Does doubling your fiber lower colorectal cancer risk?
Conflicting. Of 10 studies that test it, 4 disagree, 3 agree and 3 partly agree.
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The claim checked: To avoid colorectal cancer, double or triple your fiberHear it at 1:22:54
What do the studies say?
Conflicting
The studies split. Four of ten disagree. Among 88,757 nurses, women who ate the most fiber, 28.5 g a day, had the same risk as women who ate the least, 8.5 g. A pooled analysis of 13 cohort studies found no link. A pooled analysis adds up the results of many smaller studies. Three agree. Across ten European countries, each extra 10 g of fiber a day went with lower risk. In UK groups, people eating about 24 g a day had 30% lower odds than people eating 10 g. Three are mixed. No study had people double or triple their fiber on purpose.
Computed from 10 studies that test this point, each judged by two readers, in full.
Sources land on both sides.
10 more studies are waiting for a full read and do not count yet.
The study behind the claimThe retelling overstates it
What did the study actually test?
On tape: Burkitt went to Africa and found virtually no colorectal cancer, especially in young people, because the fiber-rich diet had everyone emptying the bowel twice a day, against every 2 or 3 days in the US; so double or triple your fiber. The paper reports colorectal cancer as rare in rural Africa (about 2% of cancers, more than ten times less common than in the West) and slower bowel transit on refined diets (28.4 against 14.5 hours). It says nothing about young people and does not report bowel movements twice a day. It treats fiber removal and refined carbohydrate together, and it says the link with cancer has 'not yet been shown to be causative'. Doubling or tripling fiber was not tested. Read from the free scanned PDF on Wiley through its OCR text layer in the browser, so the text is not in the cache. The authors_sentence was checked against the page image; the result_sentence is from the OCR layer with line-break hyphens joined. No errata found.
Epidemiology of cancer of the colon and rectum.Cancer, 1971. Ecological review and hypothesis: cancer registry rates, African hospital series and a questionnaire to 34 rural hospitals, with bowel transit-time and stool-weight measurements; no individual diet or fiber intake measured, Populations, not individuals: rural African communities and hospitals compared with Western countries (Fig. 1 rates are men aged 35 to 64); transit-time groups of African villagers and African and English schoolboys.
What was saidWhat the study did
What was given
DiffersA diet so rich in fiber that people empty the bowel twice a day; double or triple your fiberCommunities on traditional high-residue diets, fiber intake not measured
Compared with
MatchesThe US, emptying the bowel every 2 or 3 daysWestern countries on refined low-residue diets
What was measured
MatchesVirtually no colorectal cancer, especially in young peopleColorectal cancer as a share of all cancers and as age-standardized rates; rare in Africa, not absent
Who took part
MatchesAfricans in the 1950s and 1960sAfrican hospital and registry series 1939 to 1967 against Western registries
What it found
“These diseases are all rare in every community examined which exists on a high residue diet, and common in every country where a low residue diet has been adopted.”
“A relationship can be demonstrated between certain bowel diseases and diet, and although this has not yet been shown to be causative in the case of cancer, the relationship between an over-refined diet and diverticular disease can now be considered established.”
No funding statement. Presented at the National Conference on Cancer of the Colon and Rectum, San Diego, January 1971. Two readers agreed on the label, and the paper was read in full.
The point checkedConflicting
Eating much more fiber, double or triple the usual intake, lowers the risk of colorectal cancer
Agree 3
pooled analysis · 2019 · landmark
“Dose-response curves suggested that higher intakes of dietary fibre could confer even greater benefit to protect against cardiovascular diseases, type 2 diabetes, and colorectal and breast cancer.”
Carbohydrate quality and human health: a series of systematic reviews and meta-analyses. Lancet Who: Adults without chronic disease in 185 prospective studies and 58 clinical trials; colorectal cancer incidence from 22 prospective studies
PMID 30638909
trial or observed group · 2012
“After multivariable adjustments, total dietary fibre was inversely associated with colorectal cancer (HR per 10 g/day increase in fibre 0.87, 95% CI: 0.79–0.96).”
Dietary fibre intake and risks of cancers of the colon and rectum in the European prospective investigation into cancer and nutrition (EPIC). PLoS One Who: Adults mostly aged 35 or older recruited 1992-2000 from 23 centres in ten European countries (EPIC)
PMID 22761771
pooled analysis · 2010
“For individuals who consumed an average of 24 g per day of fiber NSP (the highest quintile), the odds of developing colorectal cancer were 30% lower than that for individuals who consumed an average of 10 g per day of fiber NSP (the lowest quintile); for individuals who consumed an average of 18 g per day of fiber NSP as recommended by the UK Department of Health (58), the odds of developing colorectal cancer were approximately 20% lower.”
Dietary fiber and colorectal cancer risk: a nested case-control study using food diaries. J Natl Cancer Inst Who: 579 incident colorectal cancer cases and 1996 matched controls nested within seven UK cohorts (EPIC-Norfolk, EPIC-Oxford, Guernsey Study, NSHD, Oxford Vegetarian Study, UKWCS, Whitehall II)
PMID 20407088
Partly agree 3
other primary study · 2020 · landmark
“The HR for each 5-g/day increment in total fibre was 0.94 (0.86–1.03) and for each 10-g/day increment in alcohol was 1.08 (1.04–1.12).”
Diet and colorectal cancer in UK Biobank: a prospective study. Int J Epidemiol Who: UK Biobank men and women aged 40-69 recruited 2006-2010 without prevalent cancer
PMID 30993317
other primary study · 2015
“Although the association was not statistically significant for colorectal cancer overall (HR: 0.85; 95% CI: 0.70, 1.03; P-trend = 0.10), a reduced risk of distal colon cancer was observed with increased total fiber intake (HR: 0.62; 95% CI: 0.41, 0.94; P-trend = 0.03).”
Dietary fiber intake and risk of colorectal cancer and incident and recurrent adenoma in the Prostate, Lung, Colorectal, and Ovarian Cancer Screening Trial. Am J Clin Nutr Who: US adults aged 55-74 in the screening (intervention) arm of the Prostate, Lung, Colorectal, and Ovarian Cancer Screening Trial
PMID 26269366
trial or observed group · 2006
“Inverse associations were also suggested between colon cancer risk and intakes of protein, fat, vitamin C, and total dietary fiber (trend p for OR2 < 0.10).”
Dietary risk factors for colon and rectal cancers: a comparative case-control study. J Epidemiol Who: Japanese outpatients aged 20-79 at Aichi Cancer Center Hospital (HERPACC), 2001-2004: newly diagnosed colon or rectal cancer cases and cancer-free controls matched for age, sex and year
PMID 16710081
Disagree 4
other primary study · 1999 · landmark
“As compared with women in the lowest decile (median intake, 8.5 g per day), women in the highest decile (median intake, 28.5 g per day) had a relative risk of colorectal cancer of 1.01 (95 percent confidence interval, 0.71 to 1.43).”
Dietary fiber and the risk of colorectal cancer and adenoma in women. N Engl J Med Who: 88,757 US female registered nurses aged 34 to 59 with no history of cancer, inflammatory bowel disease or familial polyposis (Nurses' Health Study)
PMID 9895396
other primary study · 2005
“In conclusion, we did not find support for a linear inverse association between dietary fiber intake and risk of colorectal cancer in a pooled analysis of 13 prospective cohort studies.”
Dietary fiber intake and risk of colorectal cancer: a pooled analysis of prospective cohort studies. JAMA Who: Men and women in 13 prospective cohort studies in North America and Europe (Pooling Project of Prospective Studies of Diet and Cancer)
PMID 16352792
other primary study · 2007
“Conclusions: In this large prospective cohort study, total dietary fiber intake was not associated with colorectal cancer risk, whereas whole-grain consumption was associated with a modest reduced risk.”
Dietary fiber and whole-grain consumption in relation to colorectal cancer in the NIH-AARP Diet and Health Study. Am J Clin Nutr Who: 291,988 men and 197,623 women aged 50-71 from 6 US states and 2 metropolitan areas (NIH-AARP Diet and Health Study)
PMID 17490973
other primary study · 2020
“However, after adjustment for folate, associations for dietary fiber intake attenuated and became null (HRQ5 vs. Q1: 0.96; 95% CI: 0.88, 1.04; P-trend = 0.40).”
Whole grain and dietary fiber intake and risk of colorectal cancer in the NIH-AARP Diet and Health Study cohort. Am J Clin Nutr Who: 478,994 US adults aged 50-71 (285,456 men, 193,538 women) in the NIH-AARP Diet and Health Study
PMID 32619213