Clinical and Metabolic Studies

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Results of animal experiments as well as epidemiological observations encouraged many research workers to perform clinical studies on humans. It appears the influence of dietary fiber components added to the diet is different. A summary of studies with wheat bran and cellulose by Anderson and Chen (10) indicates the influence of these fiber sources may depend on the dose of fiber. However, in the majority of studies with wheat bran, the content of fiber was not specified. Due to production technology wheat bran may consist of 16% to 45% dietary fiber. Due to the lack of detailed characteristics of the wheat bran used most results are difficult to interpret. However, in humans and experimental animals wheat bran (as opposed to oat bran) has no hypocholesterolemic effect.

It should be mentioned that Hillman et al. (14) demonstrated the consumption of diet supplemented with lignin by persons with hypercholesterolemia is associated with a decrease of total cholesterol concentration in the blood. However, in persons without disorders of lipid metabolism lignin has no such effect. Studies performed in vitro demonstrate that lignin binds bile acids (15). The hypocholesterolemic action of lignin may be the result of binding bile acids in the intestinal lumen. This mechanism is similar to cholestyramine or plant sterols that have a marked hypocholesterolemic effect in persons with hypercholesterol-emia (16). Although different doses of fiber were used in these separate studies it appears from these summaries that the water insoluble components of dietary fiber are relatively ineffective in lowering the total cholesterol concentration in blood in humans, while the water soluble components are moderately effective.

It should be stressed that it is very important to distinguish between the effects of food rich in fiber and diets containing dietary fiber components as an added ingredient. Guar gum and pectin are fiber sources that were commonly "S

added to the diet as purified preparations of dietary fiber. It was found that the hypocholesterolemic effect of such preparations depends on the dose of fiber and the forms in which they were administered (biscuits, gelatin capsules and water gel form). The most effective are fibers (pectin) in water gel form without sugar. Additionally, the hypocholesterolemic effect of fiber (pectin and beta-glucans) was more pronounced in patients with hypercholesterolemia than in persons without lipid metabolism disorders.

Palmer and Dixon (17) reported that a dose of pectin below 10 grams per day is ineffective in lowering cholesterol levels. In turn, enlarging the dose of pectin to 40-50 grams per day did not multiply its hypocholesterolemic activity. In addition, a 40-50 gram per day dose caused unfavorable side effects, particu-

The Role of Dietary Fiber

larly flatulence, and sometimes diarrhea and vomiting (18). The most often-used dose of pectin was 15 g/day. Some authors suggest that such a dose of pectin included in the diet is optimal (19). Ershoff and Wells (20) demonstrated that pectin derivatives, such as pectic acid, galacturonic acid or polygalacturonic acid, at levels of 5% of diet did not affect serum and liver cholesterol concentrations in cholesterol fed rats. Likewise, methylated polygalacturonic acid had no hypo-cholesterolemic activity. Thus it would appear that only intact pectin has hypo-cholesterolemic activity, which is partially determined by its methoxyl content.

Table 1 Influence of Oat Products on Lipoprotein Cholesterol Level in Humans

Percent of

Percent of

control

control

Dose of fiber

Cholesterol

Cholesterol

References

Fiber source

(g/day)

HDL

LDL

Kirby et al.,

Oat bran

27

0

-1.4

1981 (23)

Kestin et al.,

Oat bran

95

-2.8

-6.8

1990 (24)

Anderson et

Oat bran

110

+ 10.4

- 12.1

al., 1991

(25)

Leadbetter et

Oat bran

30

-5.1

- 2.5

al., 1991

(26)

Oat bran

60

-4.5

1.7

Oat bran

90

-8.9

- 4.0

Lepre et al.,

Oat bran

60

+2.3

-3.0

1992 (27)

Uusitupa et al.,

Oat bran

29.8

-1.3

- 4.8

1992 (28)

Whyte et al.,

Oat bran

123

+4.9

-5.6

1992 (29)

Kashtan et al.,

Oat bran

100

-9.8

- 12.4

1992 (30)

Poulter et al.,

Oat bran

50

+3.4

-8.1

1993 (31)

He et al., 1995

Oat bran

<25

0

-3.8

(32)

Oat bran

25-90

+ 2.5

-16.8

Oat bran

>90

-7.7

Bartnikowska

Our results demonstrated that in patients with hyperlipidemia, high me-thoxylated pectin exerts significantly higher hypocholesterolemic activity than low methoxylated pectin. In persons with hyperlipidemia, the degree of hypocholesterolemic activity of pectin depends on the blood total cholesterol levels. Pectin exerts higher hypocholesterolemic activity in patients with hypercholesterolemia than in persons without disorders of lipid metabolism (2).

Similar results were obtained with oat preparations (meal, bran) rich in beta-glucans. Inclusion of oat preparation to the diet of persons with hypercholes-terolemia caused significant reduction of blood total cholesterol, whereas in persons without disorders of lipid metabolism, the hypocholesterolemic effect of oat preparations was low (21). Similarly, the hypocholesterolemic effect of oat depends also on the dose of oat fiber, but exceeding such a dose does not give an appropriate hypocholesterolemic response.

In order to evaluate if oat products decrease total cholesterol concentration in blood, Ripsin et al. (22) performed a meta-analysis of the results of clinical studies with oat. This analysis provided strong support for the hypothesis that approximately 3 g per day of soluble fiber from oat products can lower total cholesterol level by 0.13 to 0.16 mmol/l, and that the reduction is greater in persons with initially higher blood cholesterol levels. From the summary of results of clinical studies with oat bran (23-32), it appears soluble beta-glucans decrease LDL cholesterol without marked influence on HDL cholesterol. As a result, the ratios of HDL cholesterol to LDL cholesterol as well as the ratio of HDL cholesterol to total cholesterol were increased.

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