Colorectal cancer risk factors

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] ; Associate Editor(s)-in-Chief: Fahad Hasan, M.D.[2] Saarah T. Alkhairy, M.D.

Overview

Colorectal cancer (CRC) risk reflects hereditary, personal/medical, and modifiable lifestyle factors. About 70% of cases are sporadic. Roughly 20–25% show familial clustering without an identified syndrome. Approximately 5–10% arise from defined hereditary syndromes, chiefly Lynch syndrome and familial adenomatous polyposis (FAP). Lynch syndrome and classic FAP carry the highest individual lifetime risk: 15–52% and nearly 100%, respectively. Modifiable exposures account for most population-attributable risk. These include excess body fatness, alcohol, red and processed meat, smoking, sedentary behavior, and low fiber or whole-grain intake. They are also increasingly implicated in the rising incidence of early-onset colorectal cancer (EOCRC, diagnosis before age 50). This chapter covers risk stratification only. Diagnostic criteria, genetic testing algorithms, and surveillance protocols for FAP and Lynch syndrome are covered in their own microchapters.

Summary of Risk Factors

Risk factor	Type	Approximate magnitude	Strength of evidence
Lynch syndrome (MLH1, MSH2, MSH6, PMS2, EPCAM)	Hereditary, nonmodifiable	Lifetime CRC risk 15–52%, varies by gene	Established
Classic familial adenomatous polyposis (APC)	Hereditary, nonmodifiable	Near 100% by middle age if untreated	Established
Long-standing ulcerative colitis or Crohn's colitis	Medical	About 2–3-fold overall; cumulative risk about 1%, 3%, and 7% at 10, 20, and 30 years	Established
IBD with concomitant primary sclerosing cholangitis	Medical	More than 3-fold higher odds of CRC/dysplasia than IBD without PSC	Established
Family history of CRC in a first-degree relative	Nonmodifiable	Increased; risk rises with more affected relatives and with younger age at their diagnosis	Established
Personal history of advanced adenoma or CRC	Medical	Increased metachronous neoplasia risk	Established
Higher body fatness (BMI, waist circumference, waist-to-hip ratio)	Modifiable	Strong evidence of increased risk; EOCRC adjusted OR 1.93	Strong (convincing/probable)
Alcohol; red and processed meat	Modifiable	Increased risk	Strong (convincing/probable)
Greater adult-attained height	Nonmodifiable	Increased risk	Strong (convincing/probable)
Cigarette smoking	Modifiable	Increased risk	Established
Sugar-sweetened beverages	Modifiable	EOCRC RR 2.18 for ≥2 servings/day versus <1 serving/week	Emerging
Diabetes mellitus or insulin resistance	Medical	Associated with increased risk; EOCRC data inconsistent	Probable
Antibiotic exposure, microbiome alteration, early-life or in-utero exposures	Environmental	Not quantified	Hypothesized, not validated

Hereditary and Genetic Risk Factors

• Lynch syndrome (hereditary nonpolyposis colorectal cancer) is the most common hereditary CRC syndrome. It accounts for about 3% of all CRC and is enriched 2–3-fold among early-onset cases.
  • It is caused by autosomal dominant germline pathogenic variants in the mismatch repair genes MLH1, MSH2, MSH6, or PMS2, or by EPCAM deletions. The result is microsatellite instability.
  • Median age at CRC diagnosis is 45–60 years, compared with about 67 years for sporadic CRC. Extracolonic cancer risk is also increased (endometrial, ovarian, gastric, urothelial).
• Familial adenomatous polyposis is caused by autosomal dominant germline APC mutations and affects about 1 in 10,000 people. Classic FAP confers near 100% CRC risk by middle age without colectomy. Attenuated FAP (AFAP) has fewer polyps and later onset.
• Other polyposis and predisposition syndromes collectively account for 1–2% of CRC. These include:
  • MUTYH-associated polyposis (biallelic, autosomal recessive)
  • Polymerase proofreading-associated polyposis (POLE/POLD1)
  • Serrated polyposis syndrome (RNF43)
  • Pathogenic variants in SMAD4, BRCA1/2, ATM, and PALB2
• Age at onset is the strongest clinical clue to hereditary disease. About 1 in 5 patients with EOCRC carries a pathogenic cancer-susceptibility variant.

Personal and Medical History

• Family history: Risk rises with the number of affected first-degree relatives and with younger age at their diagnosis. Risk is highest with multiple affected relatives or a relative diagnosed before age 50–60, especially with advanced adenomas.
• Personal history of adenomas or CRC: Multiple adenomas, adenomas ≥1 cm, or villous/tubulovillous histology increase metachronous risk.
• Inflammatory bowel disease:
  • Long-standing ulcerative colitis and Crohn's colitis increase CRC risk about 2–3-fold overall. Limited proctitis is excluded. The mechanisms are chronic inflammation and field cancerization.
  • Contemporary population-based cumulative risk is about 1%, 3%, and 7% at 10, 20, and 30 years of disease. Older estimates were 2%, 8%, and 18%. The decline is attributed to better medical therapy and surveillance colonoscopy.
  • Risk increases with greater anatomic extent, longer duration, family history of CRC, and concomitant primary sclerosing cholangitis (PSC). PSC more than triples CRC/dysplasia odds.
  • Pediatric-onset IBD raises the overall cancer rate about 2-fold in Crohn's disease and about 2.5-fold in ulcerative colitis. This excess is driven mainly by gastrointestinal cancers.
• Other conditions associated with increased risk:
  • Abdominal or pelvic radiation, including in childhood cancer survivors
  • Acromegaly, which carries an increased prevalence of adenomatous polyps
  • Immunosuppression, including solid organ transplant recipients
  • Ureterosigmoidostomy, with neoplasia near the ureterocolic anastomosis

Modifiable and Lifestyle Risk Factors

• The WCRF/AICR Third Expert Report graded the following as strong evidence (convincing or probable) of increased risk:
  • Higher body fatness
  • Greater adult-attained height
  • Alcohol
  • Red and processed meat
• The same report graded the following as protective:
  • Physical activity
  • Whole grains and dietary fiber
  • Dairy products and calcium supplements
• Cigarette smoking is an independent risk factor.
• Sugar-sweetened beverages are an emerging risk factor, including for early-onset disease.
• Migration studies support a causal role for environment over genetic background. CRC risk among immigrants from non-Western countries rises toward host-country rates with longer residence, and dietary Westernization can raise risk within one generation.
• Diabetes mellitus and insulin resistance are associated with increased risk. The plausible mechanism is hyperinsulinemia acting as a growth factor for colonic mucosa.
• Clustering of behaviors: Combinations of unhealthy behaviors carry about 2.3–2.9-fold higher CRC risk than the lowest-risk profile. Examples are smoking plus heavy alcohol use plus inactivity, or obesity plus poor diet. Higher physical activity appears to partially offset other adverse exposures.

Early-Onset Colorectal Cancer

EOCRC is a distinct epidemiologic entity with rising global incidence. Most cases are sporadic, but about 15–20% carry an identifiable hereditary syndrome, chiefly Lynch syndrome. This proportion is higher than in later-onset disease. Risk factors largely mirror later-onset CRC, with distinct signals from large cohorts:

• Obesity: adjusted OR 1.93 (95% CI 1.15–3.25) in the Nurses' Health Study II.
• Western dietary pattern (processed/red meat, butter, high-fat dairy, refined grains): OR 1.67 (95% CI 1.18–2.37) for early-onset high-risk adenomas in the highest versus lowest quintile.
• Sugar-sweetened beverages: RR 2.18 (95% CI 1.10–4.35) for ≥2 servings/day versus <1 serving/week.
• Sedentary behavior: prolonged television viewing is associated with increased EOCRC risk, especially rectal cancer.
• Alcohol and tobacco: established risk factors for EOCRC and advanced colorectal neoplasia.
• Vitamin D: total intake ≥450 IU/day is associated with lower risk (HR 0.49, 95% CI 0.26–0.83). This is observational data.
• Antibiotic exposure, gut microbiome alteration, and early-life or in-utero exposures are hypothesized contributors. They are not validated for clinical risk stratification.

Areas of Uncertainty

• Obesity and diabetes in EOCRC: Large prospective cohorts show about 2-fold risk. Some case-control studies using BMI measured near diagnosis find no association, likely reflecting reverse causation.
• IBD-associated risk by ethnicity or region: Unresolved, and confounded by unequal access to surveillance and therapy.
• Isolated lifestyle risk factors: Major guidelines do not define a separate risk-adapted screening start age for patients with obesity or heavy alcohol use who lack a family history or hereditary syndrome.

Clinical Pearls and Pitfalls

• The younger the age at CRC diagnosis, the higher the pretest probability of a hereditary syndrome. Consider germline testing in patients diagnosed before age 50, even without a strong family history.
• A negative family history does not exclude Lynch syndrome or FAP. De novo mutations and incomplete family histories can mask both.
• Do not counsel IBD patients with outdated figures (e.g., 18–30% by 30 years of pancolitis). These overstate risk relative to surveillance-era data.
• Do not overlook concomitant PSC when stratifying IBD patients for surveillance intensity.
• Limited ulcerative proctitis does not carry the elevated CRC risk of more extensive colitis.
• Rectal bleeding, iron deficiency anemia, or a change in bowel habits in adults under 50 warrants a low threshold for colonoscopy.