Testing the ability of oral nicotinamide riboside to replenish heart tissue levels of oxidized nicotinamide adenine dinucleotide using heart biopsies taken at the time of surgery.

INTERVENTION: Determining if oral supplementation of the NAD+ precursor nicotinamide riboside (NR) at a dose of 1g twice per day is sufficient to replenish left ventricular myocardial NAD+ levels and to improve mitochondrial respiration. Supplement Details: Generic name: Nicotinamide riboside (NR). Supplier: Chromadex. Dose administered: 1g twice/day of NR. Duration of administration: one month. Mode of administration: oral capsule. Compliance strategies: drug tablet return, verbal confirmation CONDITION: Cardiovascular ‐ Other cardiovascular diseases Heart failure with preserved ejection fraction (HFpEF); ; Heart failure with preserved ejection fraction (HFpEF) PRIMARY OUTCOME: NAD+ levels in left ventricular myocardial biopsy retrieved at the time of cardiothoracic surgery, using our established NAD+ assay on liquid chromatography mass spectrometry[One month after commencing treatment] SECONDARY OUTCOME: NAMPT mRNA levels in left ventricular myocardium tissue biopsy, retrieved at the time of cardiothoracic surgery, measured using qRT‐PCR ; ; [one month after commencing treatment] INCLUSION CRITERIA: 1. All participants who have equal to or greater than Grade 2 diastolic dysfunction 2. Male and female 3. Adults equal to or greater than 18 years of age 4. Concomitant diseases: all patients eligible regardless of concomitant disease. Except for end‐stage renal disease (CKD equal to or greater than 4) or end‐stage liver disease or malignant cancer. 5. Willingness to provide informed consent and willingness to participate. 6. Agreement of the treating cardiothoracic surgeon.