When insulin resistance affects the brain, it is deleterious. It interferes with metabolism, promotes inflammation, and inhibits the cleanup process of proteins from the brain. So those proteins accumulate and cause toxicity. We see this in Alzheimer's Disease, where there is an accumulation of A-beta protein and Tau protein. Brain insulin resistance interferes with the learning process and causes cognitive decline not only in Alzheimer's, but in many other neurodegenerative diseases, like Parkinson's disease. It ultimately causes cell death and loss of brain tissue. With Parkinson's, insulin resistance causes a dysfunction in the mitochondria, the energy factories of the cell and damages cell DNA. The same changes occur in patients with diabetes, who don't have Parkinson's. So truly, insulin resistance is one of the root causes of this brain disease. It's also associated with strokes, as was shown in the North Manhattan Study, or NOMAS. The study recruited a cohort of 1509 non-diabetic participants from Northern Manhattan and followed them for an average of 10 years. The study was significant, because as opposed to the Minnesota Coronary Experiment, where more than 90% of patients were Caucasian, NOMAS was more representative of the US general population, with inclusion of Caucasians, African Americans, and Hispanics.
The study found that insulin resistance was a marker of increased risk of incidence stroke, even in non-diabetic individuals. So your risk of stroke is increased years before you develop diabetes. And you may not even know that you're insulin resistant, because the insulin sensitivity test is not one of those tests routinely done in your checkups.
These results of association between stroke and insulin resistance are not limited to the US population. In a large study done in China, they looked at beta cell dysfunction, the cells that make insulin, and the association with the recurrence of stroke, disability from stroke, and death from strokes. There was a strong positive correlation, the more beta cell dysfunction, or insulin resistance, the more your risk of recurrent strokes, disabilities of strokes, and mortality from stroke.
Insulin resistance has also been indicted in mood disorders and depression.
In another study they recruited almost 500 subjects and looked at whether there was an association or correlation between the presence of depression, its severity, and the presence of insulin resistance. This study determined a strong positive correlation between the two. Moreover, sugar is known to affect the brain in similar ways to cocaine. Both are addictive, and some studies have shown that sugar is even more addictive than cocaine. In functional MRI studies, the same centers that lit up in the consumption of cocaine lit up with consumption of sugar.
This explains why it is so hard to quit sugar.