Dietary Fiber Intake and Risk of Colorectal Cancer

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Dietary Fiber Intake and Risk of Colorectal Cancer

• Rebecca McEntee, MD
• March 29, 2006

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Incidence and Mortality

• Colorectal cancer is the third most common
  cancer, and has shown a gradual decline or
  leveling in incidence and mortality over the past
  decade.
• Also, the third-leading cause of cancer related
  mortality, leading to an estimated spending of
  8.4 billion on treatment in 2004.
• Source Surveillance, Epidemiology, and End
  Results (SEER) Program and the National Center
  for Health Statistics.

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What about dietary fiber?

• Inconsistent results from various epidemiological
  studies
• Case control and ecological studies suggest an
  inverse relationship between fiber intake and
  colon cancer risk.
• Prospective cohort studies and randomized
  clinical trials do not support the association.

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Study Objective

• To evaluate the association between dietary
  fiber intake and risk of colorectal cancer by
  reanalyzing the primary data collected in 13
  prospective cohort studies included in the
  Pooling Project of Prospective Studies of Diet
  and Cancer

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Methods

• Inclusion Criteria
• At least an incidence of 50 cases of colorectal
  cancer
• Assessment of typical dietary patterns
• Completion of a validation study of the dietary
  assessment method used
• Assessment of usual dietary fiber intake

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Methods

• Study population habits assessed via food
  frequency questionnaires characterizing baseline
  intake of foods and nutrients, as well as
  non-dietary risk factors.
• Colorectal cancer incidence monitored by
  self-administered questionnaires, medical record
  review, and/or cancer registry linkage

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Methods

• Statistical Analysis
• Study- and sex-specific relative risks (RR) and
  2-sided 95 confidence intervals (CI)
• Age-adjusted and multivariate analyses
• Primary comparison done by dividing fiber intake
  into quintiles and comparing risk of colorectal
  CA between top and bottom quintiles and testing
  for trend over quintiles

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What did they find?

• In follow-up periods of 6 to 20 years, 725,628
  persons developed 8081 cases (2776 M, 5305 F) of
  colorectal cancer.
• Median energy-adjusted dietary fiber intake
  ranged, across studies, from 14 to 28 g/day M and
  13-24 g/day F

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Results

• No statistically significant heterogeneity
  between studies for the highest quintiles
• Variations in study-specific results likely
  secondary to random variation

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Results

• After adjusting for age only, fiber intake
  significantly associated with a lower risk of
  colorectal cancer (RR0.84 95 CI, 0.77-0.92
  p.002 for trend)
• After adjusting for non-dietary factors, folate,
  multivitamin use, and total energy intake, still
  slightly statistically significant (RR0.88 95
  CI, 0.82-0.96 p.01 for trend)

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Results

• After further adjustment for red meat, total
  milk, and alcohol intake, no longer statistically
  significant!! (RR0.94 95 CI, 0.86-1.01 p.75
  for trend)

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Results

• Not significantly modified by sex, age at
  diagnosis, length of follow-up, or site of
  cancer, or source of fiber
• Similar findings in both European and North
  American studies

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Does the source of fiber matter?
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What about low dietary fiber intake?

• After adjusting for age, smoking status, and
  correction for measurement error, dietary intake
  of less than 10g/day (reported by 11 of all
  participants) vs more than 10g/day showed a RR
  2.16 (95 CI, 1.12-4.16)

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Study strengths

• Examined a focused, relevant clinical question
  using a large, pooled study population
• Reanalyzed primary data, considering
  dose-response relationships and potential
  confounding factors through a variety of
  multivariate models

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Study limitations

• Fiber intake assessed via a single, self-reported
  questionnaire
• Potential for inaccurate characterization of
  usual dietary intake
• Unable to assess the effects of cumulative
  lifetime fiber intake
• Did not assess soluble vs. non-soluble fiber

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Study limitations

• Lacks description of the methods used to locate
  studies for inclusion
• Many subgroup analyses performed none specified
  a priori. Thus the single significant finding
  for participants with lt 10 g/day is questionable.

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Whats the bottom line?

• No support for a linear inverse relationship
  between high dietary fiber intake and risk of
  colorectal cancer
• May be more important to avoid a low fiber diet
  (lt10g/day)

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Reference

• Park Y, Hunter DJ, Spiegelman D, Bergkvist L,
  Berrino F, van den Brandt PA, Buring JE, Colditz
  GA, Freudenheim JL, Fuchs CS, Giovannucci E,
  Goldbohm RA, Graham S, Harnack L, Hartman AM,
  Jacobs DR, Kato I, Krogh V, Leitzmann MF,
  McCullough ML, Miller AB, Pietinen P, Rohan TE,
  Schatzkin A, Willett WC, Wolk A,
  Zeleniuch-Jacquotte A, Zhang SM, Smith-Warner SA.
  Dietary fiber intake and risk of colorectal
  cancer a pooled analysis of prospective cohort
  studies. JAMA. 2005 294 2849-2857.