The prevalence of excessive weight and obesity worldwide has been increasing markedly over the past decades . Although the prevalence of obesity remains low in many Asian countries compared with Western countries, many Asian countries are experiencing a dramatic rise in the incidence of obesity . Excessive weight and obesity are associated with higher risk of many cancers, including colorectal cancer; however, few studies have focused on the association between obesity and outcomes of cancer patients. Moreover, in Asian populations, only two published studies have examined the effect of obesity on prognosis in patients with colorectal cancer [14, 18].
The results of the limited number of studies regarding the influence of BMI on the outcomes of patients with colorectal cancer are often unclear, even if conducted in the same population. For instance, in a study that comprised a total of 509 Korean colorectal cancer patients, excessive weight had a favorable influence on OS compared with the results obtained for normal-weight participants (P = 0.001), whereas there was no significant difference in PFS (P = 0.735) . Contrary to this finding, Moon et al. reported that there was no association between BMI and OS (P = 0.210), but the overweight group showed a borderline decrease in disease-free survival compared with normal-weight participants (P = 0.064) . These discrepancies may result from the difference of BMI category between the two study populations, aside from the disparity in sample sizes. In the first study, the participants were divided into normal-weight and overweight groups based on BMI of 23 kg/m2, which is the criterion set for Asian populations. In the latter study, the participants were divided based on a BMI of 25 kg/m2 according to WHO criteria for excessive weight. In our study, we divided 525 Chinese participants based on BMI above or below 23 kg/m2 according to the WHO classification for Asian populations. BMI had no influence on OS or PFS of patients who underwent surgery for colorectal cancer, which is inconsistent with the two previous studies.
The influence of BMI on the prognosis of patients with colorectal cancer is not clear, with the few studies that have addressed this question showing somewhat disparate findings. Dignam et al. observed that BMI greater than 35 kg/m2 at diagnosis was associated with an increased risk for recurrence and death from colon cancer . In the study by Sinicrope et al., obesity was further categorized as class 1 (BMI = 30 to 34.9 kg/m2) and class 2/3 (BMI ≥35 kg/m2) . In the class 2/3 obesity group, patients had worse PFS and OS rates compared with normal-weight patients. Moreover, patients in this group showed a trend toward worse PFS in multivariate analysis. One possible explanation for the difference between Western studies and ours relates to the inclusion of more obese participants in Western cohorts. Nevertheless, the Irish study showed no significant difference between obese and nonobese cohorts regarding survival, which is consistent with our results.
In our study, we found that both past and current smokers had higher rates of mortality (OS) than patients who had never smoked. In multivariate analysis, patients who had smoked had a 10% increase in the risk of death after surgery compared with the nonsmoking cohort. In contrast to our study, the US population-based Cancer Prevention Study II found that men and women who smoked cigarettes for 20 or more years experienced higher colorectal cancer death rates, adjusted for multiple potential confounders . However, Colangelo et al. reported that the association between cigarette smoking and colorectal cancer mortality was stronger in the younger age group (age <50 years) than in the older age group . In this study, we did not further stratify the patients by smoking status, because it was recorded only at baseline.