Participants will have baseline and follow-up visits which include collecting demographic data, history of medication use, symptoms, anthropometrics as well as a physical exam. Left ventricular ejection fraction, volumes, and regional function will be measured using a standardized echocardiogram and cardiac MRI at baseline and after six months of supplementation. At this time, blood markers of thiamin status, oxidative stress (F2 isoprostanes) and neurohormonal activation (norepinephrine and BNP) will also be measured. Participants will also complete a six-minute walk test, a 3-day dietary record, the Living with Heart Failure quality of life instrument and a 24-hour urine collection (to measure urinary thiamin excretion) at both baseline and follow-up visits. Compliance will be measured using returned pill counts and verified by measuring plasma thiamin levels which respond rapidly to thiamin supplementation.