The Brain-Muscle Axis in Neurodegeneration: Linking Dementia, Sarcopenia, and Exercise-Mediated Neuroprotection.

Age-related dementia is commonly associated with several secondary complications such as sarcopenia. To date, the correlation between dementia and sarcopenia is not completely understood. This review summarizes the correlation between dementia and sarcopenia using human clinical and observational studies. We also summarize the impact of exercise on these two conditions. We performed a narrative search with systematic elements using PubMed, Embase, Web of Science, and Google Scholar. In this review, we included studies that performed dementia assessments using appropriate methods (e.g., Mini-Mental State Examination, neuropsychological testing, brain MRI, or clinical dementia rating scales) and validated sarcopenia criteria (e.g., European Working Group on Sarcopenia in Older People). We included a total of nine studies consisting of 5217 participants (mean age 63.1-87.6 years). Of these, participants had dementia-only or mixed cognitive cohorts. Depending on diagnostic criteria, the prevalence of sarcopenia among individuals with dementia ranged from 6.8% to 68.8%. Most of the studies showed a positive association between dementia and sarcopenia. Interestingly, the higher the cognitive impairment, the higher the physical decline and signs of sarcopenia. As expected, any kind of physical activity (e.g., aerobic exercise, walking, and Tai Chi) improves cognitive function and brain health. Overall, the current literature supports that dementia is associated with increased risk of sarcopenia, supporting a brain-muscle axis in neurodegeneration. Physical activity appears to be a key modifiable factor with benefits for both cognitive and musculoskeletal health. Further longitudinal and mechanistic studies are needed to clarify causality and underlying pathways.