(corrected for fat and protein intake) and incidence of bowel cancer
(large bowel r = -0.70; r = -0.76),
but no significant relationship between nonstarch polysaccharides and
bowel cancer was found (Cassidy et al. 1994
).
In the past, when scholars studied the metabolic relationship between
multiple carbohydrates and lipids, they tended to focus on dietary
fiber. Recently, however, researchers have discovered that some starch
cannot be digested. This fraction is termed resistant starch because of
its resistance to amylase degradation (Englyst et al. 1992
). Cereals are not only a rich source of starch but also of
dietary fiber, and can reduce the danger of arterial blood diseases
(Anderson 1969
, Anderson and Gustafson 1988
, Cheng and Yu 1997
). However, different
cereal starches have different properties and structures as well as a
range of physical and chemical properties. Rice is the staple food in
many Asian cultures and also the main source of carbohydrate. Resistant
starch in cornstarch has been shown to reduce serum cholesterol in rats
(De Deckere et al. 1992
).
Englyst et al.(1992)
classified starches according to
three categories of digestibility, i.e., rapidly digestible starch,
slowly digestible starch and resistant starch. The last-mentioned
category is further differentiated by whether the starch is
inaccessible to enzymes, contains chemically resistant starch granules
or is retrograded starch.
Resistant starch lowers plasma lipids in rats, but some types do not do
so in humans (Noakes et al. 1996
) or pigs
(Topping et al.1997
). It appears that rats may differ
fundamentally from humans, because resistant starch lowers plasma
lipids in that species (Levrat et al. 1996
), whereas in
humans, resistant starch lowers fecal bile acid excretion
(Langkilde et al. 1998
). Topping et al. (1997)
published micrographs of starch granule etching, and
Marsono et al. (1993)
studied resistant starch in the
porcine large bowel. Muir et al. (1998)
examined a shift
to a high rice starch Chinese diet in humans and reported on the
unexpected nature of the changes. Chen et al. (1984)
reported that dietary propionate reduced cholesterol accumulation in
both serum and liver of cholesterol-fed rats.
Very little work has been conducted investigating the effects of
resistant rice starch levels with cholesterol supplementation on lipid
metabolism, or using scanning electron microscopy (SEM) to observe the
physical properties of undigested rice starch in feces. The objective
of this research was to investigate the effects of the intake of
different ratios of rice starch to cornstarch in the AIN-76 diet
(Bieri et al. 1977
and 1980
) containing 1 g/100 g
cholesterol on serum and liver lipids in rats.
| MATERIALS AND METHODS |
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The diet content was a modified AIN-76 diet (Bieri et al.1977
and 1980
; Table 1
) in which carbohydrates account for 63 g/100
g, protein for 20 g/100 g, and fat for 10 g/100 g. Casein and soybean
oil were used to make the total energy and the proportions of the three
major nutritional elements of each diet treatment the same. The
contents of the diets were homogenized, placed in plastic bags, tightly
sealed and refrigerated at 4°C. The test diets were fed to groups of
rats (n = 7/group) for 4 wk. During the 4-wk feeding
period, feces were collected once every 2 d and stored at -20°C
until analysis. Food was withheld for 12 h at the end of wk 4. The
rats were anesthetized with 1 g/L sodium pentobarbital and dissected.
Blood was collected from the abdominal aorta, incubated at room
temperature for 45 min and centrifuged at 4000 x g for
15 min. The serum was then stored in a freezer at -70°C. The livers
were excised, rinsed in 9 g/L NaCl solution and stored tightly sealed
at -70°C.
Fecal moisture contents were analyzed according to the AOAC (1980)
method. Resistant starch was analyzed according to the
method of Berry (1986)
. Proximate compositions of
cereals and diets are shown in Table 1
. The serum triglyceride
concentration was determined as described by Mcgowan et al. (1983)
, and the procedure of Richmond (1973)
was
used to determine serum total cholesterol concentration.
Phosphotungstic acid and magnesium were added to the serum, causing
VLDL and LDL to precipitate, and the sample was then centrifuged at
4000 x g for 15 min. The upper liquid contained HDL;
using the method of Richmond (1973)
, the HDL cholesterol
was measured.
Heparin and sodium citrate were added to the serum, causing LDL to
precipitate; the serum was then centrifuged for 15 min at 4000 x g. The upper liquid contained VLDL; using the method of
Richmond (1973)
, by subtracting the cholesterol content
of the upper liquid from the total cholesterol level, the LDL
cholesterol concentration could be calculated.
The frozen liver was defrosted according to the method of Folch (1957)
. Liver (1.5 g) was cut and extracted with 20 mL
chloroform/methanol (2:1, v/v). After the addition of 4 mL of a 0.5 g/L
CaCl solution to the extract, it was collected in a sample flask and
stored. The liver lipid extract was measured accurately into a glass
tube with a screw top, according to the method of Soloni (1971)
, compared with a triolein standard solution for
calculations and used to determine the liver triglyceride content. The
liver lipid extract was treated according to the method of
Carlson and Goldfarb (1977)
; liver total cholesterol
concentrations were determined according to the method of
Richmond (1973)
.
Fecal lipid was extracted according to the method of Carlson and Goldfarb (1977)
. After vacuum concentration, the enzymatic
method of Richmond (1973)
was used to determine fecal
total neutral steroids concentration.
Fecal lipids were extracted by addition of ethanol, then concentrated
and dried using a vacuum process. Petroleum ether was added to remove
the extract. The residue was evaporated completely; methanol was added
and the sample collected in a flask. The method of Mashige et al. (1981)
was used to determine fecal bile acids.
Fecal starch was selected using an iodine solution, i.e., iodine and sodium iodide dissolved in 50 mL purified water, with additional purified water added to make 100 mL. Dried fecal starch was then loaded onto aluminum studs and coated with gold for 3 min at 8 mA under a pressure of 13.3 Pa. The samples were scanned and examined using a Hitachi model S-2400 SEM (Tokyo, Japan).
Short-chain fatty acids (SCFA) were measured by gas-liquid
chromatography using serum samples after ethanolic extraction
(Demigné and Rémésy 1982
).
Statistical analysis.
Differences among treatment group means were assessed by one-way ANOVA (SAS Institute, Cary, NC). Group means were considered to be significantly different at P < 0.05 as determined by Duncans new multiple range test.
| RESULTS |
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Serum total (r = -0.94) and LDL cholesterol
(r = -0.67) concentrations decreased with increasing
dietary rice starch (P < 0.05). Propionate was not
detected in serum of rats fed the R and R15 diets, but rats fed the
R30, R45 and R63 diets had propionate concentrations that increased
with increasing dietary rice starch (r = 0.95;
P < 0.05) (Table 2
).
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In rats fed the R45 or R63 diet, fecal moisture contents were greater
than those of other groups. Rats fed the R30 and R45 diets had
significantly lower fecal total neutral steroid excretions than those
of the C and R15 groups (Table 2)
.
The two kinds of starch had different structures as seen by SEM
(Figs. 1
, 2
). The rice starch seemed to be an aggregation (n = 2060) of smaller granules (38 µm in diameter), whereas
the cornstarch was composed of larger (515 µm in
diameter), single granules. The compound rice starch (0.99 kg/L) was
larger in size and denser in structure than the cornstarch (0.63 kg/L).
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| DISCUSSION |
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Resistant starch, so termed because of its resistance to amylase
degradation, lowers plasma lipids in rats, but not all resistant
starches have the same effect in humans (Noakes et al. 1996
) or pigs (Topping et al.1997
). Cereals
starches, which differ greatly in structure and chemical properties,
can reduce the danger of arterial blood diseases (Anderson 1969
, Anderson and Gustafson 1988
, Cheng and Yu 1997
). This research examined the effects of the intake
of different ratios of rice starch to cornstarch on serum and liver
lipids in rats by feeding the AIN-76 diet with cholesterol supplemented
at a high level. Resistant starch in cornstarch has been shown to
reduce serum cholesterol in rats (De Deckere et al. 1992
, Morand et al. 1992
). A shift to a high
rice starch Chinese diet (Muir et al. 1998
) in humans
resulted in unexpected changes.
Examination of starch granules of different sizes from cassava and corn
suggests that the smaller the granule, the greater the extent of in
vitro digestion by bacterial
-amylases and fungal amyloglucosidase
(Franco and Ciacco 1992
).
We used SEM to observe the structure of rice and cornstarch in diets, and undigested starch in feces. The diameter of rice starch granules seemed to be an aggregation (n = 2060) of smaller granules, whereas the cornstarch was composed of large (515 µm), single granules. The compound rice starch was larger in size and denser (0.99 kg/L) in structure than cornstarch (0.63 kg/L); it was digested and absorbed more slowly. We also observed undigested rice (R64) and cornstarch (C) in feces. The 45 and 63% rice starch diets favored high propionic acid fermentations.
Propionate may inhibit the synthesis of fatty acids in the liver
(probably through competition with lactate), thereby lowering the rates
of triacylglycerol secretion (Chen et al.1984
).
Propionate also may be involved in the control of hepatic cholesterol
synthesis. It has been proposed that rice starch lowers plasma
cholesterol concentrations by inhibiting hepatic cholesterogenesis via
propionate formed through large-bowel fermentation (Chen et al. 1984
).
Resistant starch has been defined by Berry (1986)
and
Englyst et al.(1987)
as starch that escapes small
intestinal digestion, but the actual definition (and that used in Table 1
) is a chemical one. It is possible that starch, not measured by the
method of Berry (1986)
, may also reach the small
intestine. It appears inappropriate to conclude that the additional
serum propionic acid (4060 µmol/L) in rats fed R30, R45,
and R60 was derived from the modest increase in resistant starch in the
diet (see Table 1
). The 1.26% increase in rats fed RS in these three
diets vs. the other two (assuming a baseline RS value of 13.0%)
amounts to 0.10.6 g fermentable carbohydrate (19.8 g/d x %
increase in RS). However, in this study, it might be appropriate to
define RS as an enzyme-resistant starch. Such dietary fibers may be
defined in two ways, i.e., by an analytical approach and a by
physiologic approach.
Rice starch cannot be completely digested by enzymes in the small
intestine. Cheng and Yu (1997)
reported that little of
the rice starch was excreted into the feces, indicating that most of
the 63% rice starch diet was fermented by bacteria in the intestines
of rats. Resistant starch is fermented by the gut microflora into SCFA
in the large intestines (Demigné and Rémésy 1982
). Chen et al. (1984)
reported that the
propionate generated by bacterial fermentation of fibers reduced
cholesterol accumulation in both serum and liver of cholesterol-fed
rats. The hypocholesterolemic effect of propionate may be related to
altered hepatic cholesterol synthesis. Nishina and Freedland (1990)
reported that the effect of propionate on lipid
metabolism is apparently limited to inhibition of de novo fatty acid
synthesis. Because rice starch can generate SCFA by fermentation, some
researchers (Suzuki and Kajuu 1983
) determined that SCFA
can inhibit the synthesis of hepatic triglycerides and reduce serum
lipids. Nishina and Freedland (1990)
showed that
propionate can inhibit the activity of pyruvate dehydrogenase in liver
and thus reduce the synthesis of fatty acids. Hara et al. (1999)
pointed out that a decrease in hepatic cholesterol
synthesis rate contributes mainly to the lowering of plasma cholesterol
in rats.
In summary, the results presented here indicate that resistant rice starch was fermented to produce propionic acid, which reduced serum total cholesterol, serum LDL cholesterol, hepatic cholesterol and hepatic triglyceride in rats.
| FOOTNOTES |
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3 Abbreviations used: C, 63% cornstarch + 1% cholesterol (AIN-76 diet +1% cholesterol); R15, 15% rice starch + 48% cornstarch + 1% cholesterol; R30, 30% rice starch + 33% cornstarch + 1% cholesterol; R45, 45% rice starch + 18% cornstarch + 1% cholesterol; R63, 63% rice starch + 1% cholesterol; RS, resistant starch; SCFA, short-chain fatty acids; SCM, scanning electron microscopy. ![]()
Manuscript received October 13, 1999. Initial review completed November 15, 1999. Revision accepted April 4, 2000.
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