Chemosensory Impairments and Their Impact on Nutrition in Parkinson's Disease: A Narrative Literature Review.

Parkinson's disease (PD) is a neurological disorder characterized by heterogeneous symptomatology, in which the classical motor features of Parkinsonism are associated with clinically significant non-motor symptoms. Olfactory alteration, as a manifestation of PD's premotor or prodromal phase, is well known. These impairments can lead to malnutrition, decreased appetite, and depression, thereby worsening patients' quality of life. However, only a few studies clarify the mechanisms, characteristics, and clinical diagnostic and therapeutic implications of impaired taste perception. Moreover, unlike most motor features of PD, non-motor symptoms often have limited treatment options or responses. The purpose of this review is to collate and describe all relevant studies on taste and smell alterations in patients with PD and how these alterations could affect nutritional status. Our search aimed to identify English-language research articles and reviews published in peer-reviewed journals over the past two decades (2004-2024), while also including older foundational studies when relevant. Several studies show that hyposmia in PD worsens over time, potentially linked to structural changes in the brain's basal ganglia and piriform cortex. Severe hyposmia is also associated with a higher risk of dementia in PD patients and can negatively influence quality of life, affecting social interactions and nutrition. Regarding taste perception, recent studies have suggested that hypogeusia may occur even in the prodromal stage of PD, such as in patients with REM sleep disorder, although the exact mechanisms remain unclear. Additionally, research has explored the role of bitter taste receptors and their possible involvement in inflammation and α-synuclein misfolding, suggesting a link between taste dysfunction and immune system changes in PD. Attention was then focused on the gut microbiota's link to the central nervous system and its contribution to gustatory dysfunctions, as well as how the nasal microbiome influences PD progression by altering the olfactory system. Nowadays, the primary role of a correct diet in the overall treatment of PD patients is becoming increasingly important for practitioners. Diet should be included among the available aids to counteract some aspects of the pathology itself. For all these reasons, it is also crucial to determine whether these chemosensory impairments could serve as disease markers, helping to better understand the underlying mechanisms of the disease.