Fw: Arbor Clinical Nutrition Update #157: fibre and colon cancer

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ARBOR CLINICAL NUTRITION UPDATES ©

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This week we review two recent epidemiological studies on fibre and colon

cancer.

 

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NUTRITION RESEARCH REVIEW

 

 

Study 1: Fibre protects in prospective study

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High fibre intake is associated with a 25% reduction in colon cancer risk,

according to results from the gigantic EPIC epidemiological study from

Europe.

 

Subjects: 519,978 adults, chosen from a variety of sample populations at 22

centres across Europe.

 

Method: Prospective observational study. Subjects completed a dietary

questionnaire (designed to take into account the specific foods eaten in

each country). Dietary data from each centre were adjusted after

comparison with the total study sample, in order to compensate for

centre-specific bias.

 

Cancer data for the sample population was gleaned from cancer and pathology

registries, health insurance data and individual follow-up (different

sources were used across the 22 countries). Follow up was continued for an

average of 4.5 years, involving nearly 2 million person years.

 

Results: There was an inverse relationship between fibre intake and risk of

developing colon cancer. Those in the highest quintile of fibre intake (32

gm/day in women and 36 gm/day in men) had a 25% reduction in risk compared

with the lowest quintile (12.6 gm/day in women and 12.8 gm/day in men):

odds ratio = 0.75 (95% CI 0.59-0.95, p= 0.005))

 

This protective association had a clear `dose-response' curve. However,

there was no relationship between colorectal cancer and fibre intake from

individual food groups (cereal, vegetable etc.)

 

Ref: Lancet 2003; 361(9368): 1496-501

 

Study two: fibre protective in case-control study

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Subjects with colonic adenoma have lowed fibre intakes, particularly from

grains, cereals and fruits, according to newly published American results.

 

Subjects: 43,611 participants in a multi-centre, randomised trial of

different methods for screening for early cancer (Prostate, Lung,

Colorectal, and Ovarian Cancer Screening project).

 

Method: Case-control evaluation after all subjects had a

sigmoidoscopy. Fibre intake (determined by a food frequency questionnaire)

was compared in those 3,591 subjects who had a colonic adenoma detected

with those 33,971 subjects who were negative for polyps.

 

Results: Subjects on high fibre diets also exercised more, smoked less,

consumed less meat and alcohol and more folate, calcium and aspirin.

 

Multivariate analysis showed a `dose-response' inverse relationship between

fibre intake and adenoma risk. Those in the highest quintile of fibre

intake were 27% less likely to have adenoma (OR = 0.73, 95% CI: 0.62-0.86,

p= 0.002).

 

The association was significant for intake of fibre from fruits and

grains/cereals, but not fibre from legumes and vegetables - see Table.

 

 

Table: Odds ratio for colorectal adenoma: lowest vs highest decile of fibre

intake

(multivariate adjusted analysis)

 

OR 95% CI Signif.

----------

------------

Grains/cereals 0·88 (0·79-0·97) p= 0·008

Legumes 0·99 (0·78-1·20) p= 0·90

Vegetables 1·00 (0·89-1·15) p= 0·99

Fruits 0·80 (0·71-0·93) p= 0·003

 

Ref: Lancet 2003; 361(9368): 1491-95

 

 

COMMENTS:

The possibility that many `diseases of Western civilisation' - including

colon cancer - could be due to a lack of roughage in the Western diet has

been hotly debated since Burkitt proposed it three decades ago.

 

The simplicity and practicality of this idea has had much appeal, as the

wide variety of fibre enriched commercial foods and supplements proves.

 

However, scientific proof has been more difficult to assemble.

 

For example, whilst there is much epidemiological evidence linking fibre

intake and colon cancer (ref.1), some large and well conducted studies do

not support this (Ref.2).

 

The Finnish ATBC study is one. After following more than 27,000 male

smokers for 8 years, no fibre-colon cancer association was found

(ref.3). An American study of nearly 89,000 women over 16 years came to

similar conclusions (ref.4).

 

Intervention trials have also been very unconvincing. Most have studied

patients with existing colonic adenomas and attempted to prevent recurrence

through increased fibre intake, but have not succeeded in doing so

(refs.5-9). The few modest successes reported have involved multi-element

interventions or small sub-groups (refs.10-11).

 

A recent meta-analysis of the randomised controlled trial data found no

evidence that increased dietary fibre reduces either incidence or

recurrence of adenomatous polyps within a 2-4 year period (ref.12).

 

Explanations for this lack of clear cut evidence have generally been of two

types. One type of explanation focuses on the strength of the direct

evidence that fibre protects the colon from cancer development. For

example, through the beneficial effect on colon cells of the short chain

fatty acids that are produced by bacterial fermentation of fibre with in

the bowel (ref.13).

 

The reason we have not been able to prove this in clinical trials,

according to this viewpoint, is that we have not yet identified exactly

which types of fibre are involved. Nor do we know what dose of fibre

sub-types is required for a protective effect.

 

This is feasible in that fibre is a particularly complex part of food,

being not one but many things, both starch and non-starch. It may well be

that fibre from different food groups (e.g. grains, fruits and vegetables)

differs in its effect on the colon 2.

 

These two new trials are therefore helpful in that they do break down the

association into individual food group sources. Because they include people

from diverse locations, they include a wide range of diets and therefore of

types and amounts of fibre in the analysis. Unfortunately, regarding

whether the fibre-colon cancer link is specific for particular food groups,

the two studies come to opposite conclusions !

 

The other type of explanation for the contradictory evidence has been to

say that it is not fibre itself which has any protective effect, but rather

the whole pattern of the kind of diets that provide `high fibre'.

 

Such a diet is likely to be lower in saturated and animal fat and higher in

antioxidants, trace minerals and phytoestrogens, and these may be the real

protective elements (refs.2, 14).

 

What does it mean for the clinician?

At present, it is difficult to know which of these two explanations is more

likely to prove correct. Of course people take fibre supplements for many

reasons, not just related to colon cancer. But clinicians will be wise to

ensure that their patients do not seek extra fibre from supplements alone.

They should get it from a whole diet, rich in the natural food sources of

fibre, such as grains, fruits and vegetables.

 

 

References:

1. Cancer Epidemiol Biomarkers Prev 2000 Mar;9(3):239-47

2. Lancet 2003; 361(9368): 1487-88

3. Cancer Causes Control 1999 Oct;10(5):387-96

4. N Engl J Med 1999 Jan 21;340(3):169-76

5. J Clin Epidemiol 1994 May;47(5):525-36

6. N Engl J Med 2000 Apr 20;342(16):1156-62

7. N Engl J Med 2000 Apr 20;342(16):1149-55

8. Lancet 2000 Oct 14;356(9238):1300-6

9. J Natl Cancer Inst 2002 Nov 6;94(21):1620-5

10. J Natl Cancer Inst 1995 Dec 6;87(23):1760-6

11. Prev Med 1999 Mar;28(3):284-92

12. Cochrane Database Syst Rev 2002;(2)

13. Physiol Rev 2001 Jul;81(3):1031-64

14. Am J Clin Nutr 1999 Sep;70(3 Suppl):459S-463S

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