What Is "Type 3 Diabetes"? The Brain Insulin Theory, Explained Plainly

By Nora WhitleyEditor, Phytomem ReviewUpdated October 18, 20264 min read

You won't find "type 3 diabetes" on a medical chart. It is a research term for the idea that Alzheimer's disease involves the brain becoming resistant to insulin. Here is where it came from, how solid it is, and why supplement ads have picked it up.

Where the term came from

The phrase came out of the lab of Suzanne de la Monte, a neuropathologist at Brown University and Rhode Island Hospital. In the mid-2000s her group reported that brain tissue from people with Alzheimer's showed lower levels of insulin and of the receptors that respond to it, and that this got worse as the disease progressed.

In 2008 she and a colleague laid out the case in a review titled, bluntly, "Alzheimer's disease is type 3 diabetes: evidence reviewed" (de la Monte and Wands, 2008). The argument: Alzheimer's has many of the features of diabetes, but in the brain, and it can happen in people whose blood sugar is normal.

What insulin does in the brain

Most people think of insulin as the hormone that moves sugar from the blood into muscle and fat. In the brain it has other jobs. It helps neurons form and strengthen connections, which is part of how memories are laid down. It affects how brain cells handle the amyloid and tau proteins that build up in Alzheimer's. And it influences inflammation.

Brain cells can take up glucose without insulin in some regions, so "the brain starves for sugar" is too simple. The more precise version of the theory is that the brain's response to insulin weakens, and the processes that depend on that signal start to fail.

How strong is the evidence?

Parts of it are solid, and parts are still argued over.

Well supported: people with type 2 diabetes have a higher risk of developing dementia. Poor blood sugar control, high blood pressure and obesity in midlife are all linked to worse brain health later. The National Institute on Aging includes managing blood sugar among its steps for protecting cognitive health.

Supported but not settled: that the Alzheimer's brain is insulin resistant, and that this is part of the disease process rather than a side effect of it. Brain imaging shows reduced glucose use in Alzheimer's, but that could reflect neurons that are already damaged.

Still uncertain: whether treating insulin resistance prevents or slows Alzheimer's. Diabetes drugs, including intranasal insulin and newer medications like GLP-1 drugs, have been tested in Alzheimer's with mixed results so far.

The term itself is debated. Some researchers find "type 3 diabetes" a useful shorthand. Others think it oversimplifies a disease with many causes, and point out that most people with Alzheimer's do not have diabetes. Either way, it is not a diagnosis your doctor will give you. The NIA's Alzheimer's fact sheet describes the disease without using the term.

Why supplements use the phrase

"Type 3 diabetes" is a gift to supplement marketing. It sounds like a hidden diagnosis, it connects memory to something people already worry about, and it suggests that ingredients known to lower blood sugar might protect the brain.

Phytomem One is a good example. Four of its ten ingredients (berberine, Ceylon cinnamon, banaba and kudzu flower) are there because they can lower blood sugar. Berberine in particular has good human evidence for that in people with type 2 diabetes (Yin, 2008). The sales page then says these ingredients restore the brain's ability to "fuel itself," which hasn't been tested in people.

That doesn't make the ingredients pointless. If your blood sugar runs high, bringing it down is good for your brain over the long run, whichever way the type 3 diabetes debate goes. It just isn't the same as treating insulin resistance inside the brain. We look at how this plays out in the formula on how Phytomem One is supposed to work, and the product as a whole in our Phytomem One review.

What actually helps

If the brain-insulin idea makes you want to act, the steps with the best evidence are the ordinary ones:

• Get your fasting blood sugar or A1c checked if it has been a while. Prediabetes is very common after 50 and usually has no symptoms.
• Stay physically active. Exercise improves insulin sensitivity throughout the body.
• Keep blood pressure in a healthy range, and treat sleep apnea if you have it.
• If you already have diabetes, keep working with your doctor on control. The NCCIH page on diabetes and dietary supplements explains why supplements shouldn't replace medication.

Questions