Slow-SPEED UK: A Double-Blind Randomised Feasibility Trial

Inclusion criteria * Age ≥ 40 years * Objective hyposmia, defined as scoring below the 15th percentile (adjusted for age/sex) on UPSIT * Ability and willingness to provide written informed consent * Proficiency in written and spoken English sufficient to complete study procedures. * Willingness and ability to attend baseline (in-person), 9-month (remote), and 18-month (in-person) assessments. * Access to telephone or internet for interim communication. Specifically, in possession of a suitable smartphone (screen size minimum 4.6 inch; Android version 9 or iOS version 15 or newer). * Physical activity threshold: During the 4-week eligibility run-in, the mean daily step count must be \<7,000 steps/day, calculated over ≥21 valid days (a valid day = ≥10 hours wear time or ≥1,000 steps). If recruitment after the first 2 months is \<60-70% of target, and subject to TSC/Sponsor approval and REC amendment, eligibility may be broadened to \<10,000 steps/day (i.e. participants averaging 7,000-9,999 steps/day become eligible). Exclusion criteria * Clinical diagnosis of PD, dementia, or other neurodegenerative conditions * Severe or unstable medical or psychiatric illness likely to impair participation * Use of agents known to alter olfaction (e.g. intranasal zinc, chronic corticosteroids) * Current enrolment in an interventional study within the past 3 months (standard research retention guidance). * Inability to complete study procedures in English, per investigator assessment * Unable to give consent * Participants not living independently in the community. Participants in nursing homes, hospitalised persons on in a non-institutionalised setting are excluded. * Subject's personal smartphone is Fitbit-incompatible i.e. Huawei P8 Lite; Huawei P9 Lite; Xiaomi Mi 6; Huawei P20 Lite * Activity level above threshold during eligibility run-in: mean daily step count ≥7,000 steps/day (or ≥10,000 steps/day if the broadened threshold is activated).