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Vol. 7, 64 7-652. August 1998 Cancer Epidemiology, Biomarkers & Prevention 647
3 The abbreviations used are: OR. odds ratio: CI. contidence interval: GSTM I.
glutathione transferase 51 I.
Glutathione Transferase Null Genotype, Broccoli, and Lower Prevalence
of Colorectal Adenomas’
Henry J. Lin,2 Nicole M. Probst-Hensch,Andrew D. Louie, Irving H. Kau, John S. Witte,Sue A. Ingles, Harold D. Frankl, Eric R. Lee, andRobert W. Haile
Division of Medical Genetics. Department of Pediatrics, Harbor-UCLA
Medical Center, Torrance. Califomia 90502 IH. J. L.. A. D. L.. I. H. K.l;
Department of Preventive Medicine. USC/Norris Comprehensive Cancer
Center. Los Angeles. California 90033 IN. M. P-H.. S. A. I.. R. W. HI:
Department of Epidemiology and Biostatistics, Case Westem Reserve
University. Cleveland, Ohio 44109 [J. S. WI: Department of Gastroenterology.
Kaiser Permanente. Los Angeles. Califomia 9�27 IH. D. Fl; and Department
of Gastroenterology. Kaiser Permanente. Bellflower. Califomia 90706
IE.R.L.I
Abstract
Cruciferous vegetables, especially broccoli, may preventcancer through anticarcinogenic compounds. Forexample, broccoli contains isothiocyanates that inducecarcinogen-detoxifying enzymes. Glutathione transferaseenzymes conjugate isothiocyanates, leading to excretion.We hypothesized that broccoli consumption incombination with the glutathione transferase Ml(GSTMI) null genotype would be associated with a lowerprevalence of colorectal adenomas because of higherisothiocyanate levels. We used a case-control study ofmainly asymptomatic subjects aged 50-74 years whounderwent a screening sigmoidoscopy at either of twoSouthern California Kaiser Permanente Medical Centersduring 1991-1993. Cases (n 459) had a first-time
diagnosis of histologically confirmed adenomas detectedby flexible sigmoidoscopy. Controls (n 507) had nopolyp detected. Subjects had a 45-mm in-person interviewfor information on various risk factors and basicdemographic data and completed a 126-item,semiquantitative food frequency questionnaire. Bloodsamples were used for GSTMJ genotyping. Subjects with
the highest quartile of broccoli intake (an average of 3.7servings per week) had an odds ratio of 0.47 (95%confidence interval, 0.30-0.73) for colorectal adenomas,compared with subjects who reportedly never atebroccoli. When stratified by GSTMJ genotype, aprotective effect of broccoli was observed only among
Received 12/19/97: revised 5/6/98: accepted 5/12/98.
The costs of publication of this article were defrayed in part by the payment ofpage charges. This article must therefore be hereby marked advertiseuzent in
accordance with I 8 U.S.C. Section I 734 solely to indicate this fact.
I H. J. L. was supported by the UCLA Clinical Nutrition Research Unit (NIH
Grant 2P��lCA427l0): the Peg Liddle Fund at Eisenhower Medical Center.
Rancho Mirage. CA: and NIH Grants 1R01CA66782 and IROICA734O3.
R. W. H. was supported by NIH Grants IR01CA51923 and 1R01CA63699.
J. S. W. was supported by NIH Grant R29CA73270.
2 To whom requests for reprints should be addressed. at Hartxr-UCLA Medical
Center. E4. 1124 West Carson Street. Torrance. CA 90502. Phone: (310) 222-
3673: Fax: (310) 328-9921: E-mail: henryjin@humc.edu.
subjects with the GSTMI null genotype (P for trend,0.001; P for interaction, 0.01). The observed broccoli-GSTMJ interaction is compatible with an isothiocyanatemechanism.
Introduction
Cruciferous vegetables are viewed as potentially protective
against cancer ( 1 , 2). These vegetables include broccoli. brus-sels sprouts, cabbage, cauliflower, collards, kale, and kohlrabi,which are varieties of Brassica oleracea (3). Of early epide-
miological evidence for a protective effect against colorectalcancer, Graham et a!. (4) found an OR3 of 0.3 comparing
people who ate cabbage at least once per week to those whonever ate cabbage. Other studies gave similar results, based ondata for: cruciferous vegetables as a group (5. 6): broccoli.
cauliflower, brussels sprouts, and turnips (7): and cabbage,cauliflower, and sprouts (8). However, not all studies have
found inverse associations between cruciferous vegetables andcolorectal cancer (9-12).
Isothiocyanate compounds are derived from glucosino-lates in various plants. Medicinal properties of glucosinolates
are historically important. as seen in writings by Pythagoras and
Hippocrates. At least 20 such compounds were identified by thel980s (13). All cruciferous vegetables are believed to contain
glucosinolates, but brussels sprouts (6()0-3900 �.tg/g), broccoli(450-1480 ;.Lg/g), and Savoy cabbage (470-1 290 �.tg/g) havethe highest content among Brassica oleracea.
Anticarcinogens in cruciferous vegetables induce enzymes
that detoxify environmental mutagens. Broccoli extract was themost potent enzyme inducer in a murine hepatoma cell assay
(14), probably due to sulforaphane, an isothiocyanate corn-pound ( I 4, 15). Sulforaphane blocked 7, 12-dirnethylbenz(a)-
anthracene-induced mammary tumors in rats. supporting a con-clusion that isothiocyanates protect against cancer ( I 6).
Zhang et a!. ( 1 7) and Kolm ci a!. ( 18) found that GSTMIrapidly conjugates isothiocyanates to glutathione, leading to
excretion. Approximately 50% of people completely lackGSTM I enzyme. due to homozygous deletion of the gene (i.e.,the GSTMI null genotype; Ref. 19-21 ). We hypothesized thata cancer preventive effect of broccoli would be stronger with
the GSTMJ null genotype. due to potentially slower excretionof isothiocyanates. Here we show effects of broccoli consump-
tion and the GSTMJ null genotype on prevalence of colorectal
adenomas, which are precursors to cancer.
Materials and Methods
Cases and Controls. Subjects were from one of two SouthernCalifornia Kaiser Permanente Medical Centers (Belltlower or
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Table 1 Characteristics of subjects with colorectal adenomas and controls
Colorectal adenomas Control subjects
(n = 459) (n = 507)
n �/c ,j
648 GSTMJ, Broccoli, and Colorectal Adenomas
We used unconditional logistic regression to estimate ORs
Sunset) and had a sigmoidoscopy during the period fromJanuary 1, 1991, through August 25, 1993. Eligible men andwomen were ages 50-74 years, residents of Los Angeles or
Orange counties, and fluent in English. None had a history ofinvasive cancer, inflammatory bowel disease, familial polypo-sis, previous bowel surgery, symptoms suggestive of gastroin-
testinal disease, or a physical or mental problem that wouldpreclude an interview. Cases had a first-time diagnosis of one
or more histologically confirmed colorectal adenomas. Controlswere selected from subjects with no polyps and were individ-
ually matched to cases by gender, age (within a 5-year cate-gory), sigmoidoscopy date (within a 3-month category), and
center where examined.There were 628 potentially eligible cases and 689 poten-
tially eligible controls during the accrual period. Seventy cases
and 94 controls refused an interview, and 29 cases and 32
controls could not be contacted. Response rates (number inter-viewedlnumber eligible) were 84% for cases and 82% for
controls. A replacement was identified when the control subjectoriginally matched to a case could not be interviewed.
Indications for sigmoidoscopy among interviewed sub-jects were: “routine” for 45% of cases and 44% of controls;“minor symptoms” for I 6% of cases and I 3% of controls; and“not specified” for 39% of cases and 43% of controls. Average
depth of the flexible sigmoidoscope was 55 ± 1 1 (SD) cm for
cases and 59 ± 5 cm for controls. Fifteen cases had carcinomain situ. Size and number of polyps were recorded by thesigmoidoscopist.
Subjects gave information on smoking, therapeutic drug
use, physical activity, height, weight, family history of cancer,
and other factors during a 45-mm, in-person interview. Theinterview was S months after sigmoidoscopy, on average, and
exposure data referred to the period up to the sigmoidoscopy.Interviewers were unaware of case or control status for 70% of
cases and 87% of controls.
Diet Data. A 126-item, semiquantitative food frequency ques-
tionnaire (22), asking about diet during the year before sig-moidoscopy, was filled-out by 5 19 cases and 556 controls. The
questionnaire provides information on intake of the followingcruciferous vegetables: broccoli; cabbage or cole slaw; cauli-flower; brussels sprouts; and kale, mustard, or chard greens.Standard methods were used to compute nutrient intakes. Spe-
cific foods corresponding to items on the questionnaire wereselected based on the relative frequency of consumption among
participants of the 1988-1989 Nationwide Food ConsumptionSurvey, southwest region. The Nutrient Data System (NDB
version 2.4) was used as a nutrient database for foods. Data on
nutrient content of supplements were from the Harvard School
of Public Health.
Genotyping. GSTMJ genotyping was done on 977 subjectswho provided a blood specimen. Dried blood spots on blotterpaper (Schleicher and Schuell no. 903) were used for DNA
templates for 936 specimens. Liquid DNA was used for theremaining 41 specimens. PCR assays were as described (23).
Duplicate assays were done for all specimens, without knowl-edge of case or control status.
Data Analysis. The analysis included 966 subjects, because 1 1of the 977 genotyped subjects did not complete a food fre-quency questionnaire and had to be excluded. We assumed thatpersons who did not answer a specific item on the questionnaire
(including cruciferous vegetables) never ate that item, havingfound that missing responses most commonly turn out to be
zero.
Gender
MaleFemale
Ethnicity
White
Hispanic (not black)
Black
Asian/Pacific Islander
Smoking
Never
Ex-smoker
Current
297 63.9 338 67.0
162 36.1 169 33.0
263 55.1 278 54.6
77 16.6 91 17.5
74 15.9 80 15.3
45 9.8 58 11.0
168 36.7 217 42.9
205 43.1 231 44.3
86 20.2 59 12.8
Average age, years 62.0 62.0
Smoking, average pack-years 22. 1 16.7
Fruits and vegetables, servings”/day 5.5 6.1
Cruciferous vegetables, servings/week 2.9 3.5
Broccoli, servings/week 1 .2 1.5
Cabbage or cole slaw, servings/week 0.85 0.88
Cauliflower, servings/week 0.5 1 0.66
Brussels sprouts, servings/week 0.24 0.24
Kale, mustard, or chard, servings/week 0. 14 0.23
Saturated fat, g/day 25.2 22.6
Energy intake. kcal/day 2027 1924
“ Serving size = /‘ cup of each vegetable (broccoli: cabbage or cole slaw:
cauliflower: brussels sprouts; kale, mustard, or chard greens).
and controlled for the matching factors-date of sigmoidoscopy
(6-month intervals), age (5-year intervals), gender, and centerattended-as indicator variables in the model. This allowed us toinclude information on unmatched subjects in the analysis.
Unmatched controls occurred when, for example, their matchedcases did not speak English or were found to have invasivecancer at follow-up colonoscopy. Unmatched cases occurredwhen we were unable to interview an eligible control. Un-matched subjects also occurred if blood, and therefore GSTMJ
genotypes, was obtainable from only one subject of a pair.
Categories of vegetable intake as presented in the tables wereentered as indicator variables in the model.
Covariates considered for possible inclusion in the multi-variate models were: race; body mass index; vigorous leisure
time activity; intake of total energy, saturated fat, and fruits andvegetables; family history of colorectal cancer; intake of non-
steroidal anti-inflammatory drugs; and smoking. None of the
covariates changed the size of the point estimates of interest bymore than 10%. We therefore adjusted only for matching vari-ables, smoking, and intake of total energy, saturated fat, and all
other fruits and vegetables (i.e., vegetables not specified by themodel).
Subjects homozygous for the GSTMJ gene deletion were
coded as having the GSTMJ null genotype. Interaction betweenvegetable intake and GSTMJ genotype was assessed by models
containing interaction terms of vegetable intake with GSTMJ
genotype. One hypothesis is that the null genotype may beassociated with increased risk of colorectal cancer in smokers,
so that smoking might modify a vegetable-GSTMJ interaction.We did not stratify analyses by smoking status, however, be-cause no interaction between smoking and GSTMJ was found
in our original analyses (23). We did restrict the analysis to
people who never smoked to corroborate that any vegetable-
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Cancer Epidemiology, Biomarkers & Prevention 649
Table 2 ORs for colorectal adenomas by intake of crucifer ous vegetables”
Intake frequency (servings)
Never 1-3/month 1/week >1/week
Broccoli
OR l.0() 0.62 0.65 0.47 0.007 1)0395C/� CI
- 0.424)93 0.43-0.99 0.3()-0.73
Controls/Cases 72/105 152/134 136/123 147/97
Mean servings/week 0 0.5 1 3.7
Cabbage/Cole slaw
OR 1.00 1.34 1.1 1 1.40 1)33 0.7495C/� CI
- 0.96-1.86 0.75-1.66 0.87-2.27
Controls/Cases I 34/ 108 204/203 106/89 63/59
Mean servings/week 0 0.5 1 3.6
Cauliflower
OR I .00 0.79 1 .0 1 0.7 I (1.30 0.2(1
95(7� CI - 0.58-1.07 0.68-1.51 0.41-1.23
Controls/Cases 2()4/2l2 186/149 75/70 42/28
Mean servings/week 0 0.5 1 3.7
Brussels sprouts
OR 1.00 0.76 1.30 0.96 0.91 (1.9795C/� CI
- 0.55-1.06 0.75-2.24 0.38-2.40
Controls/Cases 357/332 1 1 2/84 26/34 I 2/9
Mean servings/week 0 0.5 1 3.5
Kale, mustard. chard greens
OR 1.00 0.62 0.99 0.41 0(14 0.1)495C/� CI
- 0.4()-().97 0.52-1.88 0.16-1.06
Controls/Cases 408/393 62/38 2 1/2 1 16/7
Mean servings/week 0 0.5 I 3.8
“ The effect of single vegetables (as indicator variables for categories) were adjusted for smoking (indicator variables for current smoking and ex-stiloking): matching
variables (age in 5-year categories: date of sigmoidoscopy in five categories total): clinic: gender: saturated fat (indicator variables for quartiles of saturated fat intake in
the total population): energy intake (calories as continuous variable): intake of fruits and vegetables indicator variables for quartiles of fruit and vegetable intake minus
the specific vegetable(s) of interest in the modell.I, p for trend was calculated by entering the mean number of servings/week in each group as a categorical variable in the model.
‘ P for trend was calculated by entering the intake of the respective vegetable as a continuous (ordered) variable in the model.
GSTMJ interaction was not different in this subgroup. Cigarettesmoking data were abstracted from in-person interviews, and
results of the main effect of smoking are presented elsewhere(24). Individuals who had never smoked 100 cigarettes in theirlife were coded as never smokers. Subjects who had smokedmore than 100 cigarettes were coded as ex-smokers if they quit
smoking before the sigmoidoscopy and as current smokers if
they were smoking at the time of sigmoidoscopy.Restriction of analyses to subjects with an energy intake
between 800 and 4200 kcal did not substantially alter results
and was thus omitted.
Results
Cases and controls are described in Table 1 . Male:female dis-tribution, ethnic mix, and average ages were fairly comparablefor the two groups. There was a lower percentage of smokersamong controls. On average. controls ate more fruits andvegetables per week, including cruciferous vegetables and
broccoli.
Main effects of broccoli and of cruciferous vegetables onthe prevalence of colorectal adenomas are shown in Table 2.
Earlier we presented protective effects of cruciferous vegeta-
bles and broccoli in our study population (25). Because we now
focus on interaction between cruciferous vegetables andGSTMI, we again show the main effects of cruciferous vege-tables and broccoli, but among subjects with known GSTMJ
genotype.We estimated ORs for single cruciferous vegetables across
four levels of serving intake: never; 1-3/month; 1/week;
and > I/week. Increased consumption of broccoli and kale, but
not of other cruciferous vegetables, were associated with lower
prevalence ofcolorectal adenornas (broccoli: P for trend, 0.007;kale: P for trend, 0.04). The OR comparing the highest to
lowest category of broccoli intake was 0.47 (95% CI, 0.30-
0.73). The corresponding OR for kale was 0.41 (95C/ CI,
0.16-1.06).Results from a model with vegetable intake as a continu-
ous variable (instead of indicator variables) agreed well with
the results from the model shown in Table 2. The ORs (and
95% CIs) per additional serving/week and the Ps for trend
were: broccoli, 0.91 (0.83-0.99), P 0.03; cabbage, 1.02
(0.91-I . 15), P = 0.74; cauliflower, 0.91 (0.79-1 .05). P = 0.20:
brussels sprouts, 1.00 (0.80-1.26), P = 0.97: and kale. 0.78
(0.62-0.99), P = 0.04.A main effect of the GSTMI null genotype was reported
earlier (23). The overall OR for adenomas with the GSTM/ null
genotype was 0.85 (95% CI, 0.65-1.10), consistent with no
GSTMI effect.
We used quartiles of vegetable intake to assess interaction
between GSTMI and vegetables, because there were too few
subjects in the reference category of “never” for all cruciferous
vegetables combined. For broccoli, quartiles produced the same
groupings as in Table 2. The overall risk of adenomas decreased
with increasing intake of cruciferous vegetables (Table 3). but
the effect was somewhat weaker than for broccoli (Table 2).
Analysis of quartiles was not possible for brussels sprouts andkale, because they were rarely eaten.
A protective effect of broccoli and cruciferous vegetables
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procarcinogens ______
I enzymes
DNA
mutagens ‘�z:::I:II:damage � neoplasms
GSTs, mEH. detoxified� NQO1. UGTs � mutagens *excretion
inhibit : � induce
broccoli * isothiocyanates ______� conjugated � excretionGSTM1 isothiocyanatesGSTP1
650 GSTMJ, Broccoli, and Colorectal Adenomas
T able 3 ORs for cob rectal a denomas by GSTMI genotype and intake of cruci ferou s vegetab les and broccoli”
GSTMI null GSTMI non-null P’ Without GSTMI stratification
QI Q2 Q3 Q4 ‘�ir�nd QI Q2 Q3 Q4 ‘�trcnd Ql Q2 Q3 Q4 �t,c,d” �
Broccoli
OR 1(8) 0.65 0.99 0.36 0.0()l 1.30 0.80 0.61 0.74 0.43 0.01 (see Table 2 for broccoli)95C/� CI - 0.37-1.1 0.54-1.8 0.19-0.68 0.70-2.4 0.50-l.() 0.24-0.88 0.40-0.99
Controls/ 36/44 85/67 5 1/6)) 77/33 36/61 67/67 85/63 70/64
Cases
Servings/ 0 0.5 I 3.7 0 0.5 1 3.7
week
Cruciferous vegetables
OR 1(X) 1.08 0.85 0.52 0.02 1.24 1.04 0.77 0.95 0.71 0.26 1(8) 0.95 0.72 0.66 0.04 0.04
95(7� Cl - 0.64-1.8 0.5()-l.4 0.29-0.93 0.76-2.0 0.84-1.2 0.51-0.98 0.72-1.1 - 0.66-1.4 0.50-1.0 0.44-1.0
Controls/ 68/65 56/58 59/49 66/32 58/75 58/58 76/59 66/63 1 26/140 1 I 4/1 16 135/1 08 132/95
Cases
Servings/ 0.6 1.8 3.2 7.3 0.6 1.7 3.3 8.1 0.6 1.7 3.3 7.8week
‘, Effects of broccoli and cruciferous vegetables Iservings/week: sum of broccoli, cabbage/cole slaw, cauliflower, brussels sprouts. kale/mustard/chard greens (as indicator
variables for quartiles)l were adjusted for smoking (indicator variables for current smoking and ex-smoking): matching variables (age in 5-year categories: date of
sigmoidoscopy in live categories total): clinic: gender: saturated fat (indicator variables for quartiles of saturated fat intake in the total population): energy intake (calories
as a continuous variable): intake of fruits and vegetables indicator variables for quartiles of fruit and vegetable intake minus the specific vegetable(s) of interest in the
model I.
1’ p for trend was calculated from entering the mean number of servings/week in each quartile (calculated separately for GSTMI null and non-null genotypes) as a categorical
variable in the model. If the P for trend was calculated from entering broccoli and cruciferous vegetable intake as an ordered (continuous) variable into the model, we
obtained values of 0.04 (null genotype) and 0.26 (non-null genotype) for broccoli and 0.02 (null genotype) and 0.38 (non-null genotype) for cruciferous vegetables.‘ P for interaction was calculated as the likelihood ratio test comparing the model with three vegetable intake � GSTMI interaction terms to the model not containing any
interaction terms. We also investigated the interaction by entering vegetable intake as a continuous variable into the model. so that we had to also enter only one interaction
term between GSTMI and vegetable intake into the model. We obtained: P for interaction, 0.30 for broccoli: and P for interaction, 0.16 for cruciferous vegetables..1 p for trend was calculated by entering the mean number of servings/week in each category as a categorical variable in the model.
(. P for trend was calculated by entering the intake of the respective vegetable as a continuous (ordered) variable into the model.
Fig. I. Proposed relationships among broccoli, isothio-
cyanates, and mutagen metabolism. Isothiocyanates in-
hibit P450 enzymes (phase I enzymes) that transform
procarcinogens to mutagens (32). Isothiocyanates also
induce phase 2 enzymes (GSTs. NQOI. ,nEH. and UGTs),
which detoxify mutagens ( 14). Glutathione transferases
GSTMI and GSTPI conjugate isothiocyanates and divert
them from the enzyme induction pathway to excretion
( 17, 18). Lack of GSTM I is hypothesized to favor the
pathway that leads to enzyme induction, thereby protect-
ing against neoplasms. GSTs, glutathione transferases;
NQOJ, NAD(P)H:quinone oxidoreductase: mEH. micro-somal epoxide hydrolase: UGTs. UDP-glucuronosyl-
transferases.
was observed only among subjects with the GSTMJ null gen-otype, as judged from statistically significant Ps for trend(based on using mean intake of vegetable in each quartile as thecategorical variable). For broccoli, the P for trend was 0.001 forsubjects with the GSTMI null genotype, compared with 0.43 forsubjects with non-null genotypes. For cruciferous vegetables,
values of P for trend were 0.02 and 0.71, respectively. Theinteraction between GSTMI genotype and vegetable intake
reached statistical significance for broccoli (P = 0.01) but notfor cruciferous vegetables combined (P 0.26). The lowest
adenoma prevalence was among subjects with broccoli con-sumption in the highest quartile and with the GSTMI null
genotype (OR, 0.36; 95% CI, 0.19-0.68).We also assessed interaction by using vegetable intake as
a continuous variable in the model. For broccoli, the Ps fortrend were 0.04 and 0.26 for the GSTMJ null and non-null
genotypes, respectively. For cruciferous vegetables, corre-sponding Ps for trend were 0.02 and 0.38. Ps for interactionbased on the continuous models were 0.30 for broccoli and 0.16for cruciferous vegetables.
Discussion
Our aim is to identify mechanisms by which cruciferous veg-
etables protect against colorectal cancer. Although fiber and
folate may contribute (26-29), these vegetables also contain
nonnutrient compounds that block cancers in laboratory rats.
Examples are 3,3’-diindolylmethane, indole-3-carbinol, and
brassinin, found in brussels sprouts, cabbage, and cauliflower
(30, 31). Here we focused mainly on broccoli, because it
contains the isothiocyanate sulforaphane, a potent inducer of
carcinogen-detoxifying enzymes (14).
People consume several milligrams of isothiocyanates or
glucosinolates per day. Protection against cancer by these corn-
pounds is attributed to inhibition of phase 1 enzymes (32) and
induction of phase 2 enzymes (Fig. I ). Induction of detoxifying,
phase 2 enzymes has been called the electrophile counterattack
response (33, 34). However, glutathione transferases GSTM1
and GSTP1 conjugate isothiocyanates, leading to excretion (17,
18, 35). Roughly half the population lack GSTM 1 due to gene
deletion. This common genetic variant led to our strategy of
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Cancer Epidemiology, Biomarkers & Prevention 651
indirectly assessing isothiocyanate levels in relation to colorec-
tal adenoma prevalence.
Subjects with high broccoli intake and the GSTMI null
genotype had the lowest adenoma prevalence. These subjectscould have higher isothiocyanate levels, due to high intake and
slow excretion. A GSTMJ interaction was observed only for
broccoli, perhaps due to the larger range of broccoli intakes
compared with other vegetables. Average broccoli intake was
1 .7 servings per week, compared with 0.97 for cabbage or cole
slaw; 0.70 for cauliflower; 0.27 for brussels sprouts; and 0.24
for kale, mustard, or chard greens.
The protective effect of broccoli could be due to other
compounds whose levels are higher in broccoli than in other
cruciferous vegetables. For example, broccoli has a higher
content of carotenoids. However, adjustment for carotenoids,
folate, vitamin C, and fiber in our previous analysis did not
substantially change protective effects of broccoli or crucifer-
ous vegetables (25).The GSTMJ null genotype conceivably could have raised
the risk of adenomas, instead of lowering the risk, through
production of less GSTM 1 for mutagen removal. However,
GSTM 1 is only one of several enzymes that detoxify electro-
philes. Lack of GSTMI may not matter for mutagen removal if
these compounds can be quickly cleared by other enzymes.
Also, GSTM1 is not inducible in the colon (36) or in hepato-
cytes (32).
Earlier reports showed increased risks of bladder and lung
cancer in smokers with the GSTMJ null genotype (37-39).
However, our previous work did not find a higher risk for
colorectal adenomas in smokers with the null genotype (23).
Analyses of broccoli and GSTMJ effects in the subgroup of
people who never smoked did not change results, again sup-
porting an interpretation of no interaction between GSTMI andsmoking for colorectal adenomas.
Strengths of the study were screening asymptomatic sub-jects for a first-time diagnosis of adenomas, a fairly large
sample size, and a response rate greater than 80% for both casesand controls. Diet effects from our study population (24, 29, 40,
4 1) agree with other published studies, supporting the validity
of our results. Weaknesses of the study were reliance on diet
questionnaires, the potential for recall bias, detection of prey-
alent rather than incident adenomas, and assessment of adeno-
mas in only the left colon. Some 50% of colon cancers occur in
the proximal segment (42). Up to 15-17% ofcontrols who have
no polyp by sigmoidoscopy may have a polyp in the right colon
(43, 44). If etiologies of left- versus right-sided adenomas are
different (45-47), then our study is an unbiased analysis of
left-sided adenomas. If etiologies are the same, our estimates of
effect would be biased toward the null.
In summary, prevalence of colorectal adenomas was low-
est among people with high broccoli intake and the GSTMI null
genotype. Slower conjugation and excretion of protective iso-
thiocyanates, due to lack of GSTM 1 enzyme, is a plausible
mechanism. Studies on enzyme variants and cancer risk usually
focus on carcinogens, like those in cigarette smoke. In contrast,
our work suggests a novel gene-environment interaction in-
volving an anticarcinogen. If confirmed, the result suggests that
people with the GSTMI null genotype may benefit more from
broccoli.
Acknowledgments
Chun-Ya Han. Bruce K. Lin, and David B. Lee provided technical assistance.
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1998;7:647-652. Cancer Epidemiol Biomarkers Prev H J Lin, N M Probst-Hensch, A D Louie, et al. prevalence of colorectal adenomas.Glutathione transferase null genotype, broccoli, and lower
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