The ageing brain runs on its blood supply.
Around forty-five percent of dementia is potentially preventable, and blood pressure and movement are the biggest levers, because the most common dementia is the most vascular one.
Most of it is not written in stone.
Resting blood flow to the brain falls slowly across adult life, and the small vessels stiffen and leak as the years stack up. The most common forms of decline are the most vascular ones, and they track the things you can measure and move. Around forty-five percent of dementia is potentially preventable through modifiable risks, led by blood pressure and physical activity.1
Lower the supply or raise the inflammation and a brain ages faster; protect the vessels and you buy years. The levers are on What helps.
The first light to go out.
When blood flow to the brain drops, the hippocampus is the first place to feel it. It is the structure that turns today into a memory you keep, and it is the worst-defended ground in the brain. Its CA1 neurons start dying after about five minutes without flow, while the cells next door hold on.2 It runs the most expensive job in the head, laying down new memories, on the thinnest supply there is: roughly a third fewer capillaries than the cortex, a weak on-demand top-up, and for about half of us no real backup line.3 Built low on reserve, it goes first.
The brain does not switch off all at once. It fails in a fixed order: protein-making slows, then signalling goes silent, and only at the lowest flow do the cells die.4 Between silence and death sits the middle ground that matters most: neurons quiet, yet still alive and normal on a scan. The lights are on, but nothing is being saved.
The established order brain tissue fails in as blood flow drops, mapped in research on ischemia, tissue starved of blood. The death step at the floor is near-total flow loss, far below the everyday dips this page is about. The shaded band is the recoverable window the diagram marks.
Researchers find the same pattern in every setting they have measured. Lower hippocampal blood flow goes with worse memory in heart failure, in Alzheimer’s, in ageing, and after COVID alike.5 Whether low flow is the whole cause or only part of it barely matters here, because the thing you do about it is the same either way.
You are two people. One lives each moment. The other keeps it, and is the only one who will remember the day happened at all.6 Every day still happens to the first. But a day that is never written down leaves the second nothing: lived once, then gone. Your sense of being one continuous person is built from that saved thread,7 so when the saving fails, what thins out is not just your recall. It is the felt length of your life. No one can put a number on the years, and total amnesia is rare. What is common is quieter: days that blur, weeks with no landmark, a whole year that afterward feels like it barely happened.
The hours were spent. Fewer of them were kept.
The same fragility cuts the other way. Because the hippocampus runs on blood, everything that protects blood flow protects memory. Twenty minutes of aerobic exercise raises hippocampal flow by about ten to twelve percent.8 Sleep does the saving itself: deep sleep is when the day gets filed, and one bad night drops your ability to form new memories by about forty percent.9 Steady blood pressure cuts the risk of mild cognitive impairment.10 You cannot regrow the hippocampus, but you can defend its supply.
Four numbers behind the canary: how little stopped flow it takes, how thin the supply already is, what one lost night costs, and what twenty minutes returns.
The conditions of the ageing brain.
From the most vascular dementia to whether your own memory is ageing normally. Each carries its verdict and evidence.
Vascular dementia the most vascular
Vascular cognitive impairment is the dementia most directly tied to blood flow: stiffened, leaky small vessels dysregulate perfusion, the white matter is injured, and the resulting white-matter lesions predict future dementia, raising the risk of the vascular kind by about 73 percent across a 36-study review of 19,040 people.11 On its own it is the second most common dementia, roughly 15 to 20 percent of cases, but the true vascular contribution is far larger once mixed pathology is counted, since vascular damage turns up in up to about half of Alzheimer brains.12 Because the cause is vascular, it is preventable: in SPRINT-MIND, targeting systolic blood pressure under 120 slowed white-matter-lesion growth and cut mild cognitive impairment.1310 Two caveats: SPRINT-MIND stopped early and never proved a drop in full dementia by itself, and since most older brains carry mixed pathology, treating blood pressure lowers risk and slows the damage without promising any one person escapes dementia.
Alzheimer’s disease strong case
In Alzheimer’s, low brain blood flow appears early. ASL and SPECT imaging show reduced perfusion in the posterior cingulate, precuneus and temporoparietal cortex, often before atrophy or symptoms appear, with parietal flow already down by roughly a quarter at the mild-cognitive-impairment stage.1415 The small vessels and the blood-brain barrier start leaking early, beginning in the hippocampus, and this damage predicts later cognitive decline even when amyloid and tau still look normal, especially in carriers of the APOE4 gene.1617 Controlling vascular risk is one reason the 2024 Lancet Commission estimates that addressing modifiable factors, several of them vascular, could prevent about 45 percent of dementia.1 Low flow is part cause and part consequence, because the brain throttles supply as neurons fail, and Alzheimer’s has many drivers beyond its vessels.18
Dementia with Lewy bodies a diagnostic marker
In DLB, blood flow and metabolism drop in the occipital lobes at the back of the brain, often while a patch of posterior cingulate stays relatively spared (the cingulate island sign). That back-of-brain pattern is the single most useful feature separating DLB from Alzheimer’s on perfusion SPECT and FDG-PET, and it is written into the formal diagnostic criteria.1920 But the low flow is a downstream consequence of alpha-synuclein (Lewy body) pathology, not a vascular cause, and the occipital cortex itself is largely intact, functionally silenced by lost inputs rather than destroyed, which is why dopamine-transporter imaging is a far stronger marker than blood flow.21 Because DLB and Parkinson’s disease dementia are the same synucleinopathy, their imaging patterns are hard to tell apart.22
Parkinson’s disease an amplifier
Parkinson’s is primarily a disease of dopamine-making cells and misfolded alpha-synuclein, but the brain’s blood supply gets pulled in. Imaging shows the cortex running underperfused, strongest toward the back of the brain, and this deepens once Parkinson’s progresses to dementia.2324 On top of this, about one in three people with Parkinson’s has orthostatic hypotension, so standing drops their blood pressure and pulls brain blood flow down with it, causing dizziness, foggy moments and falls.2526 Comorbid small-vessel damage adds to it, and regular aerobic exercise has the best evidence for protecting both flow and function.27 One caveat: in Parkinson’s the perfusion changes mostly follow the neurodegeneration and feed back on it, rather than starting it.
Normal cognitive ageing real but modest
Resting blood flow to the brain drops slowly across adult life, roughly half a percent a year, with the frontal cortex and grey matter losing the most. That compounds to something like a fifth lower flow by the seventies, alongside a drop in vascular reserve.2829 That slow decline tracks the hallmark change of normal ageing, slower processing speed, and cardiovascular fitness blunts it, though the imaging evidence that exercise raises flow is positive but patchy by region.3031 No one yet knows whether lower flow drives the cognitive slowing or only mirrors an ageing brain that asks less of its blood.32
Keep your brain better supplied.
Most research on cerebral blood flow never reaches the people it could help. We read it and send only what changes what you can do: a new way to raise your own blood flow, or a finding that moves the advice on this page.