The SELECT Trial: Semaglutide and Cardiovascular Outcomes

SELECT enrolled 17,604 adults with obesity (BMI ≥30 kg/m²) or overweight (BMI ≥27 kg/m²) with established cardiovascular disease or high cardiovascular risk, but crucially, without type 2 diabetes or prior use of GLP-1 agonists. The trial randomised participants to semaglutide 2.4 mg weekly or placebo, with concomitant lifestyle counselling (dietary and exercise guidance) provided to all. The trial was intended to assess whether semaglutide could reduce cardiovascular events in the non-diabetic obesity population—a population previously lacking robust evidence for cardiovascular benefit from weight loss interventions.

SELECT enrolled a mean baseline weight of approximately 110 kg (BMI approximately 34.7 kg/m²) and required documented cardiovascular disease or multiple cardiovascular risk factors. The trial was designed with a median follow-up of 2.6 years, considerably longer than the STEP weight loss trials, enabling assessment of durable cardiovascular benefit. Results were reported in November 2023 at the American Heart Association conference and published in 2024.

SELECT's primary endpoint was a composite of cardiovascular death, nonfatal myocardial infarction, nonfatal stroke, and hospitalization for unstable angina. Secondary endpoints included individual components of the composite, hospitalization for heart failure, and mortality. The trial enrolled participants with mean age 61 years, approximately 55% male, with baseline obesity prevalence of nearly 100%. Approximately 73% had established cardiovascular disease (prior myocardial infarction, stroke, or coronary intervention); approximately 27% had high cardiovascular risk without prior events. No participants had type 2 diabetes at baseline, distinguishing SELECT from prior cardiovascular outcomes trials in diabetes populations (SUSTAIN-6, LEADER, etc.).

Baseline cardiometabolic parameters included mean systolic blood pressure approximately 130 mmHg, LDL cholesterol approximately 90 mg/dL (with ~80% on statins), and triglycerides approximately 135 mg/dL. These characteristics reflect a real-world cardiovascular risk population typical of heart disease clinics.

SELECT demonstrated a significant reduction in the primary composite endpoint: 6.7% of semaglutide recipients experienced the primary outcome versus 8.9% of placebo recipients (hazard ratio 0.73, 95% CI 0.66–0.82; p<0.0001). This represented a 27% relative risk reduction. The absolute risk reduction was 2.2 percentage points over a median 2.6-year follow-up period—corresponding to approximately 50 major cardiovascular events prevented per 1,000 participants treated for approximately 2.6 years. By conventional thresholds, this was a clinically and statistically significant benefit.

Individual components of the composite outcome showed benefits: semaglutide reduced nonfatal myocardial infarction (hazard ratio 0.78, 95% CI 0.65–0.94) and nonfatal stroke (hazard ratio 0.73, 95% CI 0.58–0.92). Cardiovascular death showed a non-significant trend toward benefit (hazard ratio 0.86, 95% CI 0.63–1.18). Notably, hospitalization for unstable angina was also reduced (hazard ratio 0.66, 95% CI 0.45–0.97). These benefits occurred in the absence of type 2 diabetes, differentiating SELECT from diabetes-centered outcomes trials and expanding the therapeutic rationale for semaglutide beyond glycaemic control.

SELECT participants receiving semaglutide achieved mean weight loss of approximately 10.4% from baseline versus 1.5% with placebo—a 8.9 percentage point difference. Weight loss was accompanied by improvements in systolic blood pressure (approximately 3 mmHg greater reduction with semaglutide), triglycerides (approximately 10% reduction), and LDL cholesterol (modest reductions). These secondary cardiometabolic improvements likely contribute to the overall cardiovascular benefit observed, though the relative contribution of weight loss versus direct GLP-1 signaling effects remains uncertain.

Regarding safety, gastrointestinal adverse events (nausea, vomiting, diarrhoea, constipation) occurred in approximately 25% of semaglutide recipients versus 10% with placebo. These events were generally mild-to-moderate and led to treatment discontinuation in approximately 4% of the semaglutide group (versus 2% placebo). Serious adverse events, including acute pancreatitis and gallbladder-related events, occurred at similar frequencies in both groups. Notably, heart rate increased by approximately 1.5–2 bpm with semaglutide, though serious arrhythmias were uncommon in both groups.

The SELECT trial's demonstration of cardiovascular benefit in non-diabetic obesity without acute weight loss necessarily being the sole driver suggests that GLP-1 receptor agonism may confer direct cardioprotective effects. Potential mechanisms include: (1) improved insulin sensitivity and reduced metabolic inflammation; (2) direct anti-inflammatory effects of GLP-1 signaling; (3) improved endothelial function and vasomotor tone; (4) reduction in cardiac arrhythmia risk through electrophysiological effects; and (5) modulation of atherosclerotic plaque composition. These mechanisms remain speculative, as mechanistic studies in SELECT participants were limited; however, preclinical research supports each of these pathways.

Clinically, SELECT's findings suggest that semaglutide may be considered for cardiovascular risk reduction in obese individuals with established cardiovascular disease or high cardiovascular risk, independent of diabetes status or HbA1c elevation. This represents a substantial expansion of the therapeutic indication beyond traditional diabetes and obesity management, positioning GLP-1 agonists as potential cardiovascular protective agents. However, SELECT was not powered to assess benefit in primary prevention (low-risk obese individuals), and extrapolation to that population remains speculative.