Inclusions:
1. Age ≥ 18 years old and ≤ 80 years old \& willingness to be randomized to either diet.
2. NYHA class I - III for at least 3 months.
3. Ejection fraction ≥40% by biplane 2D, or 3D echo, or CMR
4. Echo findings of abnormal or indeterminant diastolic function or right heart catheterization (RHC) data: At rest: mean pulmonary capillary wedge pressure (PCWP) \> 15 mmHg. pulmonary vascular resistance (PVR) \< 3 Wood Units.
5. Stable medical therapy for at least 3 months as determined by the treating physician (no new cardiac or diabetic medications within 3 months of enrollment or during enrollment and dosage should be stable for 1 month prior to enrollment).
6. Dose of oral diuretics changes allowed but must be stable for 1 week prior to randomization.
7. Body Mass Index (BMI) ≥ 25 or Type 2 Diabetes Mellitus or prediabetes (fasting glucose of 100 - 125 mg/dL or glycated hemoglobin (A1C) 5.7-6.4% or metabolic syndrome.
8. Ability to participate in exercise treadmill testing.
9. Ability to sign written consent.
Exclusions:
1. Women who are pregnant, current breast-feeding, or have intention to become pregnant while in the study.
2. Known allergy or sensitivity to gadolinium-based contrast agents.
3. Implanted pacemaker, cardioverter defibrillator, Cardiac resynchronization therapy, left ventricular assist device.
4. Other metallic implants/aneurysm clips that are contraindicated in MRI.
5. Claustrophobia
6. History of severe kidney disease of estimated glomerular filtration rate (eGFR) \<30 ml/kg/1.73m2.
7. Type I diabetes.
8. History of diabetic ketoacidosis.
9. Prior diagnosis of oxygen dependent pulmonary disease.
10. Body Mass Index (BMI) \< 25.
11. Recent acute myocardial infarction or acute coronary syndrome (30 days).
12. Or recent (within 30 days) or planned (within 30 days) cardiac revascularization History of left main disease, severe triple vessel disease, coronary artery bypass graft surgery.
13. Left ventricular ejection fraction \< 50%.
14. Uncontrolled systemic systolic blood pressure (SBP)/diastolic blood pressure (DBP) hypertension (SBP \>180 or DBP \>110 mmHg).
15. Severe stenotic or regurgitant valvular heart disease, expected to lead to surgery during the study period.
16. Persistent atrial fibrillation.
17. History of uncontrolled or untreated ventricular arrhythmias.
18. Cardiovascular diseases or treatments that increase the unpredictability of or change the subject's clinical course, independent of heart failure.
19. Heart transplant or listing for heart transplant.
20. Cardiomyopathy based on infiltrative diseases (e.g. amyloidosis), accumulation diseases (e.g. haemochromatosis, Fabry disease), muscular dystrophies, cardiomyopathy with reversible causes (e.g. stress cardiomyopathy), hypertrophic obstructive cardiomyopathy or known pericardial constriction.
21. Acute decompensated heart failure requiring intravenous diuretics, vasodilators, inotropic agents or mechanical support within 1 week of screening and during the screening period prior to randomization.
22. Hemoglobin of \<9 g/dL at screening.
23. Major surgery (major according to the investigator's assessment) performed within 90 days prior to screening, or major scheduled elective surgery (e.g. hip replacement) within 90 days after screening.
24. Acute or chronic liver disease, defined by serum levels of transaminases or alkaline phosphatase more than three times the upper limit of normal at screening.
25. Gastrointestinal surgery or gastrointestinal disorder that might interfere with diet. Prior bariatric surgery allowed if weight-stable for past 3 months.
26. Any documented active or suspected malignancy or history of malignancy within 2 years prior to screening, except appropriately treated basal cell carcinoma of the skin, in situ carcinoma of the uterine cervix, or low-risk prostate cancer (subjects with pre-treatment prostate-specific antigen levels of \<10 ng/mL, and biopsy Gleason scores of ≤6 and clinical stage T1c or T2a).
27. Presence of any disease other than heart failure that results in a life expectancy of \<1 year (in the opinion of the investigator).
28. History or recurrent severe hypokalemia, potassium \< 3.0 mg/dL.
29. Current enrollment or completion within 30 days of an investigational device or drug study.
30. Chronic alcohol or drug abuse or any condition that, in the investigator's opinion, will make the subject unlikely to fulfill the study requirements or complete the trial.
31. Any other clinical condition that might jeopardize subject safety during participation in this study or prevent the subject from adhering to the study protocol.
32. Unable or unwilling to follow guidelines of assigned diet group, including inability to purchase food.
33. Unable to participate in the comprehensive diet program, including biometric data acquisition and data entry.
34. The subject cannot currently be on a low-carb diet plan. 30-day washout would be required.
35. Patient has to have stable weight over the past 3 months (± 5% total body weight). If no weight was recorded in the past 3 months, will have 1 month lead in time for wash out.
36. Refusal to consent.
