When compared to controls, fasting individuals with PD exhibited a significant reduction in the concentrations of total ghrelin (SMD: −0.72, 95% CI −1.22, −0.22; Z = 2.84; P = 0.005) and active ghrelin (SMD: −0.42, 95% CI −0.71, −0.13; Z = 2.82; P = 0.005). The gene discussed is GHRL; the disease is Parkinson disease.