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Cigarettes and alcohol in relation to colorectal cancer within the Singapore Chinese Health Study
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Content
CIGARETTES AND ALCOHOL IN RELATION TO COLORECTAL
CANCER WITHIN THE SINGAPORE CHINESE HEALTH STUDY
by
Wan Hua Tsong
A Thesis Presented to the
FACULTY OF THE GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
MASTER OF SCIENCE
(APPLIED BIOSTATISTICS AND EPIDEMIOLOGY)
May 2005
Copyright 2005 Wan Hua Tsong
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
UMI Number: 1427959
Copyright 2005 by
Tsong, Wan Hua
All rights reserved.
INFORMATION TO USERS
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®
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ACKNOWLEDGEMENTS
The expertise of Dr. Mimi Yu, Dr. Sue Ingles, and Dr. Mariana Stem
has been invaluable. The advice of Dr. Jianmin Yuan, Dr. Canlan Sun, Dr.
Manuela Gago-Dominguez, and Dr. Esteban Castelao has been helpful. The
assistance o f Donna C. Murdock, Xuejuan Jiang, and Eunjung Lee has been
helpful. The work of Dr. Mimi Yu, Dr. Canlan Sim, Dr. Woon-Puay Koh,
Kazuko Arakawa, and several others who have contributed to the creation
and management o f the Singapore Chinese Health Study were crucial to the
completion of this thesis.
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TABLE OF CONTENTS
ACKNOWLEDGEMENTS ii
LIST OF TABLES iv
ABSTRACT V
INTRODUCTION 1
METHODS 5
RESULTS 9
DISCUSSION 20
CONCLUSION 26
BIBLIOGRAPHY 27
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iv
LIST OF TABLES
Table 1. Distribution of selected characteristics by level of 10
drinking and smoking, Singapore Chinese Health Study, 1993-
2002.
Table 2. Cigarette smoking in relation to colorectal cancer 11
risk, Singapore Chinese Health Study, 1993-2002.
Table 3. Duration and amount of smoking in relation to 14
colorectal cancer, Singapore Chinese Health Study, 1993-2002.
Table 4. Alcohol consumption in relation to colorectal 16
cancer, Singapore Chinese Health Study, 1993-2002.
Table 5. Beverage type in relation to colorectal cancer, 17
Singapore Chinese Health Study, 1993-2002.
Table 6. Alcohol and smoking in relation to colorectal 18
cancer, Singapore Chinese Health Study, 1993-2002.
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ABSTRACT
V
The association of cigarette smoking and alcohol consumption with
colorectal cancer was assessed using the Singapore Chinese Health Study.
This study involves a population-based, prospective cohort of over 60,000
Singapore Chinese. After an average of 7 years of follow-up, 637 incident
cases of colon (n=392) and rectal (n=245) cancer had developed. Cigarette
smoking was positively associated with rectal but not colon cancer risk.
Alcohol consumption was positively associated with both colon and rectal
cancer risk. The joint effect of heavy cigarette smoking and daily alcohol
consumption was associated with a statistically significant 5.3 fold increase
in rectal cancer risk.
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1
INTRODUCTION
Over the past three decades, the incidence of colorectal cancer among
the Singapore Chinese has steadily increased to surpass the incidence among
United States Caucasians (International Agency for Cancer Research, vol. 4-
8). During the period from 1973 to 1977, annual age-standardized colorectal
cancer incidence rates for Singapore Chinese men and women were 29 and
21 per 100,000, respectively (International Agency for Cancer Research, vol.
4). By the period from 1993 to 1997, these same rates for men and women
had risen to 44 and 31 per 100,000, respectively (International Agency for
Cancer Research, vol. 8).
Cigarette smoking and alcohol consumption are suspected risk factors
for colorectal cancer. However, due to the high correlation between smoking
and drinking, one exposure may act as a confounder for the other and the
joint exposure may be a more relevant measure o f risk than the separate
exposures adjusted for each other. A meta-analysis of 25 cohort-based
studies on cigarette smoking and colorectal cancer indicates that smoking
increases the risk of developing colorectal cancer by 34% (23%-45%)
(Chyou 279; Engeland 503; Heineman 730; Kato 278; Klatsky 1010; Knekt
137; Limburg 205; Nordlund 627; Nyren 1304; Olsen 401; Sandler 1331;
Sanjoaquin 119; Shimizu 1042; Singh 765; Sturmer 1179; Terry 481; Terry
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2
587; Tiemersma 389; Tulinius 868; Van der Hel 297; Wakai 326; Wei 440;
Wu 689; Yamada 783). However, only eleven of these studies adjusted for
alcohol consumption; and among these eleven studies, residual confounding
can not be ruled out as a source of bias given the high correlation between
smoking and drinking. Only two studies (Otani 1497; Murata 561) have
investigated smoking among nondrinkers. However, both studies have failed
to reach statistical significance for these estimates. It is possible that this
failure could be due to the use of the smoking status variable which does not
take into account the effects of dosage and duration.
A meta-analysis of 18 prospective studies on alcohol and colorectal
cancer indicates that drinking alcohol increases the risk of colorectal cancer
by 47% (30%-67%) (Chen 1304; Cho 607; Chyou 280; Flood 556; Glynn
219; Goldbohm 100-101; Hamack 154-155; Klatsky 1010; Olsen 400; Otani
1496; Pedersen 863; Sanjoaquin 119; Shimizu 1041; Singh 765; Su 70; Wei
440; Wu 689; Yamada 783). Fourteen of these studies have investigated
alcohol consumption adjusted for smoking; however, residual confounding
cannot be ruled out as a source of bias for these studies given the high
correlation between smoking and drinking. Only two studies (Cho 610;
Otani 1497) investigated alcohol consumption among never smokers and
their results are inconsistent. One study (Cho 610) failed to find a
statistically significant association between alcohol and colorectal cancer
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3
among never smokers. The other study (Otani 1497) did find a statistically
significant positive association between alcohol consumption and colorectal
cancer. However, this association was for drinking 300 or more grams per
week (about 3.6 drinks per day). Further investigation is necessary to
determine whether there is a risk o f colorectal cancer for lower levels of
drinking among never smokers.
There are just two studies (Murata 561; Otani 1497) that have
investigated the joint effect of cigarettes and alcohol. These studies have
already been mentioned above in the discussion of cigarette smoking.
Although both studies find statistically significant associations for the joint
effect [relative risk (RR) ~ 3 for both], this effect is for current smokers. It
may be possible that the estimate is higher when the effects o f smoking
dosage and duration are taken into account.
The Singapore Chinese Health Study is a population-based cohort
study of diet and cancer, initiated in 1993. This study is the first non-
Japanese, cohort-based study to present both the separate and joint effects of
cigarettes and alcohol in relation to colorectal cancer. Although Japan and
Singapore are both Asian nations, the levels of smoking and drinking in
Japan are very different from those in Singapore. From 1998 to 1999, the
prevalence of smoking in Japan was 33% (Mackay 96). In 1999, the
prevalence of smoking in Singapore was 15% (Mackay 100). From 1961 to
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4
2000, the level of per capita alcohol consumption in Japan ranged between
3.87-7.27 liters per year (World Health Organization). This range for
Singapore was 1.81-2.90 liters per year (World Health Organization).
Furthermore, this study would be the first study to provide joint effect risk
estimates for smoking and drinking using a smoking variable that takes into
account the effects of dosage and duration.
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5
METHODS
DATA COLLECTION
The Singapore Chinese Health Study consists of 63,257 men and
women recruited between April 1993 and December 1998 from government-
built housing estates. Over eighty percent of Singaporeans reside in these
facilities and emigration from Singapore is negligible (Singapore Department
o f Statistics 18). Eligible persons were between the ages of 45 and 74 and
spoke either Cantonese or Hokkien, the two major dialects of the Singapore
Chinese. The Institutional Review Boards of the University of Southern
California and the National University o f Singapore granted approval for this
study.
Data were collected by trained interviewers in the homes of
participants. Dietary data were collected using a structured, 165-item food
frequency questionnaire in reference to the past 12 months from the date of
interview. Data for smoking were collected in reference to the lifetime of the
participant. For age at the start of smoking, participants were asked to choose
from four age ranges: 14 or younger, 15-19, 20-29, and 30 or older. For the
number of cigarettes smoked per day, participants were asked to choose from
six options: 6 or less, 7-12, 13-22, 23-32, 33-42, and 43 or more. Data for
alcohol consumption were collected in reference to the past 12 months from
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6
the date of interview. For all alcoholic beverage types, participants were
asked to choose from eight frequency options: never or hardly, once a
month, 2-3 times a month, once a week, 2-3 times a week, 4-6 times a week,
once a day, and 2 or more times a day. Participants were then asked to
indicate the corresponding number of drinks consumed. Volume amounts
were provided on the questionnaire to standardize answers between
participants. For the given volume amounts, the ethanol content for one
drink was 13.5 grams for beer, 10.85 grams for rice wine, 11.68 grams for
grape wine, and 10.85 grams for hard liquor. A total alcohol index using
drinks per week or drinks per day was created by summing the product of
frequency and amount across beverage types.
Incident cancer cases and deaths were ascertained by records linkage
with the Singapore Cancer Registry and the Singapore Registry of Births and
Deaths. There were 1,933 exclusions for a cancer history at baseline. The
present analyses utilize baseline questionnaire data for 61,324 cohort
participants. As of December 31, 2002, an average of 7.1 years of follow-up
were accumulated. Colon cancer was defined by ICD -0 2nd edition codes
C18.0-C18.9. Rectal cancer was defined by ICD-0 2nd edition codes C19.0,
C19.9, C20.0, and C20.9. O f the 61,324 cohort participants with no cancer
history at baseline, 637 developed colon (n=392) or rectal (n-245) cancer.
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7
O f these 637 colorectal cancer cases, 598 were adenocarcinomas, 14 were
other carcinomas, 6 were carcinoids, and 19 were of unknown histology.
STATISTICAL ANALYSES
Relative risk estimates, 95% confidence intervals, and corresponding
p-values were computed by Cox regression using PROC TPHREG on S AS
version 9.0. Person-years were calculated from the date of interview to the
earliest of the following events: diagnosis of colon or rectal cancer, death, or
December 31,2002.
P-values for subsite and gender differences were calculated using a
Wald's chi-square with one degree of freedom. Wald's chi-square values
were calculated using the square of the two-sample t-test for unequal
variances with the colon and rectal beta parameters as the two means. Meta
analysis results were calculated using the META function on Stata version
8.0 (StataCorp, College Station, TX).
Potentially confounding, nutrient covariates were identified by
stepwise selection separately for each subsite from a list of quartile variables
that met two criteria: 1) association with either colon or rectal cancer (p for
trend < 0.25) and 2) correlation with either smoking or drinking (r 0.10).
Beta-cryptoxanthin (ug/1000 Kcal), n-3 polyunsaturated fatty acids from fish
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8
and shellfish (g/1000 Kcal), lycopene (ug/1000 Kcal), and caffeine (mg/1000
Kcal) were identified by stepwise selection for colon cancer using entry and
removal significance levels set at p = 0.15. Carbohydrate (g/1000 Kcal) was
identified by stepwise selection for rectal cancer using entry and removal
significance levels set at p = 0.15.
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9
RESULTS
Among men, 63% of drinkers were smokers and 35% of smokers
were drinkers. Among women, 13% of drinkers were smokers and 13% of
smokers were drinkers. Most never smokers and nondrinkers were women
while most smokers and drinkers were men (table 1). Never smokers and
light drinkers were more highly educated and younger at baseline than their
respective smoking and drinking counterparts. Smokers and nondrinkers had
a slightly higher prevalence of diabetes than their respective smoking and
drinking counterparts. The majority of smokers and heavy drinkers were
current smokers while the majority of nondrinkers and light drinkers were
never smokers. On average, smokers drank more than never smokers, and
drinkers smoked more than nondrinkers. Heavy drinkers began smoking at a
slightly earlier age than light drinkers and nondrinkers.
For either men or women, smoking was associated with rectal cancer,
but not colon cancer (table 2). Among women, the number of cigarettes
smoked per day was inversely associated with colon cancer (p=0.05) and
smoking at or after age 15 was associated with a statistically significant
inverse risk [relative risk (RR) 95% confidence interval (Cl) = 0.50 (0.27-
0.93)] of colon cancer. Subclinical symptoms of colorectal cancer may have
caused some cases to change their living habits, including quitting smoking.
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Table 1. Distribution of selected characteristics by level of drinking and smoking, Singapore Chinese Health Study, 1993-2002.
Cigarette smokers Alcohol drinkers
Never Light Heavy1 Nondrinker <7 drinks/wk 7+ drinks/wk
(N=42,585) (N=16,435) (N=2,304) (N=49,652) (N=8,773) (N=2,899)
Column percentages
Gender
Men 27.1 83.1 91.9 37.7 69.7 85.1
Women 72.9 16.9 8.1 62.3 30.3 14.9
Dialect
Cantonese 48.2 42.3 35.9 45.7 50.4 42.7
Hokkien 51.8 57.7 64.1 54.3 49.6 57.3
Level of education
No formal education 29.9 20.7 22.6 29.8 15.4 18.1
Primary school 40.6 51.9 61.0 43.4 47.0 52.9
Secondary school and on 29.5 27.4 16.4 26.8 37.6 28.9
Diabetes
No 91.3 90.6 90.8 90.2 94.7 94.4
Yes 8.7 9.4 9.2 9.8 5.3 5.6
Smoking status
Never 100 NA NA 74.0 56.6 30.2
Former NA 35.8 34.6 9.9 15.2 15.6
Current NA 64.2 65.4 16.1 28.2 54.2
Mean (± standard deviation)
Age (years) 55.6±7.9 58.2±8.1 58.1±7.7 56.7±8.0 54.9±7.6 55.7±7.5
Body mass index (kg/m2 ) 23.3±3.3 22.8±3.3 22.9±3.2 23.2±3.3 23.0±3.2 22.6±3.2
Age at start of smoking NA 21.4±5.6 12.0±0.0 20.4±6.1 20.4±5.9 19.3±5.8
Cigarettes/day NA 16.1±11.0 25.9±10.8 4.35±9.38 7.42±11.10 14.7±13.8
Drinks/week 0.43±2.30 2.10±6.38 3.40±8.84 NA 1.66±1.61 15.9±11.8
Heavy smokers began smoking before age 15 and smoked 13 or more cigarettes/day. Light smokers are all other smokers.
NA=not applicable. ______________________________________________________________________________________
o
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Table 2. Cigarette smoking in relation to colorectal cancer risk, Singapore Chinese Health Study, 1993-2002.
Colorectal cancer Colon cancer Rectal cancer
Exposure Person-years Cases RR (95% Cl) Cases RR (95% Cl) Cases RR (95% Cl)
Total
Smoking status
Never smoker 307,186 385 1.00 263 1.00 122 1.00
Former 44,784 97 0.97 (0.76-1.24) 50 0.77 (0.55-1.08) 47 1.36(0.93-1.99)
Current 85,084 155 0.99 (0.80-1.23) 79 0.77 (0.58-1.03) 76 1.43 (1.03-1.99)
Cigarettes/day
Never smoker 307,186 385 1.00 263 1.00 122 1.00
<13 51,712 96 0.96 (0.76-1.22) 55 0.83 (0.61-1.13) 41 1.24 (0.85-1.81)
13-22 49,474 106 1.10(0.86-1.40) 49 0.78 (0.56-1.09) 57 1.72(1.20-2.46)
23+ 28,681 50 0.84 (0.61-1.15) 25 0.64 (0.42-1.00) 25 1.21 (0.76-1.94)
p for trend 0.66 0.03 0.05
Age at starting
Never smoker 307,186 385 1.00 263 1.00 122 1.00
15+ 106,812 195 0.94(0.77-1.14) 106 0.78 (0.60-1.01) 89 1.25 (0.91-1.71)
<15 23,056 57 1.21 (0.90-1.63) 23 0.74 (0.47-1.16) 34 2.16(1.42-3.28)
p for trend 0.54 0.05 0.001
Men
Smoking status
Never smoker 81,937 131 1.00 81 1.00 50 1.00
Former 39,267 92 1.08 (0.82-1.42) 47 0.87 (0.61-1.26) 45 1.44 (0.95-2.17)
Current 69,593 130 1.05 (0.81-1.35) 67 0.88 (0.63-1.24) 63 1.31 (0.89-1.93)
Cigarettes/day
Never smoker 81,937 131 1.00 81 1.00 50 1.00
<13 37,583 75 1.05 (0.79-1.40) 45 1.01 (0.70-1.46) 30 1.11 (0.70-1.76)
13-22 44,179 97 1.17(0.89-1.53) 44 0.85 (0.59-1.24) 53 1.68(1.13-2.49)
23+ 27,098 50 0.92 (0.66-1.29) 25 0.74 (0.47-1.17) 25 1.22 (0.75-1.99)
p for trend 0.91 0.16 0.08
All risks estimates were calculated by Cox regression and adjusted for age (years), gender (total set only), dialect group (Hokkien, Cantonese),
interview year, level of education (no formal, primary school, secondary school or higher), body mass index (<20, 20-<24, 24-<28, 28+ kg/m2),
history of diabetes (no, yes), family history of colorectal cancer (no, yes), and alcohol consumption (nondrinker, <7, 7-<14, 14+ drinks/wk).
(table 2 continued on next page)
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(continued) Table 2. Cigarette smoking in relation to colorectal cancer risk, Singapore Chinese Health Study, 1993-2002.
Colorectal cancer Colon cancer Rectal cancer
Exposure Person-years Cases RR (95% Cl) Cases RR (95% Cl) Cases RR (95% Cl)
(continued) Men
Age at starting
Never smoker 81,937 131 1.00 81 1.00 50 1.00
15+ 89,414 173 1.02 (0.81-1.28) 95 0.90 (0.66-1.21) 78 1.21 (0.84-1.75)
<15 19,446 49 1.29 (0.92-1.81) 19 0.80 (0.48-1.33) 30 2.11 (1.32-3.38)
p for trend 0.24 0.33 0.005
Women
Smoking status
Never smoker 225,249 254 1.00 182 1.00 72 1.00
Former 5,517 5 0.46 (0.19-1.12) 3 0.39 (0.12-1.22) 2 0.64 (0.16-2.61)
Current 15,490 25 0.97 (0.64-1.46) 12 0.63 (0.35-1.14) 13 1.87(1.03-3.41)
Cigarettes/day
Never smoker 225,249 254 1.00 182 1.00 72 1.00
<13 14,128 21 0.84(0.54-1.32) 10 0.55 (0.29-1.04) 11 1.61 (0.84-3.07)
13-22 6,879 9 0.76 (0.39-1.49) 5 0.58(0.24-1.41) 4 1.24 (0.45-3.40)
p for trend 0.29 0.05 0.27
Age at starting
Never smoker 225,249 254 1.00 182 1.00 72 1.00
15+ 17,397 22 0.73 (0.47-1.13) 11 0.50 (0.27-0.93) 11 1.33 (0.70-2.54)
<15 3,610 8 1.20 (0.59-2.44) 4 0.82 (0.31-2.23) 4 2.20 (0.80-6.08)
p for trend 0.54 0.07 0.10
All risks estimates were calculated by Cox regression and adjusted for age (years), gender (total set only), dialect group (Hokkien, Cantonese),
interview year, level of education (no formal, primary school, secondary school or higher), body mass index (<20, 20-<24, 24-<28, 28+ kg/m2),
history of diabetes (no, yes), family history of colorectal cancer (no, yes), and alcohol consumption (nondrinker, <7, 7-<14, 14+ drinks/wk).
13
These changes have may biased risk estimates. Thus, analyses were repeated
after excluding the first 2 years of follow-up (40 cases and 67,658 person-
years excluded). The trend for cigarettes smoked per day was no longer
significant (p=0.13) and the inverse association for smoking at or after the
age of 15 was no longer significant [RR 95% Cl = 0.59 (0.31-1.13)] for
women. Male smokers who began smoking before the age o f 15 had the
highest rectal cancer risk. They were 2.11 times as likely as never smokers to
develop rectal cancer after alcohol adjustment. Repeating the analysis after
excluding the first two years of follow-up in men did not yield different
results. Adjusting for alcohol in men slightly reduced both colon and rectal
cancer risk estimates. However, adjusting for alcohol in women did not
change colon cancer risk estimates and slightly increased rectal cancer
estimates.
The analysis of the combined effect of dosage and duration for
cigarette smoking (table 3), confirms a positive association between smoking
and rectal cancer risk. Since the number of year smoked is assessed once at
baseline and affected by age at the time of interview, age at the start of
smoking was used to give a better sense of smoking duration. There was a
lack of a significant positive association between smoking and colon cancer
risk. Male smokers who began smoking before the age of 15 and smoked 13
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Table 3. Duration and amount of smoking in relation to colorectal cancer, Singapore Chinese Health Study, 1993-2002.
Age at starting
15+ <15
Site Cigarettes/day Cases RR(95% Cl) Cases RR(95% Cl)
Total - 263 colon and 122 rectal cases among never smokers
Colorectal
<13
13+
81 0.96(0.75-1.24) 15
114 0.92(0.73-1.17) 42
0.97 (0.58-1.64)
1.33 (0.95-1.87)
Colon
<13
13+
49 0.88(0.64-1.21) 6
57 0.71 (0.51-0.97) 17
0.57 (0.25-1.30)
0.81 (0.49-1.36)
Rectal
<13
13+
32 1.14(0.76-1.73) 9
57 1.34(0.93-1.92) 25
1.82 (0.91-3.64)
2.35 (1.47-3.77)
Men - 81 colon and 50 rectal cases among never smokers
Colorectal
<13
13+
65 1.07(0.79-1.44) 10
108 0.99(0.76-1.28) 39
0.95 (0.50-1.82)
1.42 (0.98-2.05)
Colon
<13
13+
41 1.07(0.73-1.57) 4
54 0.79(0.56-1.13) 15
0.60 (0.22-1.65)
0.87 (0.50-1.53)
Rectal
<13
13+
24 1.05(0.64-1.71) 6
54 1.31 (0.88-1.94) 24
1.55 (0.66-3.65)
2.34(1.41-3.87)
All risks estimates were calculated by Cox regression and adjusted for age (years), gender (total set only), dialect group (Hokkien, Cantonese),
interview year, level of education (no formal, primary school, secondary school or higher), body mass index (<20, 20-<24, 24-<28, 28+ kg/m2),
history of diabetes (no, yes), family history of colorectal cancer (no, yes), and alcohol consumption (nondrinker, <7, 7-<14, 14+ drinks/wk).
4^
15
or more cigarettes per day had the highest risk estimate for rectal cancer [RR
95% Cl = 2.34 (1.41-3.87)].
Alcohol was statistically significantly associated with colorectal
cancer risk in men, but not women (table 4). Risk estimates for the entire
cohort were slightly higher for rectal cancer than for colon cancer. Data for
alcohol by beverage type indicated that the effects of beer and liquor, but not
rice wine or grape wine were reflected in the positive associations for alcohol
(table 5). Repeating the analysis after excluding the first two year o f follow-
up did not yield different results for total alcohol or beverage type.
Data for cigarette smoking by alcohol consumption (table 6) were
consistent with unstratified data (table 1-5). All estimates for men were
higher than estimates for the total cohort, reflecting lower estimates for
women for both smoking and drinking. For men, alcohol was associated with
colorectal cancer among never smokers. Risks estimates for colon cancer
were not significant [RR 95% Cl = 1.46 (0.58-3.62)], but consistent in
direction with rectal cancer estimates [RR 95% Cl = 2.90 (1.12-7.46)]. The
effect of cigarette smoking among nondrinkers was clearly limited to rectal
cancer. Among nondrinkers, smoking increased the risk of rectal cancer by
over two-fold but had no effect in the colon [RR 95% Cl = 0.99 (0.52-1.88)].
The joint effect of smoking and drinking is thus limited to rectal cancer.
Light smokers (defined as those who smoking at age 15 or later or smoking
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Table 4. Alcohol consumption in relation to colorectal cancer, Singapore Chinese Health Study, 1993-2002.
Colorectal cancer Colon cancer Rectal cancer
Exposure Person-years Cases RR (95% Cl) Cases RR (95% Cl) Cases RR (95% Cl)
Total
Drinks/day
Nondrinker 355,569 502 1.00 321 1.00 181 1.00
<1 61,224 83 1.03 (0.81-1.31) 43 0.90 (0.65-1.25) 40 1.22 (0.85-1.73)
l-<2 10,147 28 1.83 (1.25-2.70) 16 1.83 (1.10-3.05) 12 1.84(1.02-3.34)
2+ 10,114 24 1.65 (1.08-2.51) 12 1.56 (0.86-2.82) 12 1.75 (0.96-3.21)
p for trend 0.002 0.08 0.01
Men
Drinks/day
Nondrinker 130,834 235 1.00 136 1.00 99 1.00
<1 42,765 69 1.12(0.86-1.47) 34 0.98 (0.67-1.44) 35 1.31 (0.89-1.93)
l-<2 7,586 25 2.07 (1.37-3.14) 13 1.96(1.10-3.48) 12 2.22(1.21-4.05)
2+ 9,612 24 1.76(1.15-2.71) 12 1.72 (0.94-3.13) 12 1.81 (0.98-3.34)
p for trend 0.0003 0.03 0.004
Women
Drinks/day
Nondrinker 224,735 267 1.00 185 1.00 82 1.00
<1 18,459 14 0.77 (0.45-1.32) 9 0.72 (0.37-1.42) 5 0.73 (0.29-1.80)
l-<2 3,063 3 0.88 (0.28-2.74) 3 1.32 (0.42-4.13) 21,522 person-years above
p for trend 0.40 0.70 0.49
All risks estimates were calculated by Cox regression and adjusted for age (years), gender (total set only), dialect group (Hokkien, Cantonese),
interview year, level of education (no formal, primary school, secondary school or higher), body mass index (<20, 20-<24, 24-<28, 28+ kg/m2),
history of diabetes (no, yes), family history of colorectal cancer (no, yes), and cigarette smoking (never, <13 cigarettes/day and age 15+ at start, <13
cigarettes/day and age <15 at start, 13+ cigarettes/day and age 15+ at start, 13+ cigarettes/day and age <15 at start).
o \
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Table 5. Beverage type in relation to colorectal cancer, Singapore Chinese Health Study, 1993-2002.
Colorectal cancer Colon cancer Rectal cancer
Exposure Cases RR (95% Cl) Cases RR (95% Cl) Cases RR (95% Cl)
Total
Nondrinkers 526 1.00 335 1.00 191 1.00
Beer (drinks/week) 111 1.02(1.00-1.03) 57 1.01 (0.98-1.03) 54 1.03 (1.01-1.05)
Nondrinkers 631 1.00 387 1.00 244 1.00
Rice wine (drinks/week) 6 0.98 (0.84-1.15) 5 1.04 (0.91-1.19) 1 0.46 (0.06-3.80)
Nondrinkers 633 1.00 391 1.00 242 1.00
Grape wine (drinks/week) 4 0.77 (0.47-1.26) 1 0.03 (<0.01-17.6) 3 0.96 (0.68-1.37)
Nondrinkers 586 1.00 363 1.00 223 1.00
Liquor (drinks/week) 51 1.04(1.00-1.08) 29 1.06(1.01-1.10) 22 1.00 (0.91-1.09)
Men
Nondrinkers 248 1.00 142 1.00 106 1.00
Beer (drinks/week) 105 1.02(1.00-1.03) 53 1.01 (0.98-1.03) 52 1.03 (1.01-1.05)
Nondrinkers 351 1.00 193 1.00 158 1.00
Rice wine (drinks/week) 2 0.98 (0.83-1.16) 2 1.04(0.90-1.19) 0 -
Nondrinkers 352 1.00 195 1.00 157 1.00
Grape wine (drinks/week) 1 0.70 (0.29-1.73) 0 - 1 0.89 (0.46-1.72)
Nondrinkers 309 1.00 173 1.00 136 1.00
Liquor (drinks/week) 44 1.04(1.01-1.08) 22 1.06(1.02-1.11) 22 1.01 (0.93-1.09)
All risks estimates were calculated by Cox regression and adjusted for age (years), gender (total set only), dialect group (Hokkien, Cantonese),
interview year, level of education (no formal, primary school, secondary school or higher), body mass index (<20, 20-<24, 24-<28, 28+ kg/m2),
history of diabetes (no, yes), family history of colorectal cancer (no, yes), and cigarette smoking (never, <13 cigarettes/day and age 15+ at start, <13
cigarettes/day and age <15 at start, 13+ cigarettes/day and age 15+ at start, 13+ cigarettes/day and age <15 at start), and other alcoholic beverages.
-a
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Table 6. Alcohol and smoking in relation to colorectal cancer, Singapore Chinese Health Study, 1993-2002.
Smoking intensity
Never Light1 Heavy
Site Alcohol (drinks/day) Cases RR (95% Cl) Cases RR (95% Cl) Cases RR (95% Cl)
Total
Nondrinker 332 1.00 144 0.94 (0.76-1.17) 26 1.28 (0.84-1.95)
Colorectal <1 40 1.04 (0.74-1.45) 35 0.92 (0.64-1.33) 8 1.60 (0.78-3.25)
1+ 13 1.61 (0.92-2.83) 31 1.69(1.15-2.50) 8 2.25(1.10-4.61)
Nondrinker 233 1.00 75 0.72 (0.54-0.96) 13 0.94 (0.53-1.67)
Colon <1 22 0.84 (0.54-1.32) 19 0.75 (0.46-1.22) 2 0.59 (0.14-2.38)
1 + 8 1.46 (0.72-2.98) 18 1.46 (0.88-2.42) 2 0.84 (0.21-3.40)
Nondrinker 99 1.00 69 1.44(1.02-2.05) 13 2.05 (1.11-3.78)
Rectal <1 18 1.47 (0.88-2.46) 16 1.31 (0.74-2.29) 6 3.79(1.62-8.88)
1+ 5 1.96 (0.79-4.87) 13 2.21 (1.20-4.08) 6 5.27 (2.24-12.4)
Men
Nondrinker 94 1.00 118 1.05 (0.80-1.38) 23 1.40 (0.89-2.23)
Colorectal <1 27 1.22 (0.79-1.87) 34 1.07 (0.72-1.59) 8 1.91 (0.92-3.94)
1 + 10 1.93(1.00-3.71) 31 1.99(1.32-3.00) 8 2.57(1.24-5.31)
Nondrinker 63 1.00 62 0.81 (0.57-1.16) 11 0.99 (0.52-1.88)
Colon <1 13 0.90 (0.49-1.64) 19 0.90 (0.53-1.50) 2 0.70 (0.17-2.89)
1 + 5 1.46 (0.58-3.62) 18 1.74(1.03-2.95) 2 0.97 (0.24-3.99)
Nondrinker 31 1.00 56 1.54 (0.99-2.41) 12 2.28(1.16-4.48)
Rectal <1 14 1.86 (0.99-3.51) 15 1.42 (0.77-2.64) 6 4.42 (1.83-10.7)
1+ 5 2.90(1.12-7.46) 13 2.51 (1.31-4.83) 6 5.75 (2.38-13.9)
All risks estimates were calculated by Cox regression and adjusted for age (years), gender (total set only), dialect group (Hokkien, Cantonese),
interview year, level of education (no formal, primary school, secondary school or higher), body mass index (<20, 20-<24, 24-<28, 28+ kg/m2),
history of diabetes (no, yes), and family history of colorectal cancer (no, yes).
1 Heavy smokers were those who started smoking before age 15 and smoked at least 13 cigarettes/day. Light smokers were those who began to
smoke at age 15 or later or smoked less than 13 cigarettes/day.____________ _______________________________________________ _________
oo
19
less than 13 cigarettes per day) who drank one or more drinks per day were
2.51 times as likely as nondrinking, never smokers to develop rectal cancer.
Heavy smokers (defined as those smoking before the age of 15 and smoking
at least 13 cigarettes per day) who drank one or more drinks per day were
5.75 times as likely to develop rectal cancer as nondrinking, never smokers.
Excluding the first 2 years of follow-up did not yield different results.
For all results, excluding cases with carcinoid and unknown
histologies did not yield different results. For all results, adjusting for
nutrients identified by stepwise regression as possible confounders for both
colon and rectal cancer did not yield different results.
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20
DISCUSSION
The results of this cohort study clearly show that cigarette smoking
increases the risk of rectal cancer independently o f alcohol, that alcohol
consumption increases the risk of colorectal cancer independently of cigarette
smoking, and that the joint effect of cigarettes and alcohol can produce an
almost 6 fold increase in rectal cancer risk. Furthermore, the results fail to
support an association between cigarette smoking and colon cancer, and
support associations between alcohol and both colon and rectal cancer. The
lack of a statistically significant joint effect between cigarettes and alcohol in
relation to colon cancer is a result of the lack of an association between
cigarette smoking and colon cancer.
Two other cohort-based studies (Murata 561; Otani 1497) have
investigated cigarette smoking among nondrinkers of alcohol in relation to
colorectal cancer risk. Both studies provide estimates for Japanese men.
Their results for cigarette smoking and colon cancer among nondrinkers are
consistent with the results of the present study. All three studies fail to find a
statistically significant association. The strength o f the current study is that
the relevant risk estimate is based on more cases (n=74) than the both the
Murata study (n=13) and the Otani study (n-41).
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21
The result of the present study for cigarette smoking and rectal cancer
is inconsistent with the result of the Otani study. The Murata study did not
provide risk estimates for rectal cancer. The Otani study did not find an
association between smoking and rectal cancer among nondrinkers.
However, the present study shows a strong positive association between
smoking and rectal cancer. A strength of the present study is that the relevant
risk estimate is based on more cases (n-43) than the Otani study (n=16).
Another strength o f the present study is that the variable used to assess
cigarette smoking incorporates both the effect of dosage and duration. A
limitation of the present study and the Otani study is that stable estimates
were not provided for women. The cohort for the Murata study did not
include women.
Two other cohort-based studies (Cho 610; Otani 1497) have
investigated alcohol consumption among never smokers in relation to
colorectal cancer risk. The present study and the Otani study show
statistically significant associations for both colon cancer and rectal cancer.
However, the present study shows a statistically significant association
between alcohol consumption and colorectal cancer at a consumption level of
1 or more drinks per day (or 84 or more grams per week) while the Otani
study shows this association at a consumption level of 300 or more grams per
week (or 3.6 drinks per day), despite the comparability o f the point estimates.
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2 2
The point estimate for the Otani study (RR = 2.2) is only slightly larger than
the point estimate for the present study (RR = 1.93). A strength of the
present study is that this estimate is based on more cases (n=104) than the
Otani study (n=35).
The statistically significant positive association found by the present
study is inconsistent with the result of the Cho study. The Cho study fails to
find a statistically significant association between alcohol consumption and
colorectal cancer despite utilizing pooled data for 4 cohorts in North America
and Europe. There was no evidence (p>0.2) of between-study heterogeneity
for the highest intake category or between-study heterogeneity due to gender
for the highest intake category; however, the power of the heterogeneity test
is questionable given that there were only 4 cohorts. Additionally, two of the
four cohorts composing the pooled analysis were composed of all women.
Gender-stratified, cigarette-adjusted risk estimates for alcohol in the Cho
study indicated that point estimates for the highest exposure level were
similar between men and women, but estimates for men were statistically
significant while estimates for women were not. If women drink less alcohol
than men, it is also possible that women drink for shorter durations than men.
Thus, combining data for men and women may dilute the effect measured by
dosage with the shorter duration o f exposure in women. A limitation of all
the studies mentioned is the lack of duration data for alcohol.
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23
Two other cohort-based studies (Murata 561; Otani) have investigated
the joint effect of cigarettes and alcohol. Both studies show 3 fold risk
increases for colon cancer compared to nondrinking, never smokers or
nonsmokers, but the present study does not. The present study shows no
association between the joint effect and colon cancer. It is true that the joint
effect estimate for colon cancer in the present study may be unreliable due to
there being only 2 exposed cases. However, all three studies fail to show
statistically significant risk estimates between cigarette smoking and colon
cancer among nondrinkers. For the Murata study, the risk estimate (RR=3.2)
for drinking among nonsmokers is actually higher than the risk estimate for
drinking among smokers (RR=3.1). However, for the Otani study, the joint
effect estimate (RR=3.0) is higher than the estimate for alcohol among never
smokers (RR=2.2). Nevertheless, it is still possible that the joint effect
estimate may not reflect the effect of cigarette smoking at all. Instead, if
smokers have a longer duration of alcohol consumption than never smokers,
what appears to be an increased risk due to smoking among drinkers may
actually be an increased risk due to longer duration of alcohol consumption
among smokers. This postulated effect may occur for the Otani study, but
not the present study due to the wider range of drinking observed in the Otani
study. For the narrow range of drinking observed in the present study (0 to 1
or more drinks per day), the duration o f alcohol consumption may be
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24
relatively homogeneous. However, for the wider range of drinking observed
in the Otani study (0 to 3.6 or more drinks per day), the duration of alcohol
consumption may be sufficiently heterogeneous to result in alcohol duration
differences between never smokers and current smokers.
The Otani study showed a three fold risk increase of rectal cancer for
the joint effect of cigarettes and alcohol. These results are consistent in
direction but not magnitude with the results of the present study which show
a near six fold increase in the risk of rectal cancer. Both estimates for these
two studies are based on exactly 37 cases. However, the estimate for the
Otani study applies to current smokers who drink 3.6 or more drinks per day.
The estimate for the present study applies to smokers who began smoking
before the age of 15 and smoked 13 or more cigarettes per day, and drank 1
or more drinks per day. It is possible that the higher risk estimate observed in
the present study is due to the use of a more detailed cigarette smoking
variable that isolates smokers of high dosage and high duration from all other
smokers.
The strengths of the present study lie in the prospective cohort design,
the stratification of cigarette smoking by alcohol consumption, the high case
numbers for most stratified analyses, and the use of a smoking variable
incorporating dosage and duration for the joint effect estimate. The
prospective cohort design reduces the likelihood of recall bias because
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25
exposure data are recorded before the diagnosis of cancer. The stratification
o f cigarette smoking by alcohol consumption and vice versa rules out the
possibility of residual confounding due to the high correlation between
smoking and drinking. The high case numbers provide reliable estimates for
the stratified analyses. The incorporation of dosage and duration into the
smoking variable for the joint effect estimate provides more detailed
information for smoking.
The limitations of the study were the lack of stable estimates for
women and the lack of duration data for alcohol.
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26
CONCLUSION
For this population of Singapore Chinese, cigarette smoking increases
the risk o f rectal cancer, but not colon cancer. Alcohol consumption
increases the risk of both colon and rectal cancer. The joint effect of
cigarettes and alcohol increases the risk of rectal cancer by almost 6 fold.
There is no joint effect association for colon cancer due to the lack of
association between cigarette smoking and colon cancer.
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27
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Tsong, Wan Hua
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Cigarettes and alcohol in relation to colorectal cancer within the Singapore Chinese Health Study
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Applied Biostatistics and Epidemiology
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