Metabolism and Physiological Effects of Pectins

20 Metabolism and Physiological Effects of Pectins WANDA L. C H E N O W E T H and GILBERT A. L E V E I L L E

Downloaded by CORNELL UNIV on May 24, 2017 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/bk-1975-0015.ch020

Food Science and Human Nutrition, Michigan State University, East Lansing, Mich. 48824

Non-digestible dietary carbohydrates recently have received much attention. Although most of this attention has focused on the importance of cereal sources of dietary fiber, poorly digested pectic substances derived primarily from fruit and vegetable sources likewise may be of significance in relation to human health. Pectic substances are complex, colloidal carbohydrate deriv atives which occur in or are isolated from plants. They contain a large proportion of anhydrogalacturonic acid units, most likely combined in a chain-like arrangement. The carboxyl groups of the polygalacturonic acids may be partly esterified or may form salts with various cations (1, 2). Pectin is a general term usually employed to designate water-soluble pectinic acids of varying methyl ester content and degree of neutralization capable of forming gels with sugar and acids (1). The amount of pectic substances in several common fruits and vegetables is shown in Table I. Estimation of the total amount of pectic substances in an average diet is not possible because data are available only for a limited number of foods. Comparison of the content of pectic substances in various foods is further com plicated by differences in analytical methods and incomplete descriptions of the foods. In addition to pectic substances found naturally in plant foods, pectin may be added to foods during processing. The most common use of pectin is in making jams and jellies; however pectin is an acceptable additive in a number of other foods. Pectins also have found numerous uses in the prepar ation of various pharmaceutical products. Much of what i s known about the p h y s i o l o g i c a l e f f e c t of p e c t i n has evolved as a consequence o f i n t e r e s t i n the use of p e c t i n f o r therapeutic purposes. P e c t i n and combinations o f p e c t i n w i t h other c o l l o i d s have been used e x t e n s i v e l y to t r e a t d i a r r h e a l d i s e a s e s , e s p e c i a l l y i n i n f a n t s and c h i l d r e n (3 - 6).

J o u r n a l A r t i c l e No. Station.

7046

, Michigan A g r i c u l t u r a l Experiment 312

Jeanes and Hodge; Physiological Effects of Food Carbohydrates ACS Symposium Series; American Chemical Society: Washington, DC, 1975.


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This use of p e c t i n had i t s o r i g i n i n the treatment of d i a r r h e a w i t h a d i e t of scraped apples, a home-remedy p r a c t i c e d f o r hundreds of years i n Europe and introduced i n t o t h i s country i n 1933 by Birnberg (7). The e f f e c t i v e n e s s of p e c t i n i n t r e a t i n g d i a r r h e a subsequently l e d to i n v e s t i g a t i o n s to determine the mechanism of i t s e f f e c t and i t s f a t e i n the alimentary t r a c t . Experiments i n dogs showed that when 20g p e c t i n was fed i n combination w i t h a mixed d i e t , only 10 percent of the p e c t i n could be recovered i n the f e c e s ; however, i f the same amount was fed during f a s t i n g an average of 50 percent was excreted (Table I I ) . Results obtained w i t h humans fed 50g of p e c t i n d a i l y w i t h a mixed d i e t were s i m i l a r to those i n dogs w i t h approximately 90 percent apparent decomposition of p e c t i n t a k i n g place (8, 9 ) . The degree of decomposition appears to be i n f l u enced by the r e t e n t i o n time i n the i n t e s t i n e , adjustment of the animal to the d i e t , and the degree of e s t e r i f i c a t i o n of the p e c t i n (9, 10). Tests made w i t h human subjects and dogs i n d i c a t e d a l a c k of enzymes i n s a l i v a and g a s t r i c j u i c e which could act on p e c t i n . L i k e w i s e , t r y p s i n , pepsin and rennet had no e f f e c t on p e c t i n i n v i t r o ; however p e c t i n incubated w i t h feces was r a p i d l y decomposed (11). Results of s t u d i e s i n animals and humans w i t h i l e o s t o m i e s i n d i c a t e d that the breakdown of p e c t i n occurs c h i e f l y i n the colon most l i k e l y by the a c t i o n of b a c t e r i a l enzymes ( 9 ) . I s o l a t i o n of the microorganisms which are capable of decomposing p e c t i n r e vealed that the most a c t i v e groups were A e r o b a c i l l u s , L a c t o b a c i l l u s , Micrococcus and Enterococcus (12, 13). The c h i e f products formed during b a c t e r i a l fermentation are carbon d i o x i d e and formic and a c e t i c a c i d . I f g a l a c t u r o n i c a c i d i s produced i t apparently i s broken down r a p i d l y s i n c e only a s m a l l amount i s present i n i n c u b a t i o n mixtures. Although a b a c t e r i c i d a l a c t i o n of p e c t i n has been proposed to e x p l a i n the e f f e c t i v e n e s s of p e c t i n i n t r e a t i n g d i a r r h e a , most experimental r e s u l t s do not support t h i s theory (14, 15). However, recent evidence suggests that under c e r t a i n i n v i t r o c o n d i t i o n s , p e c t i n s may have a s l i g h t a n t i m i c r o b i a l a c t i o n toward E. c o l i (16). Most of the recent i n t e r e s t i n p e c t i n has been i n r e l a t i o n to i t s e f f e c t on l i p i d metabolism. In 1957 L i n and coworkers (17) f i r s t reported that i n r a t s the a d d i t i o n of p e c t i n to a b a s a l d i e t c o n t a i n i n g c h o l e s t e r o l increased the e x c r e t i o n of f e c a l s a p o n i f i able and non-saponifiable l i p i d s and decreased the absorption of exogenous c h o l e s t e r o l . About the same time Keys and coworkers (18) proposed that the lower incidence of a t h e r o s c l e r o s i s associ a t e d w i t h " I t a l i a n - t y p e d i e t s " might be r e l a t e d to the amount of complex carbohydrates such as p e c t i n , h e m i c e l l u l o s e and f i b e r present i n the f r u i t s and vegetables abundant i n these d i e t s . Subsequent i n v e s t i g a t i o n s have thus focused on the e f f e c t of p e c t i n on serum and l i v e r c h o l e s t e r o l concentrations and the f e c a l e x c r e t i o n of l i p i d and s t e r o l s . Numerous experiments i n r a t s (Table I I I ) have shown that


314

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TABLE I PECTIN CONTENT OF VARIOUS FOODS Food

P e c t i c substances %

Apples

fresh basis II II


Bananas

0.5

-

1.6

0.7 - 1.2

Peaches

It

It

0.1 - 0.9

Strawberries

II

tl

0.6 - 0.7

Cherries

II

II

0.2 - 0.5

Green peas

II

II

0.9 - 1.4

dry matter b a s i s It tt II

6.9 - 18.6

Carrots Orange pulp

12.4 - 28.0

Potato

n

it

it

1.8 - 3.3

Tomato

tt

tt

tl

2.4 - 4.6

Adapted from "The P e c t i c Substances" ( 2 ) .

TABLE I I RECOVERY OF PECTIN FROM FECES BY URONIC ACID ESTIMATION Per Cent Pectin fed with basal d i e t : dogs

8.9

humans

8.7

P e c t i n f e d during f a s t i n g : dogs

51.2

humans

13.8 Adapted from Am. J. Dig. D i s . ( 9 ) .


Jeanes and Hodge; Physiological Effects of Food Carbohydrates ACS Symposium Series; American Chemical Society: Washington, DC, 1975. 0 Ψ Ψ >KNS) 4-(NS) Ψ Ψ + (NS) 0 *(NS)

0 Ψ + (NS)

42 28 28

28 28

28 21 28

28 28

28

18 18

12

5% 2.5% 5%

5% 10%

5% 5% 5%

5% 10%

5%

3% 3%

10%

0 1% 1%

1% 1%

1% 1% 1%

1% 1%

1%

0 1%

0

2

(25) Ψ

Ψ

τ

(23)

0 *(NS)

Ψ Ψ

τ

(33)

(36)

(22)

*(NS) Ψ Ψ

Ψ Ψ Ψ

*

(20)

Ψ Ψ

Ψ Ψ

0

(19)

(17)

0 0 Ψ

f

Ref

0 Ψ Ψ

τ

Bile Acids

NS = no s t a t i s t i c a l l y

significant difference

Results expressed as increase o r decrease compared to r e s u l t s i n r a t s on same d i e t without p e c t i n

50 mg/day

6

Dietary Pectin

500 mg/day

Dietary Cholesterol

E f f e c t of P e c t i n on Plasma and F e c a l L i p i d s i n Rats"^ Fecal Liver Plasma No. Days Total Total on d i e t C h o l e s t e r o l Lipids Sterols Lipids Cholesterol

TABLE I I I



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a d d i t i o n of 3 to 10% p e c t i n to a d i e t c o n t a i n i n g 1% c h o l e s t e r o l counteracts the increase i n l i v e r c h o l e s t e r o l and l i v e r t o t a l l i p i d induced by c h o l e s t e r o l feeding (19-25). A decrease i n serum c h o l e s t e r o l i s u s u a l l y observed as a r e s u l t of p e c t i n supplement a t i o n although i n some experiments the decrease has not been s t a t i s t i c a l l y s i g n i f i c a n t . P r o t o p e c t i n and p e c t i n s w i t h a low methoxy content do not appear t o be e f f e c t i v e i n lowering serum or l i v e r c h o l e s t e r o l . Supplements of tomato p e c t i n have been reported to produce a smaller decrease i n l i v e r c h o l e s t e r o l than c i t r u s p e c t i n (25); however c i t r u s p e c t i n N.F. and apple p e c t i n apparently a r e e q u a l l y e f f e c t i v e (19). An a n t i - h y p e r c h o i e s t e r o l e m i c a c t i o n of p e c t i n has a l s o been reported i n chickens (26-28) and swine (29). I n chickens the a d d i t i o n of 3 to 5% p e c t i n to the d i e t caused a marked increase i n f e c a l e x c r e t i o n of c h o l e s t e r o l and t o t a l l i p i d s i r r e s p e c t i v e of the c h o l e s t e r o l content of the d i e t (26, 27). I n guinea p i g s and hamsters p e c t i n apparently does not have a c h o l e s t e r o l lowering e f f e c t (30). Although long-term feeding of p e c t i n l e d to a s i g n i f i c a n t decrease i n plasma c h o l e s t e r o l i n male, but not female, r a b b i t s (31), short-term feeding had no e f f e c t (30). In most experiments p e c t i n has been found t o produce an e f f e c t on c h o l e s t e r o l and l i p i d concentrations only i n animals r e c e i v i n g added d i e t a r y c h o l e s t e r o l (19, 29, 32). However, Mokady (33) has r e c e n t l y reported that i n short term feeding experiments i n r a t s , s u b s t i t u t i o n of 10% p e c t i n f o r s t a r c h i n a c h o l e s t e r o l - f r e e d i e t caused a f i v e - t o t e n - f o l d increase i n f e c a l t o t a l l i p i d s and doubled o r t r i p l e d s t e r o l e x c r e t i o n . A s m a l l decrease i n serum c h o l e s t e r o l was observed but r e s u l t s were s i g n i f i c a n t f o r only one of the p e c t i n s t e s t e d , a h i g h molecular weight c i t r u s p e c t i n (Table I V ) . Few i n v e s t i g a t i o n s have been conducted i n humans to evaluate the e f f e c t of p e c t i n supplementation. Keys and coworkers (18) f e d middle-aged men c o n t r o l l e d d i e t s of n a t u r a l foods w i t h and without a d d i t i o n of 15g d a i l y of e i t h e r c e l l u l o s e o r p e c t i n . A three-week p e r i o d of p e c t i n supplementation r e s u l t e d i n a f a l l i n the mean c o n c e n t r a t i o n s of serum c h o l e s t e r o l to l e v e l s approximately 5% below the l e v e l on the same d i e t without p e c t i n (Table V). C e l l u l o s e supplementation, however, f a i l e d t o show any s i g n i f i c a n t e f f e c t on serum c h o l e s t e r o l concentrations. I n a study by Palmer and Dixon (34) an attempt was made to determine the e f f e c t i v e dose of p e c t i n which i s r e q u i r e d to reduce blood c h o l e s t e r o l concentrat i o n s . Sixteen men were f e d v a r y i n g amounts of p e c t i n during s i x 4-week t e s t p e r i o d s . Other d i e t a r y v a r i a b l e s and r i s k f a c t o r s a s s o c i a t e d w i t h a t h e r o s c l e r o s i s were not c o n t r o l l e d . Although not a l l subjects responded e q u a l l y w e l l t o p e c t i n supplementation, d a i l y doses of 8 to 10g p e c t i n caused a s i g n i f i c a n t decrease i n serum c h o l e s t e r o l of these men, a l l of whom had i n i t i a l values which were e i t h e r normal o r only s l i g h t l y elevated. E v a l u a t i o n of the e f f e c t of d i e t a r y p e c t i n on plasma and f e c a l l i p i d s i n humans was made i n an i n v e s t i g a t i o n conducted a t


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TABLE IV EFFECT OF PECTIN ON BLOOD CHOLESTEROL AND FECAL LIPIDS IN RATS FED A CHOLESTEROL-FREE DIET Relative Blood Cholesterol

Control 2


Low MW P e c t i n High MW P e c t i n

1

values f o r

Fecal Lipids

Total Fecal Sterols

%

%

%

100

100

100

91

458

278

76

735

372

Low M P e c t i n

86

390

222

Pectin S

83

553

270

P e c t i n MR

83

478

293

"Average values f o r 8 rats/group expressed as a percentage of c o n t r o l values ( f o r animals f e d a p e c t i n - f r e e d i e t ) MW = molecular weight; M = methoxy; S = slow s e t t i n g ; MR = medium-rapid s e t t i n g Adapted from Nutr. Metabol. (33).

TABLE V MEAN SERUM CHOLESTEROL CONCENTRATIONS IN 24 MEN FED 15gPECTIN/DAY Major source of d i e t a r y carbohydrate

Serum c h o l e s t e r o l , mg % No P e c t i n

+ Pectin

Legumes

202.4

192.7

Sucrose

221.5

211.3

Δ -9.7 -10.2

Adapted from Proc. Soc. Exper. B i o l . and Med. (18).



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the U n i v e r s i t y of Iowa (35). During an i n i t i a l p e r i o d of 4 weeks three healthy male subjects were fed c o n t r o l l e d d i e t s having a composition s i m i l a r to that of the average American d i e t . During a second 5-week p e r i o d the men r e c e i v e d the same d i e t plus 20 to 23g p e c t i n N.F. which was incorporated i n t o the foods i n the d i e t . The f i n a l 5 weeks represented a second c o n t r o l and recovery p e r i o d . R e s u l t s showed that a f t e r p e c t i n feeding there was a s i g n i f i c a n t decrease of 13% i n the mean plasma c h o l e s t e r o l compared to the value f o r the c o n t r o l p e r i o d (Table V I ) . No apprec i a b l e e f f e c t on the plasma t r i g l y c e r i d e l e v e l was observed. During the p e r i o d of p e c t i n i n g e s t i o n the men showed an increase i n t o t a l f e c a l f a t , s t o o l volume, f e c a l s t e r o l and f e c a l d i g i t o nide p r e c i p i t a b l e s t e r o l s . Several mechanisms have been proposed by which d i e t a r y p e c t i n may lower plasma and l i v e r concentrations of c h o l e s t e r o l i n v a r ious s p e c i e s . P o s s i b l e mechanisms would i n c l u d e : 1) r e d u c t i o n i n c h o l e s t e r o l absorption; 2) a l t e r a t i o n i n i n t e s t i n a l m i c r o f l o r a and 3) depression of b i l e a c i d a b s o r p t i o n or r e c i r c u l a t i o n . Experimental r e s u l t s reported by L e v e i l l e and S a u b e r l i c h (22) i n d i c a t e that the most important e f f e c t of p e c t i n i n the r a t appears to be i t s i n f l u e n c e on b i l e a c i d a b s o r p t i o n . In c h o l e s t e r o l - f e d r a t s the a d d i t i o n of p e c t i n increased f e c a l b i l e a c i d e x c r e t i o n by 32%, whereas f e c a l n e u t r a l s t e r o l e x c r e t i o n was not a l t e r e d (Table V I I ) . A d d i t i o n a l experiments w i t h i n v e r t e d i n t e s t i n a l sacs demonstrated that p e c t i n decreased i n v i t r o t a u r o c h o l i c a c i d t r a n s p o r t by 50%. Further support f o r the importance of the i n h i b i t i o n of b i l e a c i d absorption was provided by the s i m i l a r i t y i n response of c h o l e s t e r o l - f e d r a t s to p e c t i n and cholestyramine, a known i n h i b i t o r of b i l e a c i d absorption (Table V I I I ) . More r e c e n t l y , however, P h i l l i p s and B r i e n (36) suggested that there may be d i f f e r ent mechanisms f o r the a c t i o n of p e c t i n and cholestyramine s i n c e i n t h e i r experiments p e c t i n d i d not a f f e c t v i t a m i n A absorption whereas cholestyramine i s known to l i m i t absorption of the v i t a min. ^ D i e t a r y p e c t i n somewhat decreases absorption of c h o l e s t e r o l 4- C i n c h o l e s t e r o l - f e d r a t s , as evidenced by decreased deposit i o n of r a d i o a c t i v e c h o l e s t e r o l i n the l i v e r and increased f e c a l e x c r e t i o n of c h o l e s t e r o l - 4 - ^ C (22). However, impaired cholest e r o l absorption induced by d i e t a r y p e c t i n apparently i s only p a r t i a l l y r e s p o n s i b l e f o r the hypochoiesterolemic e f f e c t of p e c t i n . Since p e c t i n e f f e c t i v e l y lowers l i v e r c h o l e s t e r o l even when c h o l e s t e r o l and p e c t i n are fed s e p a r a t e l y on a l t e r n a t e days (19, 22), impaired absorption of the added c h o l e s t e r o l would not appear to be the most c r i t i c a l means by which p e c t i n exerts i t s e f f e c t . In r a t s fed a c h o l e s t e r o l - f r e e d i e t Mokady (37) r e c e n t l y reported that h e p a t i c b i o s y n t h e s i s of c h o l e s t e r o l , as measured by conversion of a c e t a t e - l - ^ ^ C to c h o l e s t e r o l , was s u b s t a n t i a l l y higher i n r a t s fed 10% p e c t i n . In v i t r o i n c o r p o r a t i o n of l a b e l i n t o t r i g l y c e r i d e s , phospholipids and t o t a l l i p i d s i n the l i v e r was a l s o s i g n i f i c a n t l y higher i n the p e c t i n - f e d animals. The


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TABLE VI CHANGES IN PLASMA AND FECAL LIPIDS OF THREE MEN IN RESPONSE TO PECTIN SUPPLEMENTATION Periods

Plasma c h o l e s t e r o l , mg %

Ï

II

Basal Diet

Basal Diet + 20-23 g Pectin

226

±

6.8

84

±

12.8

1

III Basal Diet

2

196

±

9.9

90

±

10.9

211

±

8.4


Plasma, t r i g l y c e r i d e s , mg % T o t a l f e c a l f a t , g/24 h r F e c a l s t e r o l s , g/24 h r Fecal digitonide precip-

3.4 1.06 69

± ± ±

0.31 0.12 8.3

5.1 1.23 126

± ± ±

98

±

9.5

0.41

2

3.2

±

0.41

0.12

3

0.82

±

0.15

91

±

18.5

12.4

2

i t a t e , mg/24 h r Mean

±

standard d e v i a t i o n f o r 3 s u b j e c t s .

D i f f e r s s i g n i f i c a n t l y from mean values f o r Periods I and I I I (p < 0.05). ^ D i f f e r s s i g n i f i c a n t l y from mean value f o r P e r i o d I I I (p < 0.05). Unpublished data adapted from (35).


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TABLE V I I EFFECT OF PECTIN SUPPLEMENTATION IN CHOLESTEROL-FED RATS Cholesterol (1%) Plasma c h o l e s t e r o l ,

C h o l e s t e r o l (1%) + p e c t i n (5%)

128

±

3

1

116

±

5

7.4

±

0.3

6.6

±

0.3

10.3

±

0.5

7.5

±

0.3

142

+

5

2

141

±

8

23.6

±

3.3

mg/100 ml Liver f a t , %


L i v e r c h o l e s t e r o l , mg/g F e c a l s t e r o l s , mg/day F e c a l b i l e a c i d s , mg/day ±

"*"Mean

17.9

2.4

±

2

SE of mean f o r 10 r a t s .

2 Values are means f o r 5 animals. Adapted from J. Nutr. (22).

TABLE V I I I COMPARISON OF THE EFFECT OF PECTIN AND CHOLESTYRAMINE IN CHOLESTEROL-FED RATS Liver Dietary treatment

1% c h o l e s t e r o l

Total Lipid

Cholesterol

Plasma Cholesterol

%

mg/g

mg/100 ml

7.9 ± 0.3

10.3 ± 0.5

128 ± 14

7.1 ± 0.4

7.2 ± 1.1

91 ± 4

5.8 ± 0.2

4.0 ± 0.2

86 ± 4

1% c h o l e s t e r o l + 5% p e c t i n 1% c h o l e s t e r o l + 1% cholestyramine

Mean f o r 5 r a t s * SE of mean Adapted from J. Nutr. (22).



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author suggested that the higher r a t e of h e p a t i c l i p o g e n e s i s observed i n the p e c t i n - f e d r a t s might be due to a r e d u c t i o n i n the absorption of d i e t a r y f a t , since absorbed l i p i d i s known to i n h i b i t hepatic f a t t y acid synthesis. The e f f e c t of d i e t a r y p e c t i n on the number and types of i n t e s t i n a l microorganisms has not been thoroughly i n v e s t i g a t e d ; however, the f a i l u r e of a n t i b i o t i c s to prevent the lowering of blood c h o l e s t e r o l by p e c t i n Ç19, 22) i n d i c a t e s that i n t e s t i n a l m i c r o f l o r a do not c o n t r i b u t e s i g n i f i c a n t l y to i t s e f f e c t . The e f f e c t of p e c t i n on the absorption of n u t r i e n t s other than l i p i d s has received l i t t l e a t t e n t i o n . In r a t s the u t i l i z a t i o n of 3-carotene and absorption of v i t a m i n A was not impaired by p e c t i n supplementation (36). A d d i t i o n of 12 or 24% p e c t i n to d i e t s of r a t s has been found to decrease the d i g e s t i b i l i t y of prot e i n (38). V i o l a and coworkers (39) reported that slow s e t t i n g p e c t i n (55% e s t e r i f i e d ) decreased the apparent d i g e s t i b i l i t y of p r o t e i n but d i d not impair i t s u t i l i z a t i o n . However, a d d i t i o n of 10% medium-rapid-acting p e c t i n (65% e s t e r i f i e d ) l e d to an impairment of p r o t e i n u t i l i z a t i o n as shown by a decrease i n weight gain per gram of digested p r o t e i n . Both p e c t i n preparations decreased the apparent r e t e n t i o n of calcium by approximately 30%. In summary, experimental r e s u l t s i n v a r i o u s species i n d i c a t e that ingested p e c t i n i s n e a r l y completely broken down i n the colon most l i k e l y by b a c t e r i a l enzymes. The products formed apparently are not e x t e n s i v e l y u t i l i z e d s i n c e p e c t i n makes a n e g l i b l e c o n t r i b u t i o n to the energy value of the d i e t (39). The d i g e s t i b i l i t y and u t i l i z a t i o n of p e c t i n , however, needs to be re-evaluated using more s e n s i t i v e and s p e c i f i c methods. The lowering of plasma and l i v e r c h o l e s t e r o l concentrations by p e c t i n supplementation appears to be r e l a t e d p r i m a t i l y to i t s e f f e c t on b i l e a c i d absorption. R e s u l t s showing an increased e x c r e t i o n of b i l e a c i d s would suggest the p o s s i b i l i t y of an increased h e p a t i c conversion of c h o l e s t e r o l to b i l e a c i d s thus reducing serum and l i v e r c h o l e s t e r o l concent r a t i o n s . By removing the feedback i n h i b i t o r of c h o l e s t e r o l on HMG-CoA reductase these changes would e x p l a i n the increase i n h e p a t i c b i o s y n t h e s i s of c h o l e s t e r o l which has been observed. Further i n v e s t i g a t i o n s are needed to evaluate c h o l e s t e r o l synt h e s i s and turnover as w e l l as the a c t i v i t y of enzymes important i n the r e g u l a t i o n of c h o l e s t e r o l s y n t h e s i s .


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