Weight loss is a well-recognized but poorly understood non-motor feature of Parkinson’s disease (PD). Many patients progressively lose weight as the disease advances, often alongside worsening motor symptoms and quality of life. Until now, it was unclear whether this reflected muscle loss, poor nutrition, or deeper metabolic changes. New research shows that PD-related weight loss is driven mainly by a selective loss of body fat, while muscle mass is largely preserved, and is accompanied by a fundamental shift in how the body produces energy.