Blood sugar, type 2 diabetes and your brain: the dementia link

Type 2 diabetes doesn't directly cause dementia — but chronically high blood sugar is among the strongest modifiable risk factors for it, and diabetes sits on the 2024 Lancet Commission's list of 14. High blood sugar doesn't guarantee anyone will develop dementia, but it does raise the risk, working through insulin resistance, damage to the brain's blood vessels, and inflammation. The encouraging part: blood sugar is one of the more manageable factors on the list, and the same moves that steady it — moving more, managing weight, a whole-food diet — are good for your brain too. The first step is simply knowing your numbers, and going over them with your clinician.

Why blood sugar reaches your brain

Your brain runs on glucose, and it depends on insulin to help it work properly — so it's no surprise that when the body's blood-sugar system is under strain, the brain feels it. Researchers point to a few overlapping pathways:

• Insulin resistance. In type 2 diabetes, cells stop responding well to insulin. The brain has its own insulin signalling, and when that becomes blunted, neurons may use energy less efficiently — a process thought to interfere with memory and with the way the brain clears certain proteins.
• Vascular damage. Persistently high blood sugar quietly injures blood vessels, including the tiny ones feeding the brain. That damage is directly linked to vascular dementia and can compound the changes seen in Alzheimer's — which is one reason high blood sugar and high blood pressure so often travel together and amplify each other.
• Inflammation and oxidative stress. High glucose promotes a low-grade inflammatory state and the build-up of reactive by-products, both of which appear to wear on brain cells over time.

These mechanisms are still being untangled, and the relationship almost certainly runs in more than one direction. But together they explain why blood sugar keeps showing up in the dementia risk-factor research.

The "type 3 diabetes" nickname — what it does and doesn't mean

You may have seen Alzheimer's described as "type 3 diabetes." It's a memorable phrase, and it captures something real — the idea that insulin resistance in the brain may contribute to Alzheimer's. But it's worth being clear about what it is: an informal research term, not an official diagnosis. "Type 3 diabetes" isn't recognised by the American Diabetes Association or other major health bodies, you can't be diagnosed with it, and even among researchers the classification is debated — it's still being argued whether brain insulin resistance is a driver of Alzheimer's or a downstream effect of it.

So the honest reading is that the nickname reflects a genuine, active area of science about how blood sugar and insulin affect the brain — not a settled medical fact. It's a useful shorthand for "there's a real metabolic thread here," and no more than that.

What the evidence shows

The strongest everyday signal comes from large, long-term cohort studies that track people's blood sugar over years and watch what happens. Among the most detailed is a study from Kaiser Permanente Northern California, which followed a large group of people with type 2 diabetes and looked at their HbA1c — a blood test that reflects average blood sugar over the previous few months — across many years.

The pattern was consistent: people whose HbA1c was persistently high had a higher risk of dementia than those who spent more of their time in a better-controlled range. Those whose readings sat mostly in the 9%–10% band had roughly a third higher risk, and those mostly at 10% or above had a substantially higher risk still — while better long-term control tracked with lower risk. A related Kaiser study of more than 170,000 people found that swinging blood sugar — big ups and downs in HbA1c over time — was also associated with higher dementia risk, even for people whose average looked fine.

Two caveats keep this honest. These are associations, not proof that one thing causes the other, and they describe groups rather than predicting any single person's future. But the direction is clear and it lines up with the biology: better, steadier glucose control over the long run is associated with a lower risk. Add the 2024 Lancet Commission — which counts diabetes among the 14 modifiable factors behind an estimated 45% of dementia cases worldwide — and blood sugar earns its place as a lever genuinely worth acting on.

Know your numbers — and prediabetes counts too

Because so much of the risk builds quietly over years, the single most useful thing you can do is find out where you stand. Two numbers matter most: your HbA1c and your fasting glucose. A doctor can order both, and they're the starting point for understanding your own picture — what your targets should be, and how they fit with the rest of your health, is a conversation to have with your clinician rather than something to judge from a single reading.

It's also worth knowing that risk doesn't switch on at the moment of a diabetes diagnosis. Prediabetes — blood sugar that's higher than normal but not yet in the diabetes range — sits on the same spectrum, and it's common to have it without any symptoms at all. That's not a reason to worry so much as a reason to check: catching raised blood sugar early gives you the most room to act, and the steps involved are the same ones that support your brain.

What actually helps

The reassuring thing about blood sugar is how responsive it is — to everyday changes and, when needed, to medication. The moves that help it are, almost point for point, the moves that help the brain:

• Move most days. Regular activity helps your muscles use glucose and improves insulin sensitivity — and physical activity is itself one of the best-supported factors for brain health.
• Favour whole foods. A pattern built on vegetables, fruit, whole grains, legumes and less ultra-processed food (a Mediterranean-style way of eating fits well) tends to steady blood sugar and supports the brain at the same time.
• Mind weight and sleep. Losing even a modest amount of excess weight can meaningfully improve blood sugar, and treating poor sleep or sleep apnoea helps too.
• Take medication as prescribed. If your doctor prescribes medication to manage your blood sugar, taking it consistently is what delivers the benefit. Don't start, stop, or change it on your own — that's a decision to make together with your clinician.

One question that comes up constantly here is whether the newer diabetes drugs do anything extra for the brain. The honest answer is that the semaglutide Alzheimer's trials came back flat — a reason to take a GLP-1 for the metabolic benefits it's actually prescribed for, not as a brain-health strategy.

None of this is about chasing a perfect number. It's about steady, long-term control — and every one of these steps is a conversation worth having with your doctor, who can confirm your readings, rule out other causes, and tailor a plan to you.

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This article is educational and is not medical advice, diagnosis, or treatment. Studies cited — including the Kaiser Permanente glycemic-control research and the Lancet Commission — describe risk factors and associations in general, not Solenna specifically, and "type 3 diabetes" is an informal research term rather than a recognised diagnosis. Solenna does not diagnose, prevent, treat, or cure Alzheimer's disease, diabetes, or any form of dementia; individual results vary and no outcome is guaranteed. Never start, stop, or change any medication, and make no changes to how you manage your blood sugar, without speaking to a qualified healthcare professional.