“Regular exercise physically restructures three separate systems in the brain: it grows memory-related tissue (hippocampus), maintains a healthy blood vessel network, and influences how the brain's immune cells (microglia) manage inflammation, changes that occur independent of weight loss or muscle gain.”

All four studies cited in the post exist and are accurately titled. The hippocampus study is real: exercise training increased hippocampal volume by 2%, effectively reversing age-related loss in volume by 1 to 2 years, and increased hippocampal volume was associated with greater serum levels of BDNF , in a single-blind randomized controlled trial with 120 community-dwelling older adults . The brain-volume study is also real. Fifty-nine healthy but sedentary community-dwelling volunteers aged 60 to 79 participated in a 6-month randomized clinical trial, with half in an aerobic training group and half in a toning and stretching control group, plus twenty young adults as MRI controls. The microglia study is real and is a mouse study. It provides single-cell RNA sequencing analyses of exercise and ageing effects in the mouse hippocampus, showing that exercise has a profound and selective effect on aged microglia, reverting their gene expression signature to that of young microglia. Allowing 18-month-old mice access to a running wheel also largely prevented or reverted T cell presence in the ageing hippocampus, highlighting the impact of exercise on aged microglia and peripheral immune cell presence in the ageing female mouse brain. However, the broader human literature on hippocampal volume is not settled. Hippocampal volume increase in response to aerobic exercise has been consistently observed in animal models, but the evidence from human studies is equivocal; a systematic search identified 4398 articles, of which 14 were eligible, and a random-effects meta-analysis showed no significant effect of aerobic exercise on total hippocampal volume across the 737 participants. The results provided meta-analytic evidence for exercise-induced volumetric retention in the left hippocampus, suggesting exercise may be useful for preventing age-related hippocampal deterioration. A later meta-analysis notes the heterogeneity directly: one trial reported a 1 to 2% increase in hippocampal volume after 12 months of aerobic training whereas another reported 1.9% atrophy, and two other studies showed no differences after 12 months . The flagship hippocampus study drew a published methodological objection. Critics noted that although the aerobic exercise group improved on the memory task, so did the stretching control group in whom hippocampal volume decreased, further undermining any assumed link between hippocampal volume and improved memory, and argued that the title and abstract were misleading and a major overstatement of the findings. The fourth study cited by the post does not address brain vasculature at all. It concerns systemic blood pressure: previous meta-analyses showed mean reductions of 6.0 to 12.3 mmHg in systolic and 3.4 to 6.1 mmHg in diastolic blood pressure in response to aerobic training in hypertensive individuals, and aerobic training of moderate intensity is the primary modality recommended in the management of hypertension . On the vascular mechanism specifically, the human evidence is mechanistic and indirect. A systematic review notes multiple pathways in which exercise training might benefit cerebrovascular function, including improvements in redox control and decreased inflammation contributing to increased endothelial function and cerebral angiogenesis , and a separate trial notes that in middle-aged animals, aerobic exercise enhances vascular growth factor production and promotes angiogenesis .