In this large population of generally low-income black and white adults, overall we found that BMI was not associated with CRC screening among whites, and, contrary to our initial hypothesis, that increased body size was associated with small increases in screening odds among black men and women. Previous reports examining CRC screening in relation to body size have been inconsistent. Using 2001 Behavioral Risk Factor Surveillance System (BRFSS) data, obese women were found to be less likely to have received a sigmoidoscopy within the past five years while obese males were more likely to have done so (17
)while among the 130,000 individuals (97% white) in the Cancer Prevention Study (CPS) II Nutrition Cohort, lower odds of ever having a colonoscopy or sigmoidoscopy were found among overweight and obese males and females (18
). Another study reported that overweight and obese males and females were both slightly more likely to have received CRC screening compared to their healthy weight counterparts(19
), and two found no association between CRC screening and overweight or obesity (20
). None of these studies presented results specifically among African-Americans. Two relatively small studies have focused solely on African American populations with one reporting non-statistically significant increases in CRC screening among obese African Americans (22
), and the other reporting decreased CRC screening among obese black women but not obese black men (12
Studies examining associations between obesity and CRC screening separately by both race and gender are rare, a gap in the literature that this report uniquely fills. In the few studies that have presented separate results by race and gender, it appears that white women may be the most susceptible to lower screening in relation to larger body size. Using 2005 National Health Interview Study (NHIS) data for 4,430 white and 690 black women, Leone et al. observed lower odds of colonoscopy among obese white women (OR for BMI ≥ 30 v. < 30 = 0.66 [0.50–0.85]) and higher odds among obese black women (OR=1.30 [0.83–2.96] (23
). With our much larger population of black and white women in the SCCS, we classified women into obese versus non-obese categories, as well as finer categories of BMI. When we grouped women by dichotomous obesity (BMI ≥ 30 versus 18.5–29.9 kg/m2
), the OR of 1.05 [0.97–1.14] for obese black women was similar to the OR for of 1.06 [0.96–1.18] for obese white women. However, comparing BMI ≥ 40 vs 18.5–24.9 kg/m2
, the OR were 1.13 [0.98–1.29] for black and 0.99 [0.83–1.19] for white women. In addition, among the subset of white women with income >$25,000, the OR for BMI ≥ 40 vs 18.5–24.9 kg/m2
was 0.66 [0.44–1.00], while no parallel decline was seen among black women (OR=1.05 [0.73–1.49]). Hence both the SCCS and NHIS data suggest that racial differences may exist, with lower CRC screening associated with obesity only among moderate to upper income white women.
These differences by race may reflect issues related to negative body image, an issue that has been shown to be more pervasive among white women than black women (24
). If white women are more self-conscious about their weight, this may prevent them from receiving certain medical procedures, particularly those that are more invasive such as sigmoidoscopy or colonoscopy. Reasons for the observed difference in screening rates between obese and non-obese individuals in higher but not lower-income women are unclear. It may be that among women of the lowest means, simply meeting the basic needs of food, shelter, and medical care when absolutely necessary is the sole focus while body size is a stronger influence among women with greater means because basic needs have been met. This is an area deserving of further inquiry.
The overall differences observed between the NHIS and CPS populations and the SCCS could likely be related to SES differences. The CPS study population is of higher educational attainment and has more health-conscious behaviors than the general population, and the NHIS is designed to be nationally representative. In contrast, the SCCS has a much higher proportion of low income, low educational attainment, and uninsured individuals than either the NHIS or CPS.
Among black men, we observed higher CRC screening rates among overweight and obese SCCS participants. It has been reported that overweight and obese individuals utilize health care more often than those of healthy weight and thus have more opportunities to discuss cancer screening with their physician (25
). It is also possible that medical practitioners are recommending CRC screening more often to patients who are overweight because excess weight is a known risk factor for CRC. However, we did not observe increased CRC screening among overweight white participants and there is not reason to expect that increased medical visits or screening recommendations for obese individuals would be limited to blacks.
One limitation of this analysis was that we were able only to examine CRC screening by sigmoidoscopy or colonoscopy as information on fecal occult blood testing (FOBT) or barium enema was not obtained as part of the SCCS baseline survey. There is some evidence to indicate that FOBT is used more commonly among individuals of low-income, low education, and without health insurance (26
); notably, though, the proportion of screening exams done by FOBT is relatively low and has been dropping in recent years (26
). In studies that have examined endoscopy alone versus FOBT and endoscopy together in relation to BMI, associations have generally been similar for the two screening measures; ORs for endoscopy alone versus FOBT or endoscopy by categories of BMI were very similar in an analysis of 2000 NHIS data (21
)as were adjusted CRC screening rates using BRFSS data (27
). In contrast, using 2005 NHIS data, Leone et al. reported an association between obesity and colonoscopy but not FOBT (23
). On balance, the relatively low proportion of CRC screening exams done by FOBT coupled with multiple reports that showed similar associations between obesity and CRC screening regardless of screening modality indicate that the results we found for endoscopy are likely reflective of patterns for all CRC screening.
A further limitation is that both the use of sigmoidoscopy and colonoscopy as well as body size were obtained by self-report. However, self-reported endoscopy by either sigmoidoscopy or colonoscopy had high sensitivity (>96%) and specificity (>92%) when compared with health plan records in at least one study (28
). As for self-reported body size, while are cent review indicates that among women, height tends to be over-reported and weight under-reported (29
), data from the 1999–2004 National Health and Examination Survey show that despite errors in self-report, BMI categories based on self-reported values still generally demonstrate good agreement with BMI categories from measured values (30
). Additionally, within the SCCS, BMI values calculated from self-reported height and weight were very highly correlated with BMI values calculated from medical record data overall (Pearson correlation coefficient > 0.95) as well as across strata of race and BMI, indicating that the self-reported values are generally of good quality.
Finally, it should be noted that the SCCS population, due to its unique recruitment through Community Health Centers and the resulting high proportion of low income individuals, is not reflective of the socioeconomic or race distributions of the entire US population. Thus, differences in effects compared with other studies, particular those that are nationally representative such as the NHS, may be due to the socioeconomic status of the underlying study populations. However, this study design does not detract from our ability to make within-cohort comparisons between race groups, and is in fact a major strength of this study because residual confounding due to differences in socioeconomic status between race groups is minimized by design, and additionally because low-income individuals have traditionally been understudied.
In summary, in this large population of black and white adults, being overweight or obese does not appear to play a deterring role in the receipt of CRC screening in any race or sex group, except perhaps among the subset of upper income white women. Given that only half of adults over the age of 50 are currently being screened for colorectal cancer, efforts to increase screening rates in all individuals should remain the focus of public health initiatives.