Contribution of behavioral risk factors and obesity to socioeconomic differences in colorectal cancer incidence

Abstract

Background: Health behaviors are known risk factors for colorectal cancer and are more common in low socioeconomic status (SES) populations. We evaluated the extent to which behavioral risk factors and body mass index (BMI) explain SES disparities in colorectal cancer incidence, overall and by tumor location.

Methods: We analyzed prospective National Institutes of Health-AARP Diet and Health Study data on 506 488 participants who were recruited in 1995-1996 from six US states and two metropolitan areas and followed through 2006. Detailed baseline data on risk factors for colorectal cancer, including health behaviors, were obtained using questionnaires. SES was measured by self-reported education and census-tract data. The outcome was primary incident invasive colorectal adenocarcinoma. Poisson regression was used to estimate the association between SES and risk of incident colorectal cancer, with adjustment for age, sex, race and ethnicity, family history, and state of residence. The model estimates were used to derive percentage mediation by behavioral risk factors; bias-corrected 95% confidence intervals were obtained through bootstrap techniques.

Results: Seven-thousand six-hundred seventy-six participants developed colorectal cancer during follow-up. SES differences in prevalence of physical inactivity, unhealthy diet, smoking, and unhealthy weight each explained between 11.3% (BMI) and 21.6% (diet) of the association between education and risk of colorectal cancer and between 8.6% (smoking) and 15.3% (diet) of the association between neighborhood SES and risk of colorectal cancer. Health behaviors and BMI combined explained approximately 43.9% (95% CI = 35.1% to 57.9%) of the association of education and 36.2% (95% CI = 28.0% to 51.2%) of the association of neighborhood SES with risk of colorectal cancer. The percentage explained by all factors and BMI combined was largest for right colon cancers and smallest for rectal cancers.

Conclusion: A substantial proportion of the socioeconomic disparity in risk of new-onset colorectal cancer, and particularly of right colon cancers, may be attributable to the higher prevalence of adverse health behaviors in low-SES populations.

Figures

Figure 1.

Conceptual model of the relationships among socioeconomic factors, health behaviors, and colorectal cancer risk. Neighborhood socioeconomic status refers to the racial composition and socioeconomic context within a given unit of aggregation for area-level socioeconomic measures. The health behaviors examined are physical activity, dietary pattern, and smoking. This grouping does not imply a causal relationship between smoking and obesity.

Figure 2.

Associations between socioeconomic measures and colorectal cancer incidence according to tumor location before and after adjustment for behavioral factors and BMI: NIH-AARP Study, 1995–2006. A) Association of educational level with colorectal cancer incidence. The top panel shows incidence rate ratios (IRR) and 95% confidence intervals (CIs) for colorectal cancer risk by tumor location for persons with varying levels of education (PG = postgraduate, Col = college degree, SC = some college, >HS = more than high school, HS = high school diploma) before adjustment, and the bottom panel shows the same associations after adjustment for health behaviors (Mediterranean diet score, physical activity, and smoking) and body mass index. B) Association of neighborhood SES with colorectal cancer incidence. The top panel shows incidence rate ratios (IRR) and 95% confidence intervals (CIs) for colorectal cancer risk by tumor location for persons in varying levels of neighborhood deprivation (by quintile: Q1, least deprived to Q5, most deprived). The vertical lines represent an IRR of 1.0 (referent). All Wald χ2 two-sided P values for trend are less than .01 except those marked by an asterisk (* means ≤.05) or a dagger († means >.05). Model 1 was adjusted for age, sex, race and ethnicity, family history of colorectal cancer in a first-degree relative, and state of residence, which were entered as categorical variables in the models except age (continuous, measured from the baseline date).