Parkinson's disease (PD) is the second most frequently appearing neurodegenerative disease. It is a progressive disease of the central nervous system (CNS) and affects around 1% of people over 60 years old. During the progression decline of substantia nigra and deficits of dopamine are observed. The diagnosis is usually based on the motor symptoms such as resting tremor, bradykinesia, muscle stiffness, and postural instability. Common intercurrent symptoms are psychiatric problems like depression or dementia (1). Pharmacotherapy, for example, L-dopa or deep brain stimulation (DBS) are usually used to reduce the motor symptoms (2).

From many years the influence of insufficient vitamin D3 levels in human is investigated. In recent publications it was proved that the deficiency of vitamin D3 may lead to generation of reactive oxygen species that influence negatively on mitochondria. That may lead to increased muscle atrophy (3,4). Deficiency of vitamin D3 may be also connected with depression, dementia or the progression of neurodegenerative diseases (5). Moreover, recently studies proved that PD patients have low concentration of serum vitamin D3 (5), increased serum homocysteine (6) and abnormalities in kynurenine pathway (7).

It has beed proved that many forms of physical activity in PD patients improves mobility, static and dynamic balance but also may reduce the non-motor symptoms (8,9).

Participants will be divided randomly into 2 groups: a control group and supplemented group.

Supplementation will last for 12 weeks.

Supplementation of vitamin D3 will be based on the participants Body Mass Index (BMI). The BMI will be measured by using the TANITA scale. The doses will be as followed: for BMI under 25 - 4000 International Units (IU)/day, for BMI between 25 and 30 - 5000 IU/day, and for BMI over 30 - 6000 IU/day. The placebo and control group will receive matching placebo treatment. Vitamin D3 as well as placebo will be in the same bottles with no labels on them.

Participants will be prompted to do 3500 steps per day in the first week of the research and finish the research with making 8000 per day. Participants will be obligated to measure the amount of steps by their phones.

There will be three meetings with the participants. On the first one there will be blood collection, participants will obtain questionnaires, about their daily activity and diet, to complete at home. Participants will have to sign the agreement of taking part in the research and perform functional tests. Also they will get their first supplementation bottles. On the second one the questionnaires will be collected, functional tests will be performed and supplementation will be replenished. On the third one the blood will be collected for the second time and functional tests will be performed again.

The questionnaire about daily activity will be International Physical Activity Questionnaire - Short Form.

The questionnaire about diet will be "Kwestionariusz QEB opracowany przez Zespół Behawioralnych Uwarunkowań Żywienia, Komitet Nauki o Żywieniu Człowieka, PAN". (Only polish version is available)

The blood will be collected into test tubes with clot activator, then centrifuged at 4000×g for 10 min at 4oC. The obtained serum samples will be stored in dark at -80oC until the analysis.

Investigators want to measure the changes in concentrations in blood of 25-hydroxycholecalciferol (25-OH D3), vitamin D3 binding protein, homocysteine, calcium, parathormone, interleukin-2 (IL-2), interleukin-6 (IL-6), interleukin-15 (IL-15), C-Reactive Protein (CRP), Tumor Necrosis Factor Alpha (TNF-α), serotonin, and the chosen markers of kynurenine pathway.

The functional tests will be assessed: 6 minute walk test (6MWT), test up&go (TUG) and 10 m walk test (10MWT). All tests will be performed in the on-phase of their usual anti-PD medications. All tests will be explained and demonstrated before they will be taken. If the first attempt will go well participant will follow to the next one. If any mistake will appear, participant will wait for a minute and try again. During every meeting the procedure of each test will be remind to the participants.

The 6MWT will be administered in a 15m line at the corridor. Participants will be instructed to go back and forth along without running the 15m line as far as they can in 6 min. The total distance will be measured after 6 min.

In TUG participants will be supposed to stand up from the chair, walk 3m, turn around, go back to the chair and sit down as fast as they can without running.

In 10MWT investigators will measure the time which takes the participants to walk through 10m form standing position.

Inclusion Criteria:

• agreement to take part in a research,
• Subthalamic nucleus deep brain stimulation (STN-DBS) treatment,
• willingness to work.

Exclusion Criteria:

• supplementation of vitamin D before the research,
• serious comorbidity (like tumour, cerebrovascular disease, cardiorespiratory compromise, forced dementia).