Evidence from investigators' group has shown that chronic obstructive pulmonary disease (COPD) patients have impairment of antioxidant defenses which are caused by a defect in activity of Nrf2. This trial focuses on sulforaphane, a derivative of cruciferous vegetables, which is a potent stimulator of Nrf2 activity. The investigators want to investigate whether ingestion of sulforaphane by COPD patients will increase Nrf2 activity and expression of downstream antioxidants. Accordingly, the investigators are conducting a placebo-controlled randomized proof of principle trial of two oral doses of sulforaphane, 25 and 150 micromoles, for 4 weeks in 90 COPD patients. The investigators' goal is to establish a safe and tolerable dose of sulforaphane that effects in vivo antioxidants via Nrf2, then the investigators will have a novel candidate treatment for longer-term efficacy trials.