How brain microglia shape your Alzheimer's disease risk

Up to fifty percent of people who live to age 85 will develop some form of dementia, but according to neurologist David Perlmutter, author of the book Brain Defenders, most Alzheimer's cases are preventable. In conversation with Dr. Mark Hyman, Perlmutter explains why the brain's own immune system is the missing piece for understanding cognitive decline.

The cells that protect or destroy the brain

Microglia are the brain's immune cells. Under normal conditions, they protect neurons and keep brain tissue healthy. But when they become chronically activated, they switch roles: instead of protecting, they start destroying the connections between neurons, a shift Perlmutter describes as the cell turning into its own evil twin.

Today it is possible to observe this activation with a type of scan called a TSPO scan, which shows activated microglia in people with Alzheimer's, Parkinson's, major depression, post-traumatic stress, and even long COVID. That suggests many seemingly different neurological and psychiatric conditions share a common underlying mechanism.

Metabolism decides your brain's fate

According to Perlmutter, microglia change behavior when their metabolism shifts, and that cellular metabolism directly mirrors the metabolism of the rest of the body. Keeping blood sugar, insulin, and blood pressure in healthy ranges does not just protect the heart, it also keeps the brain's immune cells working in your favor.

Inflammation that originates anywhere in the body, including from a leaky gut, can reach the brain through the blood brain barrier and activate microglia. That is why Perlmutter insists that gut microbiome health is directly connected to brain health.

Why the amyloid theory falls short

For decades, the dominant explanation for Alzheimer's was the buildup of beta amyloid plaques in the brain. Perlmutter directly challenges that theory, citing a recent analysis of 17 studies involving more than 20,000 people that evaluated drugs designed to clear beta amyloid: clinical benefit was minimal, while up to a quarter of treated patients experienced brain bleeding or swelling. For Perlmutter, this strengthens the case for looking at neuroinflammation and metabolism as the real drivers.

Signs you can measure yourself

Perlmutter recommends tracking a handful of simple, accessible markers as part of a prevention strategy:

• Fasting glucose and insulin: early indicators of insulin resistance, a key risk factor for Alzheimer's.
• Blood pressure: keeping it in check reduces vascular inflammation that can affect the brain.
• Waist to hip ratio: a simple way to estimate visceral fat, which is linked to greater systemic inflammation.
• Brain-specific blood markers: such as p-tau 217, now available through clinical testing.

Cases that show reversal, not just slowing

Perlmutter shares the case of a patient with significant cognitive decline who improved markedly after starting a supervised ketogenic diet, alongside addressing heavy metals and gut issues. Not every case responds the same way, but results like this support the idea that targeting metabolism can, in some cases, not just slow but partially reverse the damage.

Conclusion

Perlmutter's central message is that the brain is not a fixed black box: microglial health depends heavily on lifestyle. Paying attention to diet, physical activity, sleep, the microbiome, and environmental toxin exposure in your thirties and forties, long before the first memory lapses appear, is the strongest strategy for protecting memory long term.