I sat down with Dr. Courtney Conley and what she told me stopped me cold…your walking speed can predict dementia up to seven years before the first cognitive symptom appears, which is why scientists are arguing it should be added to blood pressure and temperature as the sixth vital sign. We got into the mechanism behind it: BDNF, hippocampal neurogenesis, and why 4,000 steps a day can cut dementia risk by roughly 50% and I walked away convinced this is the most under-prescribed medicine of the 21st century.