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References: Parkinson's, Alzheimer's, and the Gut

  1. Parkinson's Disease - National Institute of Neurological Disorders and Stroke (NINDS) - The US government's detailed guide to Parkinson's symptoms, dopamine neuron loss (60 to 80% or more lost before symptoms), Lewy bodies, risk factors, and treatment with carbidopa-levodopa. Source for most of Sections 14.3 and 14.10.

  2. Parkinson's Disease: Causes, Symptoms, and Treatments - National Institute on Aging (NIA) - A shorter, family-friendly overview. It notes that many people had sleep problems, constipation, and loss of smell before stiffness and tremor began.

  3. Alzheimer's Disease Fact Sheet - National Institute on Aging (NIA) - Explains amyloid plaques, tau tangles, and how the disease progresses. Good background for Section 14.11. (The chapter's 2026 count of Americans living with Alzheimer's comes from the Alzheimer's Association's 2026 Facts and Figures report, linked in the text.)

  4. Definition and Facts for Constipation - National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) - Defines constipation and shows how common it is (about 16 in 100 adults, and about 33 in 100 adults aged 60 and older). A helpful reminder that constipation alone is not a warning sign of Parkinson's.

  5. Constipation preceding Parkinson's disease: a systematic review and meta-analysis - Adams-Carr KL, et al. Journal of Neurology, Neurosurgery & Psychiatry, 2016 - Pools 9 studies of 741,593 people and finds that constipation roughly doubles the odds of a later Parkinson's diagnosis and can come more than 10 years earlier.

  6. Idiopathic Parkinson's disease: possible routes by which vulnerable neuronal types may be subject to neuroinvasion by an unknown pathogen - Braak H, Rüb U, Gai WP, Del Tredici K. Journal of Neural Transmission, 2003 - The paper that proposed a trigger might enter through the gut and travel up the vagus nerve, comparing the spread to a falling row of dominoes. The abstract is readable on its own.

  7. Brain-First versus Gut-First Parkinson's Disease: A Hypothesis - Borghammer P, Van Den Berge N. Journal of Parkinson's Disease, 2019 - Proposes the two subtypes modeled in this chapter's MicroSim and explains which early symptoms each predicts. Open access.

  8. Transneuronal Propagation of Pathologic α-Synuclein from the Gut to the Brain Models Parkinson's Disease - Kim S, et al. Neuron, 2019 - The mouse study analyzed in this chapter's claim box: misfolded alpha-synuclein injected into the gut spread to the brain, and cutting the vagus nerve blocked it.

  9. Vagotomy and Parkinson disease: A Swedish register-based matched-cohort study - Liu B, et al. Neurology, 2017 - The large Swedish study used in Your Gut Check. It found no overall link between vagotomy and Parkinson's, with only a hint of lower risk after truncal vagotomy. Free full text.

  10. Appendectomy and risk of Parkinson's disease: a systematic review and meta-analysis - Chin HL, Tsang YS, Shi H. Frontiers in Neurology, 2025 - The newest pooled analysis of 9 studies, finding no overall link between appendix removal and Parkinson's risk. (A correction was published in 2026.)

  11. Gut microbiome alterations in Alzheimer's disease - Vogt NM, et al. Scientific Reports, 2017 - The small study comparing 25 people with Alzheimer's to 25 matched controls. A good example of an interesting correlation that can't show cause. Open access.

  12. Assessment of heterogeneity among participants in the Parkinson's Progression Markers Initiative cohort using α-synuclein seed amplification: a cross-sectional study - Siderowf A, et al. Lancet Neurology, 2023 - The 1,123-person study of the spinal fluid seed test in this chapter's At the Bedside section. Note the lower positive rate in people with LRRK2-linked Parkinson's and the authors from Amprion, the test's developer. (The book #1 refresher on the aging brain is linked in Section 14.2.)