Metabolic syndrome (MS) is a cluster of risk factors for cardiovascular disease with increasing prevalence worldwide and insulin resistance is central to its pathophysiology and multi-organ deleterious effects. One of the most affected organs, the heart, undergoes a remodeling process with an increase in fibrous tissue that impairs global cardiac function. Considering that myocardial fibrosis increases myocardial stiffness, one important determinant of diastolic function, it probably contributes decisively to subclinical left ventricular diastolic dysfunction (DD) and heart failure with preserved ejection fraction in patients with MS.

Since insulin resistance is a dominant player in the pathophysiology of MS, improvement of the metabolic profile of these patients with metformin might be associated with favorable remodeling of myocardial structure and an improvement in myocardial function. Metformin is a widely used drug to treat type 2 diabetes mellitus and is considered an option in the treatment of high-risk non-diabetic patients with MS, in addition to lifestyle counseling including a healthy diet and physical activity.

In this way, we aim to: i) assess if treating non-diabetic patients with MS and DD with metformin in addition to lifestyle counseling decreases cardiac fibrosis and improves diastolic function and assess its impact in functional capacity and health-related quality of life (HRQoL); ii) evaluate if biomarkers of cardiac remodeling and inflammation are predictive factors of response to metformin treatment in these patients.

This is a prospective, randomized, open-label, blinded-endpoint (PROBE) trial (scheduled follow-up of 24 months) with 2 arms: lifestyle counseling only and lifestyle counseling plus metformin (maximum dose of 1000mg twice daily).

The primary endpoint will be change in change in mean of septal and lateral early diastolic mitral annular velocities (E') (at the end of the 24 months of follow-up).

The secondary endpoints will include a composite of major cardiovascular events; diastolic function parameters at rest; plasma levels of insulin, glucose, insulin resistance index, NTproBNP, high-sensitivity C-reactive protein, tumor necrosis factor-α (TNFα), tissue inhibitor of matrix metalloproteinase type 1 (TIMP1) and growth differentiation factor-15 (GDF-15); functional capacity; epicardial, pericardial and abdominal adipose tissue volumes, and coronary calcium score; HRQoL.

Inclusion Criteria:

• Non-diabetic adults aged between 40 and 64 years fulfilling the American Heart Association/National Heart, Lung and Blood Institute diagnostic criteria of metabolic syndrome (at least 3 of the following: waist circumference ≥102 cm (males) or ≥88 cm (females); fasting triglycerides≥150 mg/dL or on drug therapy for decreasing triglycerides; fasting HDL-cholesterol ˂40 mg/dL (males) or ˂50 mg/dL (females) or on drug therapy for increase HDL-c; systolic blood pressure ≥130 mmHg or diastolic blood pressure ≥85 mmHg or on antihypertensive drug therapy; fasting glycemia≥100 mg/dL
• Echocardiographic evidence of left ventricle diastolic dysfunction at rest (mean E'˂10,2 cm/s if 40-59 years and ˂7,2 cm/s if 60-64 years).

Exclusion Criteria:

• diagnosis of diabetes mellitus according to the American Diabetes Association criteria;
• previous diagnosis of ischemic heart disease;
• moderate or severe cardiac valvular disease;
• left ventricle ejection fraction lower than 50%
• pericardial disease;
• uncontrolled atrial or ventricular tachyarrhythmias;
• chronic kidney disease (estimated creatinine clearance lower than 60 mL/min);
• significant liver disease (aspartate aminotransferase or alanine aminotransferase equal or above 2.5 times the upper limit of normal);
• females who are pregnant, planning to become pregnant or who admit sexual activity without appropriate contraception;
• lactation;
• unable to perform cardiopulmonary exercise test;
• recent (less than 1 month) change in drug therapy.