The Vascular Brain

Your brain is about two per cent of your body weight and takes roughly twenty per cent of your cardiac output. It has almost no capacity to store energy. It cannot tolerate more than a few minutes without perfusion. Every thought you have is paid for, in real time, by blood arriving through a network of vessels that gets progressively smaller until it is delivering oxygen through walls one cell thick.

This makes the brain the most perfusion-dependent organ in the body. And it means the single most important thing you can do for your brain in midlife has nothing to do with your brain. It has to do with your arteries.

The silent damage

The damage that matters here is the slow injury to blood vessels caused by the classic cardiovascular risk factors: high blood pressure, elevated cholesterol, diabetes and insulin resistance, and smoking. These are usually discussed in terms of the heart, but they act on blood vessels everywhere, and the brain’s vessels are among the most vulnerable. The same processes that fur up a coronary artery also stiffen, narrow and damage the vessels supplying the brain.

We tend to picture the result as stroke — sudden, dramatic, obvious. Stroke matters, but it is not the main event in dementia prevention.

The main event is small vessel disease, and it is almost entirely silent. Over years, these risk factors damage the tiny penetrating arterioles that supply the deep white matter. Those vessels stiffen, narrow, and leak. The white matter they supply becomes chronically underperfused. On MRI this shows up as white matter hyperintensities — bright patches that radiologists often report, almost apologetically, as “in keeping with age”.

They are not simply in keeping with age. They are in keeping with a lifetime of vascular risk — the blood pressure, the cholesterol, the glucose, the cigarettes — much of which was modifiable.

White matter is the wiring. It carries signal between the processing regions of the cortex. When it degrades, the brain does not lose knowledge — it loses speed and efficiency. Which is why the cognitive signature of small vessel disease is slowed processing, poorer attention, and reduced executive function: precisely the pattern most people file under normal ageing, as I argued in the last article.

There is a second mechanism worth knowing about. Vascular disease and Alzheimer’s pathology are not separate tracks. Impaired perfusion appears to reduce the brain’s capacity to clear amyloid, and vascular injury lowers the threshold at which a given amount of Alzheimer’s pathology produces clinical symptoms. In autopsy series, most dementia in older people is mixed — vascular damage and neurodegeneration together. The vascular component is the part we know how to treat.

Blood pressure: the evidence

Blood pressure is the most studied modifiable risk factor in this field, and two things about it stand out.

The first is timing. Midlife hypertension is a far stronger predictor of later dementia than late-life hypertension. Moderate-quality evidence puts midlife hypertension at a 1.19 to 1.55-fold excess risk of cognitive disorders, and the window of intervention appears to peak in the forties. This is the classic pattern of this whole series: the damage is done decades before the symptoms.

It also means that treating blood pressure at seventy-five, while worthwhile for other reasons, is not where the dementia prevention lever sits. The lever sits at forty-five.

The other thing is how much the evidence has firmed up. For years the honest answer was that the observational data was strong but the randomised data was equivocal. SPRINT MIND, the large trial designed to test this, randomised participants to a systolic target below 120 versus below 140. It found a significant reduction in mild cognitive impairment — a hazard ratio of 0.81 — but the reduction in probable dementia did not reach significance. The trial had been stopped early because the cardiovascular benefit was so clear, which left it underpowered for dementia.

That gap has now largely closed. The China Rural Hypertension Control Project randomised 326 villages and nearly 34,000 people aged over forty with uncontrolled hypertension to either usual care or intensive management targeting below 130/80, delivered by trained non-physician health workers. Over four years the intervention group’s systolic pressure fell 22 mmHg further than usual care, and the results published in Nature Medicine last year showed a 15 per cent reduction in all-cause dementia and a 16 per cent reduction in cognitive impairment.

That is randomised evidence that lowering blood pressure lowers dementia risk. It is the first trial of its size to demonstrate it cleanly, and it was achieved with cheap, generic drugs delivered by village health workers.

I have written about home measurement and what the targets actually mean in more detail elsewhere on the site.

Cholesterol: the new entry

The 2024 Lancet Commission added high LDL cholesterol to its list of modifiable dementia risk factors for the first time, and gave it a population attributable fraction of seven per cent — putting it, alongside hearing loss, among the largest single contributors globally.

The evidence behind that decision came from cohort studies involving more than a million participants, a Mendelian randomisation meta-analysis of twenty-seven studies, and pooled UK cohort data in adults under sixty-five showing that each 1 mmol/L increase in LDL was associated with roughly an eight per cent increase in all-cause dementia incidence.

The Mendelian randomisation element matters. It uses genetic variants that affect lifetime LDL levels to approximate a natural randomised trial, which gets around much of the confounding that plagues observational lipid research. It is the closest thing we have to causal evidence outside a trial.

Note again the age qualifier: the association is strongest for LDL measured in midlife, from around age forty. Same window.

The commission’s recommendation is to detect and treat high LDL from midlife onward. If your LDL is elevated and you are in your forties or fifties, this is the decade where treating it changes your trajectory.

The rest of the vascular picture

Three other items deserve a mention.

Diabetes and high blood glucose are among the most important of all, and they damage the brain through more than one route. Part of that damage is vascular — chronically elevated glucose injures the same small vessels as hypertension and cholesterol, which is why it belongs on this list. But glucose and insulin also affect the brain in ways that go beyond the vascular, and that fuller story is the subject of the next article in this series.

Atrial fibrillation raises dementia risk substantially, and not only through overt stroke. Silent cerebral emboli and beat-to-beat variability in cerebral perfusion both appear to contribute. If you have palpitations, an irregular pulse, or a smartwatch that has flagged possible AF, that is worth investigating properly rather than ignoring.

Smoking remains on the Lancet list and works largely through this same vascular pathway. There is nothing novel to say about it except that it is the single highest-yield thing a smoker can do for their brain.

What this means

There is a line in the lipid literature that puts it as well as anything: what is good for the heart is good for the brain.

I would go further. The vascular contribution to dementia is the part of the problem we understand best, measure most easily, and treat most effectively. We have cheap generic drugs with decades of safety data. We have a randomised trial in 34,000 people showing it works. And the intervention window is wide open in the exact decades when most people assume there is nothing to worry about.

If you do nothing else from this series, do this part.

Practical steps for this week

Measure your blood pressure properly. A home monitor, seated, arm supported, twice each morning and evening for seven days, discarding day one. The average is what matters, not any single reading. Bring it to your GP.
Know your LDL — and ask for an ApoB. If you have never had your cholesterol checked, or not for years, get it done. ApoB is a better measure of the risk and worth adding to the request; I explain why in the ApoB article.
Check your pulse. Thirty seconds, at rest. If it is irregular, or if a device has flagged possible atrial fibrillation, get an ECG. Untreated AF is a preventable cause of both stroke and dementia.
If you smoke, this is the risk factor to attack first. Nothing else on the list offers a comparable return.