Exercise in Alzheimer's Disease: Functional Biomarkers

Alzheimer's disease (AD) is the leading cause of dementia worldwide, affecting more than 50 million people, a figure projected to exceed 150 million by 2050, with a high economic and social burden that increases with disease severity. Beyond the accumulation of amyloid-β and hyperphosphorylated tau in the brain, AD involves systemic alterations even in early stages, including the progressive loss of muscle mass and strength (sarcopenia), chronic low-grade inflammation and disruption of the gut-brain axis. In this context, physical exercise has emerged as a high-impact adjuvant therapy capable of inducing relevant neurobiological and systemic adaptations. Among the available training modalities, velocity-based resistance training (i.e., controlling the execution velocity of each repetition to individualize the training load) has been shown to be safe and effective in older adults. However, the effects of executing resistance exercises at high velocity with moderate loads (e.g., 40-60% of one-repetition maximum \[1RM\]) versus moderate velocity with high loads (e.g., 60-80% 1RM) remain unexplored in people with AD. These exercise-induced adaptations can be monitored through circulating biomarkers that capture different biological responses: brain-derived neurotrophic factor (BDNF), a key mediator of neuronal survival and synaptic plasticity; the testosterone/cortisol ratio, an indicator of anabolic-catabolic balance; phosphorylated tau 217 (p-tau217), one of the most specific plasma markers of AD pathology; and Klotho, a protein linked to healthy aging with neuroprotective and anti-inflammatory functions. In parallel, the gut microbiome has gained prominence as a key modulator of the muscle-gut-brain axis. People with AD show reduced microbial diversity, an expansion of pro-inflammatory species and a reduction of short-chain fatty acid-producing bacteria, which compromise intestinal and blood-brain barrier integrity and amplify systemic inflammation. Although animal models consistently show that exercise promotes beneficial taxa and is associated with cognitive improvements, human studies have focused mainly on healthy older adults, and evidence in people diagnosed with AD is very limited. Moreover, molecular changes should translate into clinically relevant benefits. The World Health Organization's intrinsic capacity framework (cognition, locomotion, vitality, psychological and sensory domains) offers an integrative approach linking biological mechanisms with functional outcomes, together with sarcopenia, frailty, functional independence and caregiver burden. Therefore, this randomized controlled trial will compare two velocity-based resistance training modalities with usual care in older adults with mild-to-moderate AD, including a cognitively healthy reference group, to determine which modality has the greatest impact on the muscle-gut-brain axis, biological aging and intrinsic capacity, and whether these effects persist after detraining.