Weight loss is a common but still poorly understood feature of Parkinson’s disease. Many people living with the condition gradually lose weight as the illness progresses, often at the same time that movement problems and daily functioning worsen. Until recently, scientists were unsure why this happens. Some believed it might be caused by muscle loss, reduced appetite, or poor nutrition. However, new research suggests a different explanation. The study shows that most of the weight loss in Parkinson’s disease comes from a reduction in body fat rather than muscle. At the same time, the body appears to shift the way it produces energy, indicating bigger metabolic changes.
Although Parkinson’s disease is mainly known as a disorder of the brain, researchers increasingly believe it also affects the body’s metabolism. Many patients experience fatigue and declining nutritional status, but medical advice has usually focused on encouraging people to eat more calories. The new findings challenge this approach. They suggest that weight loss may not simply result from eating less food. Instead, the body’s normal systems for producing energy may not be working efficiently. When these systems struggle, the body may change how it fuels itself, leading to gradual fat loss even if calorie intake does not fall dramatically.
The research was led by Professor Hirohisa Watanabe from Fujita Health University in Japan, along with colleagues Dr Atsuhiro Higashi and Dr Yasuaki Mizutani. Their goal was to understand exactly what happens in the body when people with Parkinson’s disease lose weight. To investigate this, the team studied 91 people with Parkinson’s disease and compared them with 47 healthy volunteers. They analysed body composition using a technique called bioelectrical impedance analysis, which can measure fat mass, muscle mass, and other components separately. At the same time, they examined blood samples using advanced metabolic testing to track how the body processes energy.
The results revealed a clear pattern. Compared with healthy participants, people with Parkinson’s disease tended to have lower body weight and a lower body mass index. A loss of body fat mainly explained this difference. Muscle mass, however, was largely preserved, especially in the earlier stages of the disease. The prevalence of muscle loss was similar to what is normally seen in ageing adults. According to Professor Watanabe, this finding changes how doctors should think about weight loss in Parkinson’s disease. Instead of muscle wasting, the condition appears to involve a selective reduction in fat stores.
Further analysis showed that this fat loss may occur because the body’s normal energy systems are not functioning properly. The researchers found lower levels of certain metabolic compounds that are involved in processes such as glycolysis and the Krebs cycle, which normally help cells convert glucose into energy. When these pathways become less efficient, the body cannot rely on its usual carbohydrate-based energy production. As a result, it begins to activate alternative pathways. The study found higher levels of molecules linked to ketone bodies and amino acid breakdown, suggesting the body turns to fat and protein as emergency energy sources.
This metabolic shift was not the same for everyone. Patients who were thinner and those with more advanced disease tended to show higher levels of ketone-related compounds, indicating a stronger reliance on fat breakdown for energy. According to the researchers, this may mean that weight loss is a visible sign of a deeper energy problem occurring inside the body. The findings also suggest that simply increasing calorie intake may not fully solve the issue if the body cannot efficiently use glucose for energy. Understanding these hidden metabolic changes may help doctors develop better nutritional strategies and new treatments aimed at supporting the body’s energy systems in people with Parkinson’s disease.
More information: Atsuhiro Higashi et al, Metabolic profiles associated with fat loss in Parkinson’s disease, Journal of Neurology Neurosurgery & Psychiatry. DOI: 10.1136/jnnp-2025-336929
Journal information: Journal of Neurology Neurosurgery & Psychiatry Provided by Fujita Health University
