There are a number of nutrients that slow Parkinson’s disease in laboratory models and in humans, even if the majority of scientific clinical studies do not conclusively prove it their efficacy.
Two neurotransmitters, dopamine and serotonin, are involved in causing Parkinsonian symptoms. A deficit in dopamine is what causes the motor symptoms of Parkinson’s: the shuffling gait, uncontrollable tremors, and rigid facial and body muscles. Low levels of serotonin are a marker for the severity of the disease.
Some amino acids (protein building blocks) support dopamine production. Others improve enzyme function for dopamine synthesis. There are also a few that provide antioxidant properties for mitochondrial function.
- Acetyl-l-carnitine daily with R-alpha lipoic acid: This has been shown in a cellular model  of Parkinson’s disease and also in a rat model  of Parkinson’s disease to protect against the decay of mitochondria and improve brain function.
- Tryptophan: Because L-dopa treatment seems to down-regulate brain tryptophan levels, many patients on L-dopa get improved mood and functional abilities when given tryptophan or 5-HTP (150-300 mg twice daily), the precursors for serotonin. Serotonin neurotransmitter pathways are involved in mood, sleep, memory and cognition.
- N-acetyl-cysteine: 100-250 mg daily. This antioxidant helps regenerate glutathione and is, therefore, thought to protect against mitochondrial destruction in Parkinson’s disease. Higher doses (5,000 mg daily) can be used for a month for quick symptom reversal and then tapered.
- Tyrosine, D-phenylalanine: I have read that supplementation with the amino acids tyrosine (1-3 grams) with D-phenylalanine (100-250 mg) taken twice daily before food for four weeks was found to significantly improve rigidity, walking, speech and depression but not tremors in 15 patients with Parkinson’s disease. 
- L-methionine: You can take 2 grams daily and then gradually increase up to 5 grams a day for several months before tapering dose back down to 1 gram a day to help with symptoms.
- Taurine with magnesium: Has been reported to reduce tremors in Parkinson’s patients.
- Chelators DMSA  or EDTA: Used along with antioxidant supplements  if there is reason to suspect heavy metal toxicity from previous exposure to lead, cadmium, mercury or arsenic. There are oral forms of EDTA that are slow and safe but not yet evaluated in clinical trials.
- Coenzyme Q10: Deficiency is expected as we age and accelerated in chronic disease states. CoenzymeQ10 deficiency is found in about one-third of Parkinson’s cases . In a study of 80 patients with early Parkinson’s, doses of 1,200 mg daily of Coenzyme Q10 markedly curtailed the progression of their disease.  The patients in this study were followed for 16 months or until disability requiring treatment with levodopa had developed.
- NADH: 2.5 to 5 mg every other morning to help regenerate glutathione in the neurons of Parkinson’s disease. 
- Melatonin: 1 mg one hour before bed 5 days per week. In laboratory animals given MPP (1-methyl-4-phenylpyridinium) to induce Parkinson’s, melatonin prevented symptoms. (The toxin MPP can lead to Parkinson’s by causing oxidative destruction of the substantia nigra nerve cells.)
Much like the items I discussed here in relationship to Alzheimer’s, these phytonutrients are safe and may be useful in many cases
- Ashwagandha leaf extract: Greatly reduces physiological and neurological performance abnormalities in animal models of Parkinson’s disease in a dose-dependent fashion when treated for one week,  three weeks  and up to nine weeks. 
- Black tea (extract) and coffee: A meta-analysis of studies showed that caffeine intake is associated with a reduced risk of developing Parkinson’s disease in men.  Other study authors found that black tea extract protected and even reversed damage to neurons in rats with Parkinson’s disease and suggested that daily black tea consumption slows disease progression and delays the onset of Parkinson’s disease.  A large study reported that there were 10.5 cases of Parkinson’s disease per 10,000 person-years among non-coffee drinkers, and only 1.7 cases for those drinking more than 28 ounces.
- Rutin: This flavonoid is found in many foods including buckwheat, oranges, grapefruits, lemons, apples, mulberries, cranberries and (clingstone) peaches. In animal models of Parkinson’s disease, rutin extract protected dopaminergic neurons from oxidative stress  and Yerba mate (contains rutin) reduced Parkinson’s disease signs. 
- Ginkgo biloba extract: Has neuroprotective effects shown in animal models of Parkinson’s disease.
- Resveratrol: Slowed oxidative damage and dopamine depletion in an animal model of Parkinson’s disease.
- Mucuna pruriens: An herb that may work as an antioxidant and dopamine promoter.
Foods that can help
Try adding some foods to your diet that have tryptophan, which is converted to serotonin. These include:
- Soy products
- Vitamin B6 50-100 mg daily with zinc 30 mg daily: Enhances the effectiveness of concurrent treatment with L-dopa or Sinemet (L-Dopa and Carbidopa) in about 40 percent of patients. Best taken in the evening if dopamine precursors are taken in the morning. Also, vitamin B6 reduces tremors. A study of 60 Parkinson’s disease patients had a significant reduction of tremors (12 had complete disappearance) using 300-400
mg daily. 
- Vitamins C and E: Patient reports and small studies show the benefits of vitamin C (3,000 mg daily) and vitamin E (1,200 IU daily) supplementation along with other antioxidants used together, even delaying the time (by 5.5 years) until levodopa treatment became necessary in earliest stages of the disease in one group compared to a group without supplementation. 
- Selenium: 100 micrograms daily. This antioxidant can help increase levels of glutathione.
- Wheat germ oil (octacosanol): 1-2 oz. daily has been reported to reduce some symptoms (unless taking L-Dopa).
- Evening primrose oil: Has been reported to reduce tremors.
Other Interventions To Consider
Electro-acupuncture: Chinese researchers reported  in 2011 that 100 Hz stimulations daily at two acupuncture sites (ST36 and SP6) greatly protected the substantia nigra in a mouse model of Parkinson’s disease. They advocated electro-acupuncture as a promising therapy for treating Parkinson’s disease.
Physical exercise: Resistance training  and cardiovascular exercise  improve symptoms of Parkinson’s disease. Endurance exercise improves cognitive function in aging mice  and aging people  with Parkinson’s disease.
Mental exercise: Many studies demonstrate that older adults improve their expected decline in thinking skills with certain mental exercises  both in Alzheimer’s patients   as well as Parkinson’s disease patients.  Listening to calming music improves gait and gait-related activity,  but brain exercises in general don’t seem to improve most other symptoms in Parkinson’s patients. Helpful brain exercises include: Start a new hobby, travel, get a part-time job, attend adult community seminars and events on interesting topics, volunteer, learn a foreign language, learn a new talent or skill, or buy fun. You can find interesting mind games here and here.
 Zhang H, Jia H, Liu J, Ao N, Yan B, Shen W, Wang X, Li X, Luo C, Liu J. Combined R-alpha-lipoic acid and acetyl-L-carnitine exerts efficient preventative effects in a cellular model of Parkinson’s disease. J Cell Mol Med. 2010 Jan;14(1-2):215-25.
 Aliev G, Liu J, Shenk JC, Fischbach K, Pacheco GJ, Chen SG, Obrenovich ME, Ward WF, Richardson AG, Smith MA, Gasimov E, Perry G, Ames BN. Neuronal mitochondrial amelioration by feeding acetyl-L-carnitine and lipoic acid to aged rats. J Cell Mol Med. 2009 Feb;13(2):320-33.
 Miller AL. Dimercaptosuccinic acid (DMSA), a non-toxic, water-soluble treatment for heavy metal toxicity. Altern Med Rev. 1998 Jun;3(3):199-207.
 Flora SJ, Mittal M, Mehta A. Heavy metal induced oxidative stress & it’s possible reversal by chelation therapy. Indian J Med Res. 2008 Oct;128(4):501-23.
 Mischley LK, Allen J, Bradley R. Coenzyme Q10 deficiency in patients with Parkinson’s disease. J Neurol Sci. 2012 Jul 15;318(1-2):72-5.
 Shults CW, Oakes D, Kieburtz K, et al. Effects of coenzyme Q10 in early Parkinson disease: evidence of slowing of the functional decline. Arch Neurol. 2002 Oct;59(10):1541-50.
 Ma Y, Chen H, He X, Nie H, Hong Y, Sheng C, Wang Q, Xia W, Ying W. NAD+ metabolism and NAD(+)-dependent enzymes: promising therapeutic targets for neurological diseases. Curr Drug Targets. 2012 Feb;13(2):222-9.
 Rajasankar S, Manivasagam T, Surendran S. Ashwagandha leaf extract: a potential agent in treating oxidative damage and physiological abnormalities seen in a mouse model of Parkinson’s disease. Neurosci Lett. 2009 Apr 17;454(1):11-5.
 Ahmad M, Saleem S, Ahmad AS, Ansari MA, Yousuf S, Hoda MN, Islam F. Neuroprotective effects of Withania somnifera on 6-hydroxydopamine induced Parkinsonism in rats. Hum Exp Toxicol. 2005 Mar;24(3):137-47.
 Prakash J, Yadav SK, Chouhan S, Singh SP. Neuroprotective role of Withania somnifera root extract in maneb-paraquat induced mouse model of parkinsonism. Neurochem Res. 2013 May;38(5):972-80.
 Costa J, Lunet N, Santos C, Santos J, Vaz-Carneiro A (2010). “Caffeine exposure and the risk of Parkinson’s disease: a systematic review and meta-analysis of observational studies”. J. Alzheimers Dis.. 20 Suppl 1: S221–38
 Chaturvedi RK, Shukla S, Seth K, Chauhan S, Sinha C, Shukla Y, Agrawal AK. Neuroprotective and neurorescue effect of black tea extract in 6-hydroxydopamine-lesioned rat model of Parkinson’s disease. Neurobiol Dis. 2006 May;22(2):421-34.
 Khan MM, Raza SS, Javed H, Ahmad A, Khan A, Islam F, Safhi MM, Islam F. Rutin protects dopaminergic neurons from oxidative stress in an animal model of Parkinson’s disease. Neurotox Res. 2012 Jul;22(1):1-15.
 Milioli EM, Cologni P, Santos CC, Marcos TD, Yunes VM, Fernandes MS, Schoenfelder T, Costa-Campos L. Effect of acute administration of hydroalcohol extract of Ilex paraguariensis St Hilaire (Aquifoliaceae) in animal models of Parkinson’s disease. Phytother Res. 2007 Aug;21(8):771-6.
 Wang H, Pan Y, Xue B, Wang X, Zhao F, Jia J, Liang X, Wang X. The antioxidative effect of electro-acupuncture in a mouse model of Parkinson’s disease. PLoS One. 2011;6(5):e19790.
 Brienesse LA, Emerson MN. Effects of resistance training for people with Parkinson’s disease: a systematic review. J Am Med Dir Assoc. 2013 Apr;14(4):236-41.
 van der Kolk NM, King LA. Effects of exercise on mobility in people with Parkinson’s disease. Mov Disord. 2013 Sep 15;28(11):1587-96.
 Wrann CD, White JP, Salogiannnis J, Laznik-Bogoslavski D, Wu J, Ma D, Lin JD, Greenberg ME, Spiegelman BM.Exercise Induces Hippocampal BDNF through a PGC-1α/FNDC5 Pathway. Cell Metab. 2013 Oct 8. pii: S1550-4131(13)00377-X.
 Combs SA, Diehl MD, Chrzastowski C, Didrick N, McCoin B, Mox N, Staples WH, Wayman JCommunity-based group exercise for persons with Parkinson disease: a randomized controlled trial. NeuroRehabilitation. 2013;32(1):117-24.
 Verghese J, Lipton RB, Katz MJ, Hall CB, Derby CA, Kuslansky G, Ambrose AF, Sliwinski M, Buschke H. Leisure activities and the risk of dementia in the elderly. N Engl J Med. 2003 Jun 19;348(25):2508-16.
 Hall CB, Lipton RB, Sliwinski M, Katz MJ, Derby CA, Verghese J. Cognitive activities delay onset of memory decline in persons who develop dementia. Neurology. 2009 Aug 4;73(5):356-61.
 Wilson RS, Mendes De Leon CF, Barnes LL, Schneider JA, Bienias JL, Evans DA, Bennett DA.
Participation in cognitively stimulating activities and risk of incident Alzheimer disease. JAMA. 2002 Feb 13;287(6):742-8.
 Naismith SL, Mowszowski L, Diamond K, Lewis SJ. Improving memory in Parkinson’s disease: a healthy brain ageing cognitive training program. Mov Disord. 2013 Jul;28(8):1097-103. doi: 10.1002/mds.25457. Epub 2013 Apr 29.
 de Dreu MJ, van der Wilk AS, Poppe E, Kwakkel G, van Wegen EE. Rehabilitation, exercise therapy and music in patients with Parkinson’s disease: a meta-analysis of the effects of music-based movement therapy on walking ability, balance and quality of life. Parkinsonism Relat Disord. 2012 Jan;18 Suppl 1:S114-9. doi: 10.1016/S1353-8020(11)70036-0.