In comparison with control diets, plant-based diets improved Homeostatic Model Assessment for Insulin Resistance (HOMA-IR) (−0.97, 95% confidence interval (CI) (−1.67, −0.27), p = 0.007) and fasting insulin (−4.13 μU/mL, 95% CI (−7.22, −1.04), p = 0.009) in people with overweight/obesity. The gene discussed is INS; the disease is obesity due to melanocortin 4 receptor deficiency.