GLUTATHIONE TRANSFERASE NULL GENOTYPE, BROCCOLI, AND LOWER PREVALENCEOF COLORECTAL ADENOMAS

Cruciferous vegetables, especially broccoli, may prevent cancer throug h anticarcinogenic compounds. For example, broccoli contains isothiocy anates that induce carcinogen-detoxifying enzymes. Glutathione transfe rase enzymes conjugate isothiocyanates, leading to excretion. We hypot hesized that broccoli consumption in combination with the glutathione transferase M1 (GSTM1) null genotype would be associated with a lower prevalence of colorectal adenomas because of higher isothiocyanate lev els. We used a case-control study of mainly asymptomatic subjects aged 50-74 years who underwent a screening sigmoidoscopy at either of two Southern California Raiser Permanente Medical Centers during 1991-1993 , Cases (n = 459) had a first-time diagnosis of histologically confirm ed adenomas detected by flexible sigmoidoscopy. Controls (n = 507) had no polyp detected. Subjects had a 45-min in-person interview for info rmation on various risk factors and basic demographic data and complet ed a 126-item, semiquantitative food frequency questionnaire. Blood sa mples were used for GSTM1 genotyping. Subjects with the highest quarti le of broccoli intake (an average of 3.7 servings per week) had an odd s ratio of 0.47 (95% confidence interval, 0.30-0.73) for colorectal ad enomas, compared with subjects who reportedly never ate broccoli. When stratified by GSTM1 genotype, a protective effect of broccoli was obs erved only among subjects with the GSTM1 null genotype (P for trend, 0 .001; P for interaction, 0.01). The observed broccoli-GSTM1 interactio n is compatible with an isothiocyanate mechanism.