Using nearly 10 years of follow-up data from our Singapore Chinese cohort, we observed no association with dietary patterns for colorectal cancer. Our findings contribute to the few prospective studies of dietary patterns and cancer risk among Asian populations (Kim et al, 2005
; Cui et al, 2007
). Our finding of no association with a dietary pattern characterised primarily by vegetable intake is consistent with most prospective findings for similar dietary patterns among Japanese (Kim et al, 2005
), US (Fung et al, 2003
; Wu et al, 2004
), and Western European (Dixon et al, 2004
) populations. However, our finding of no association with our meat-based pattern was not consistent with most previous studies.
Our meat–dim sum pattern was similar to the ‘Western' dietary pattern, previously characterised by red and processed meats, sweets and desserts, French fries, refined grains (Fung et al, 2003
). Positive associations have been reported among most (Fung et al, 2003
; Dixon et al, 2004
; Kesse et al, 2006
; Flood et al, 2008
), but not all (Terry et al, 2001
; Wu et al, 2004
) US and Western European cohorts. Among the Japanese cohort, no association was observed with the Western pattern and colorectal cancer overall, but a positive association with a non-statistically significant trend was observed for colon cancer among women (Kim et al, 2005
Perhaps there is more convincing evidence for a positive association with a Western dietary pattern among non-Asian populations, because the pattern is more strongly correlated with other colorectal cancer risk factors, such as obesity and physical inactivity. Differences in meat cooking methods between Western and Asian populations may be another reason for the discrepancy. For example, in the US consuming grilled meat is a major source of exposure to the colorectal mutagens, heterocyclic amines (Bogen and Keating, 2001
), whereas these are at non-detectable levels among Chinese populations, where stir frying is the preferred meat cooking method (Turesky et al, 2007
These and previous findings from the Singapore Chinese cohort do not support hypotheses that dietary factors, whether as single nutrients, foods, or as dietary patterns, are major contributors to the recent rise in colorectal cancer risk in this population. Individual dietary factors with evidence for a role in colorectal cancer include red meat, preserved red meat, and alcohol as risk factors; and dietary fiber, garlic, dairy products, and calcium as preventive factors (World Cancer Research Fund/American Institute for Cancer Research, 2007
). Of these factors, only alcohol intake was associated with colorectal cancer in our data (Tsong et al, 2007
). Neither alcohol nor the other dietary risk factors that we have identified in our data, such as saturated and marine n-3 polyunsaturated fatty acids (Butler et al, in press
) and green tea (Sun et al, 2007
), are likely to explain the recent colorectal cancer trends among Singapore Chinese population.
As an alternative hypothesis, we propose that the rise in type 2 diabetes mellitus (T2D) prevalence in Singapore (Cheah et al, 1985
; Ministry of Health, 1998
) is a major contributing factor to the parallel rise in colorectal cancer risk. We have reported a modest positive association between T2D and colorectal cancer (Seow et al, 2006
). We have also observed a monotonic increase in risk of developing T2D across the spectrum of body mass index, with a moderate association for those among the second decile (HR=1.70; 95% CI=1.20–2.41, for 18.8–20.2 vs
) up to a HR of 7.80 (95% CI: 5.80–10.48) for the top decile (Odegaard et al, 2006
). Possible underlying biologic mechanisms by which insulin resistance may cause colorectal cancer include stimulating proliferation and reducing apoptosis in colon cells, inducing change in cell signalling pathways, such as protein kinase-C and mitogen-activated protein kinase, and alterering the insulin growth factor system, which is responsible for cell growth and differentiation (reviewed by Gunter and Leitzmann, 2006
The limitations of principal components analysis include the subjective nature of determining the number of patterns, labelling the patterns, and interpreting these patterns (Martinez et al, 1998
). However, we conducted sensitivity analyses and found a high degree of internal consistency and reproducibility with our patterns (Butler et al, 2006
). These limitations are far outweighed by the strengths of our study, including the use of an food frequency questionnaire that was developed for and validated in our population (Hankin et al, 2001
). In addition, the prospective design of our study reduced the opportunity for differential dietary recall to bias our findings.
In conclusion, neither individual nutrients or foods nor dietary patterns appear to be the underlying explanation for the rise in colorectal cancer among Singapore Chinese. Population trends, in addition to a modest association with T2D in our cohort, as well as a strong underlying biologic mechanism, all suggest that factors associated with insulin resistance, such as visceral adiposity and physical inactivity, may be appropriate targets for reducing colorectal cancer incidence in the Singapore Chinese population.