Alzheimer's disease isn't really "type 3 diabetes." However, Alzheimer's may be treatable with insulin, and controlling diabetes may reduce the risk of Alzheimer's.
Alzheimer's disease used to be a very rare condition. Type 2 diabetes also used to be a very rare condition. As type 2 diabetes has become more and more common, however, Alzheimer's disease has also become more and more common. Some commentators have even gone so far as to label Alzheimer's disease as "type 3 diabetes," caused by nothing more than blood sugar levels that have been kept too high for too long. It's really not quite that simple, but controlling diabetes may also lead to better brain health in old age.
What Is The Real Role Of Insulin In The Brain?
Insulin is the hormone that activates a process through which cells can absorb sugar. Cells all over the body use glucose as fuel. Insulin also has a role in regulating the ability of cells to absorb or release fatty acids, as well as helping cells to absorb amino acids they use to build proteins. Insulin isn't just about sugar.
The brain does not, as some commentators tell us, actually make its own insulin. However, certain parts of the brain are unusually sensitive to insulin. Specifically, cells in the parts of the brain that are involved in thinking, forming new memories, and recalling old memories are unusually sensitive to insulin, that is, they have more insulin receptors on their surfaces. In type 2 diabetes, however, these insulin receptors don't work the way they should.
What Has Insulin Got To Do With Alzheimer's?
There's a lot of moralizing about type 2 diabetes. Some people (and many doctors) offer the opinion that if you get type 2 diabetes, it's your fault. You ate too much sugar. Chances are that if you have type 2 diabetes, at some point you did eat too much sugar, and you would have been better off if you hadn't, but the processes that drive type 2 diabetes also drive your appetite, not just the other way around.
Type 2 diabetes is a disease of insulin resistance. This means that cells in the muscles and the liver and the brain shut down their insulin receptors to protect themselves from too much sugar flooding in. They avoid "burning out" with too much glucose by making their receptors less responsive to insulin. The pancreas, however, has the job of keeping blood sugar levels constant. When cells don't take sugar out of the bloodstream, it makes more insulin to force them to absorb it. Cells respond by becoming more insulin resistant, so the pancreas makes still more insulin, which makes cells even more insulin resistant, and so on. Eventually there's a lot of insulin floating around in the bloodstream that has to be broken down. That's where the problem starts.
Insulin And Amyloid
Brain cells can break down insulin that they cannot use. However, when they do that, they don't have enough enzymes left over to break down a protein called amyloid. This is the protein that makes "tangles that strangle" neurons and causes Alzheimer's. If your body makes too much insulin, then your brain can't get rid of the protein that is (almost always) associated with Alzheimer's.