Dietary Nonstarch Polysaccharide Intake and Risk of Colorectal Cancer: Findings from the Singapore Chinese Health Study

Dietary fiber or nonstarch polysaccharides (NSP) may provide protection from colorectal cancer development. Epidemiologic studies on the association between dietary fiber and colorectal cancer is inconsistent are limited on NSP as a modifiable risk factor. Using the Singapore Chinese Health Study, a population-based prospective cohort of 61,321 cancer-free middle-aged or older Chinese Singaporeans, we examined the association between dietary fiber and NSP intakes and colorectal cancer risk. Fiber and NSP intakes at baseline were obtained using a validated semiquantitative food frequency questionnaire coupled with the Singapore Food Composition Database. Cox proportional hazard regression model was used to estimate the HRs and respective 95% confidence intervals (CI) for colorectal cancer associated with dietary fiber and NSP intakes after adjusting for potential confounders. After an average of 17.5 years of follow-up, 2,140 participants developed colorectal cancer. NSP was inversely associated with the risk of colorectal cancer in a dose-dependent manner, whereas dietary fiber was not associated with risk of colorectal cancer overall or histologic subtypes. The multivariable-adjusted HRs (95% CIs) of colorectal cancer for quartiles 2, 3, and 4 of dietary NSP intake were 0.99 (0.88–1.11), 0.98 (0.87–1.11), and 0.84 (0.73–0.95), respectively, compared with the lowest quartile (Ptrend = 0.006). This inverse association was more apparent for colon cancer (HRQ4 vs. Q1 = 0.79, 95% CI: 0.67–0.93, Ptrend = 0.003) than rectal cancer (HR Q4 vs. Q1 = 0.92, 95% CI: 0.74–1.13, Ptrend = 0.53). Our findings suggested that dietary NSP but not fiber is associated with a reduced risk of colon cancer in Chinese Singaporeans. Significance: NSPs may be beneficial for colorectal cancer primary prevention.


Introduction
Colorectal cancer contributes around 10% of annual global cancer-related deaths (1). More than 1.2 million people are diagnosed with colorectal cancer each year with an increasing trend (2). By 2035, new cases of colorectal cancer are projected to reach 2.5 million worldwide (1,3). According to the 2018 data from the GLOBOCAN, colorectal cancer is the third leading cause of cancer type 2 diabetes are established risk factors for colorectal cancer (7). Agents such as aspirin and calcium supplements have been shown to have protective effects against colorectal cancer development (8)(9)(10).
Dietary fiber was introduced by Burkitt (11) as a potential protective agent for colorectal cancer and derived from the low incidence rate of colorectal cancer in certain areas of Africa where the fiber consumption is relatively high. The definition of dietary fiber can be very broad (12). Briefly, dietary fiber is defined as part of the plants that are edible but not digestible in the human small intestine (13). The amount of dietary fiber intake varies in different diets: relatively lower in the western diet and higher in rural African diet (rural African communities may consume up to 7 times more fibers than western diet; ref. (15). As of 2022, several meta-analysis or prospective cohort studies (16)(17)(18)(19)(20)(21)(22)(23)(24)(25), including the Nurses' Health Study (NHS; ref. 17), the Health Professional's Study (HPFS; ref. 18) and the Multiethnic Cohort Study (MEC ref. 23), all in the United States, as well as the European Prospective Investigation into Cancer and Nutrition (EPIC; ref. 26), examined and reported inconsistent associations between dietary intake of fiber and/or NSP and risk of colorectal cancer. In addition, some of these studies found null associations between dietary fiber intake and the risk of developing colorectal cancer (17,18,21) with a few reported some protective effects in restricted demographic subgroups (23,26) or particular foods groups containing fiber (16,19,20). On the other hand, a few studies reported negative associations between dietary NSP intake and the risk of colorectal cancer (24,25). Clinical trials also failed to demonstrate a protective effect of dietary fiber against the development of colorectal cancer (27) or recurrence of colorectal adenomas (28). The observed inverse fiber-colorectal cancer risk association could be attributable to NSP which is derived from similar food sources and highly correlated with fiber. Because there are inconsistent findings in epidemiologic literature regarding fiber and colorectal cancer risk, limited study on NSP as a modifiable risk factor and limited research in Asian populations, additional studies in this particular population with different dietary habits may help discern a differential effect of fiber components on risk of colorectal cancer.
In the current analysis, we examined the association for intake of dietary fiber and NSP with the risk of colorectal cancer in the Singapore Chinese Health Study (SCHS), an ongoing prospective cohort study of more than 63,000 Chinese men and women in Singapore. The study population has unique dietary habits from western populations studied in previous studies.

Study Population
Data used for the current analysis was derived from SCHS. The study design, subject recruitment, and data collection were described in detail previously (29). Briefly, the SCHS is an ongoing prospective cohort study for cancer and other chronic diseases. A total of 63,257 Chinese men and women were enrolled between April 1993 and December 1998 when the subjects were 45-74 years old. The study participants were residents of the Singapore governmentbuilt housing and belonged to either Hokkien or Cantonese dialect groups, which were originated from Fujian and Guangdong provinces, respectively in Southern China. All study participants provided informed consents. All study participants provided written informed consent. The study was conducted in accordance with recognized ethical guidelines (e.g., Declaration of Helsinki, CIOMS, Belmont Report, U.S. Common Rule). The Institutional Review Boards (IRB) of the National University of Singapore and the University of Pittsburgh have continuously approved the SCHS.

Dietary Assessment
Dietary assessment for each participant was completed using a validated semiquantitative food frequency questionnaire (FFQ) including 165 food items or groups that were commonly consumed by Chinese Singaporeans. Each participant was asked to choose from 8 frequency levels of food consumption from the "never or hardly ever" to "two or more times a day". The information on the portion size of each food item was also collected with the assistance of a photo album showing the small, the medium, and the large portion sizes of a given food. The FFQ was validated against a series of 24-hour dietary recall surveys (30) as well as biomarker studies (31,32). The average daily intake of 100 nutrients and non-nutrient compounds for each participant was calculated using the Singapore Food Composition Database and the correlation coefficients among most of the calorie-adjusted nutrients derived from the 24-hour dietary recalls and the FFQ ranged from 0.24 to 0.79 (30).
Three common physical activities were included: (i) strenuous sports (i.e., jogging, cycling on hills, tennis, swimming, tennis, or aerobics); (ii) vigorous work (i.e., moving heavy items, shoveling, or related labor work); and (iii) moderate activities (walking, cycling on flat ground, Tai Chi, and Chi Kung; ref. 33). Alcohol drinking was grouped into non-heavy drinker or heavy drinker, which was defined as 15 drinks/week or more for men and 8 drinks/week or more for women according to the most recent guideline by the US Center for Disease Control and Prevention (CDC; ref. 34). BMI was calculated as weight divided by height in meters and overweight (23-<27.5 kg/m 2 ) or obesity (≥27.5 kg/m 2 ) were defined following the recommendation from the World Health Organization (WHO) for Asian populations (35,36).

Assessment of Dietary Fiber and NSP
The dietary fiber and dietary NSP in this study were derived from their respective contents in commonly consumed food items reported in the FFQ using residual method adjusting for energy intake (37). Overall there were 17 food items commonly consumed by Chinese Singaporeans: (i) red meat; (ii) poultry; (iii) fish and shellfish; (iv) eggs; (v) total legumes; (vi) vegetable, fruits, or related juice; (vii) grain products; (viii) dairy products; (ix) desserts; (x) nuts and seeds; (xi) bread spread; (xii) fats and cooking oil; (xiii) sugar and candy; (xiv) non-dairy beverages; (xv) sauces and condiments; (xvi) flour and baking ingredients; and (xvii) soup stock and canned soup. Major contributors to dietary fiber were grain products (31%), fruits and their related juice (28%), and vegetables and their related juice (21%). Grain products, fruits and their related juice, and vegetables and their related juice products accounted for 24%, 28%, and 24%, respectively, of total insoluble NSP. The corresponding proportion of soluble NSP by these food items were 21%, 41%, and 21% (Supplementary Table S1).

Ascertainment of Colorectal Cancer Cases
The incident cases of cancer and death among the study participants were identified through annual record linkage analyses of all surviving cohort participants with the nationwide Singapore Cancer Registry and the Singapore Registry of Birth and Deaths, respectively. The IRB cases were defined by the International Classification of Disease-Oncology Second Edition (ICD-O-2) C18-C20 (C18 for colon cancer and C19-C20 for rectal cancer; ref. 38). The follow-up for cancer incidence and vital status of the cohort participants has been virtually complete as to date, overall only 56 original participants (<0.1%) were lost to follow-up due to their emigration out of Singapore.

Statistical Analysis
In this analysis, 1,936 participants with a history of cancer at baseline were excluded, leaving 61,321 subjects included in the final analysis. We used the t test We also conducted sensitivity analysis by excluding colorectal cancer cases and person-years observed within the first two years of follow-up postenrollment.
SAS statistical package, version 9.4 (SAS Institute) was used for all statistical analysis. All P values cited are two sided. The P values less than 0.05 were considered statistically significant.

Data Availability Statement
The limited dataset that supports the findings of this study is available on request from the corresponding author. The limited dataset is not publicly available due to privacy or ethical restrictions.

Ethics Statement
All participants of the Singapore Chinese Health Study provided written consent forms. The Singapore Chinese Health Study has been approved by the Institutional Review Boards of the National University of Singapore and the University of Pittsburgh.

Results
Overall, 2,140 colorectal cancer cases (1,355 colon cancer and 785 rectal cancer) were identified among the 61,321 participants who were free of cancer at baseline after a mean (SD) follow-up of 17.5 (5.4) years. The mean (SD) age of participants at the enrollment was 59.5 (8.0) years for colorectal cancer cases and 56.3 (8.0) years for those free of colorectal cancer.
Participants who developed colorectal cancer were older and more likely to be male, ever smokers, heavy drinkers, or to have a family history of CRC (All P's < 0.05; Table 1). Also, participants who developed colorectal cancer, on average, had a lower intake of dietary fiber and NSP compared with those who remained free of colorectal cancer (All P's ≤ 0.01).
Higher intake of dietary fiber or NSP was observed more frequently in females, Cantonese origin, those who obtained secondary or higher education, had more physical activity, history of diabetes, higher energy consumption, but less likely to smoke cigarettes or drink alcohol heavily (Supplementary Table S2 Table S3). Null association for either dietary fiber or NSP with risk of rectal cancer was also found.
In the subgroup analysis by sex, BMI, smoking status, and history of diabetes, a statistically significant inverse association for dietary NSP with colorectal cancer and colon cancer risk was observed only in men, individuals with BMI > 23 kg/m 2 , or those without a history of diabetes (all P trend < 0.05; Table 3). We conducted similar subgroup analysis which did not reveal consistent association for dietary fiber with the risk of colon or rectal cancer or combined (Supplementary Table S4).
In the sensitivity analysis, we excluded colorectal cancer cases and person-years observed within the first two years of follow-up and found similar estimates:

Discussion
In the current analysis of an ongoing prospective cohort study including 61,321 Chinese men and women in Singapore, we showed that a higher intake of NSP was associated with a lower risk of colorectal cancer and colon cancer even after adjustment for dietary fiber.   (cellulose and hemicellulose) provides the laxative effects and the soluble NSP (mixed-link β-glucans) may lower the cholesterol level and maintain healthy glucose and insulin levels (41). We found the inverse association for NSP intake was primarily for colon cancer, but not for rectal cancer. The biology behind the protective effect of NSP is not fully understood, but undigested NSP, when partially fermented by colon bacteria produces short-chain fatty acids (propionate, acetate, and butyrate), and increases the stool weight and decreases the transit time of feces in the colon (24), all thought to be protective against colorectal cancer. On the other hand, dietary fiber was not protective against the development of colorectal cancer, which is consistent with the null results by the clinical trial studies (43)(44)(45)28).
As a subtype of dietary fiber, NSP is the fraction of carbohydrates excluding starch and free sugar molecules (46,47). The other types of dietary fiber, including resistant starch and oligosaccharides, may provide other health benefits such as reducing the risk of type 2 diabetes (48), but may not offer a protective effect for colorectal cancer. This differential protective effect may be due to the sources of the food since NSPs are mainly present in primary or secondary plant cell walls (49). Surrounded by growing plant cells, primary cell walls consist of polysaccharide and structural proteins while secondary cell walls, in which mature plant cells are surrounded, contain lignin and cellulose in addition to polysaccharide and structural proteins (50). However, foods contain different types of fiber or nutrients, which is difficult to separate out (51). Future studies are warranted to determine the biological mechanisms for the differential effects of dietary NSP intake on colorectal cancer prevention.
The present study had several strengths. This was the first prospective study examining the association between dietary fiber or NSP and the risk of colorectal cancer in an Asian population. As a prospective study, information on dietary, supplement, and related lifestyle factors were collected prior to diagnosis of colorectal cancer, minimizing the potential impact of colorectal cancer diagnosis on dietary habits or lifestyle factors. We were able to comprehensively evaluate both dietary fiber and dietary NSP while monitoring for potential contributing factors. Sufficient statistical power is provided by the large sample size with long-term follow-up. Our study also had several limitations. The FFQ at baseline only captured a onetime assessment of dietary pattern among participants while the dietary pattern can change over time. Changes in the dietary pattern over time after the baseline dietary assessment might result in non-differential misclassification of both colorectal cancer cases and noncases, which could lead to the underestimation of the dietary exposure-colorectal cancer risk association toward the null (52).
In addition, endoscopic screening information was not collected in the SCHS, although there was no national colorectal cancer screening program in place when the cohort was assembled. The FFQ, including fiber component was validated by a series of 24-hour food recalls. Although we do not have specific biomarkers for NSP, it is however noted that NSP was highly correlated with fiber in our study population (r = 0.92; ref. 30). With multiple lifestyle factors examined, residual confounding may persist. In addition, there were small percentage of participants used calcium supplements (3.4% at baseline) or calcium use (5.06% at follow-up 1), therefore, assessing the potential confounding effect by calcium supplement use or aspirin use is not applicable.
In summary, this is the first prospective study that examined simultaneous dietary fiber and NSP with the risk of colorectal cancer in an Asian population and revealed that dietary NSP rather than fiber was statistically significantly associated with a lower risk of colorectal cancer. The potential protective effect of NSP, independent of dietary fiber, on colorectal cancer development warrants further study.