Parkinson's disease is a progressive condition that harms nerve cells of the brain (neurodegeneration). Current treatments for Parkinson's disease (including levodopa and deep brain stimulation) improve certain symptoms but are not thought to improve the underlying neurodegenerative disease process (they are not a "cure"). The cause of Parkinson's disease is unknown. However, some evidence suggests that tiny structures in the investigators cells called "mitochondria" might be involved. Mitochondria are the powerhouses that produce fuel for the investigators cells. Failure of these 'powerhouses' to supply the energy needs of certain nerve cells might lead to Parkinson's disease. Preliminary evidence suggests that a food called 'ketones' might be able to enhance the function of mitochondria and improve Parkinson's disease symptoms and possibly even the neurodegenerative process. In this study, the investigators would like to investigate this possibility by giving patients with Parkinson's disease dietary supplements of 'ketone esters' in a drink. The investigators will then assess if this improves symptoms of Parkinson's disease.

The study design is a prospective, double blinded, randomised, controlled trial.

Inclusion Criteria:

• Patients with Primary Parkinson's disease fulfilling UK Brain Bank criteria
• Age of onset of Parkinson's disease symptoms > 40 years old
• Duration of symptoms over 2 years

Exclusion Criteria:

• Dementia
• Active psychosis
• Deep brain stimulation or apomorphine infusion
• Severe motor fluctuations
• Significant metabolic or uncontrolled medical cormorbidity