September 5, 2012 — People with a relatively low socioeconomic status account for a disproportionate number of colorectal cancers in the United States. Now, for the first time, a large prospective, observational study has shed light on the degree to which behavior and body mass contribute to this disparity.
Over one third of the excess risk…could be explained by differences in…behavioral risk factors.
“This study showed that over one third of the excess risk of invasive adenocarcinoma of the colon and rectum resulting from low [socioeconomic status] could be explained by differences in…behavioral risk factors, particularly in an unhealthy diet,” conclude the authors, led by Chyke A. Doubeni, MD, MPH, from the Department of Family Medicine and Community Health at the University of Pennsylvania Perelman School of Medicine in Philadelphia.
In addition to diet, Dr. Doubeni and colleagues found that physical inactivity, smoking, and being overweight are likely contributors to this risk.
In their study, published online September 5 in the Journal of the National Cancer Institute, the authors looked at health behaviors, obesity, and colorectal cancer risk among Americans of all socioeconomic statuses.
They used the National Institutes of Health-AARP Diet and Health Study as their data source. Specifically, they looked at middle-aged and elderly people from 6 states (California, Florida, Louisiana, New Jersey, North Carolina, and Pennsylvania) and 2 metropolitan areas (Atlanta, Georgia and Detroit, Michigan). All of the participants enrolled in the study in 1995/96 and were followed through 2006. Health behaviors of the participants were determined using questionnaires.
Of the 506,488 study participants, 7676 developed colorectal cancer during the 10-year follow-up period.
How Class and Behavior/Body Mass Are Related
The authors evaluated the socioeconomic status of the participants in 2 ways: by census-tract data, which revealed “neighborhood socioeconomic status,” and by self-reported educational level (less than high school vs high school and more than high school).
On the basis of data from other studies on colorectal cancer and behavior, Dr. Doubeni and his team used statistical modeling to estimate the likely percentage of colorectal cancers mediated by behavioral risk factors.
They found that differences in socioeconomic status in the reported levels of physical inactivity, unhealthy diet, smoking, and unhealthy weight each explained between 11.3% and 21.6% of the association between education and risk for colorectal cancer, and between 8.6% and 15.3% of the association between neighborhood status and risk for colorectal cancer. Diet was found to have the biggest impact of all the health behaviors.
Overall, the combination of health behaviors and body mass index (BMI) explained approximately 43.9% (95% confidence interval [CI], 35.1% to 57.9%) of the association between risk for colorectal cancer and education and 36.2% (95% CI, 28.0% to 51.2%) of the association between the risk and neighborhood socioeconomic status.
In short, somewhere between one third and nearly one half of colorectal cancers among either low-income or less-than-high-school-educated Americans might be attributable to obesity and unhealthy behaviors.
However, a pair of experts not involved with the study do not find these results to be a cause for despair.
Instead, the study “demonstrates the intricate interplay” of socioeconomic and behavioral factors affecting colorectal cancer risk, write John Z. Ayanian, MD, and John M. Carethers, MD, in an accompanying editorial. Dr. Ayanian is from the Department of Health Care Policy at Harvard Medical School in Boston, Massachusetts, and Dr. Carethers is from the Department of Internal Medicine at the University of Michigan in Ann Arbor.
Public health practitioners can learn from these results, they believe. The study “underscores the need for more effective public health strategies to improve nutrition and physical activity in the United States and thereby curb the rising tide of obesity, particularly for those with less education and in disadvantaged communities,” the editorialists write.
Colon Cancer by Location
The study accounted for the anatomic location of the participants’ cancers (proximal colon, distal colon, or rectum), which resulted in one of the study’s “key findings,” according to the editorialists.
The health behaviors and BMI explained 95% of the association between education and the incidence of proximal colon cancer, but only 38% of the association between education and distal cancer and 24% of that between education and rectal cancer, Dr. Ayanian and Dr. Carethers point out.
That is a dramatic difference, they note. However, the editorialists think that these contrasting results for proximal and more distal cancers might “reflect the impact of an important omitted variable — colorectal cancer screening by socioeconomic status.”
Colorectal cancer screening has been shown to be more effective in reducing cancer incidence and mortality in the distal colon and rectum than in the proximal colon, the editorialists explain. Thus, this finding might have an easy explanation, they note.
“Because adults who are less educated and from less affluent communities are less likely to be screened, the greater effectiveness of screening for distal colorectal cancer may explain why socioeconomic gradients were much steeper for these anatomic sites than for proximal cancer,” they write.
The study was funded in part by the National Cancer Institute. The study authors and editorialists have disclosed no relevant financial relationships.
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- Lifestyle and Reducing Your Risk of Bowel Cancer (familyhistorybowelcancer.wordpress.com)