Using isothiocyanate excretion as a biological marker of Brassica vegetable consumption in epidemiological studies: evaluating the sources of variability.

OBJECTIVE Brassica vegetable consumption (e.g. broccoli) leads to excretion of isothiocyanates (ITC) in urine. We evaluated the consistency of ITC as a biomarker for dietary Brassica vegetable consumption across the types of vegetables and methods of preparation used in Western societies, and across consumption levels. DESIGN A single-armed behavioural intervention with duplicate baseline assessment and post-intervention assessment. Urinary ITC excretion and estrogen metabolites were measured from 24-hour urine samples. Dietary intake was measured by a 24-hour recall. SETTING The behavioural intervention facilitated daily Brassica intake among participants by providing peer support, food preparation instruction, guided practice in a teaching kitchen, and other information. SUBJECTS Thirty-four healthy free-living postmenopausal women who recently had a negative screening mammogram at the University of Massachusetts Medical Center. RESULTS Urinary ITC excretion and total Brassica intake followed the same pattern over the intervention. The ITC biomarker significantly predicted Brassica intake when Brassica consumption averaged about 100 g day-1, but not when Brassica consumption averaged about 200 g day-1. Urinary ITC levels were somewhat higher when more raw vegetables were consumed as compared to lightly cooked vegetables, while the types of Brassica consumed appeared to have only a small, non-significant effect on urinary ITC levels. CONCLUSION Urinary ITC excretion would be a good exposure biomarker among populations regularly consuming a vegetable serving/day, but may be less accurate among populations with greater intake levels or a wide range of cooking practices.