Alzheimer’s disease (AD) is the most common form of dementia characterized by memory loss, impaired thinking, and behavior that usually develops in severity and interferes with daily life. The greatest risk factor is increasing age and it is a progressive disease, meaning its symptoms worsen over time. More specifically, a protein fragment known as beta-amyloid takes the place of normal brain cells, destroying synapses and then clumping into plaques and causing nerve cell death. Synapses are contact points via which nerve cells relay signals to one another. A key feature of AD is the loss of these synapses and directly linked with deterioration of brain function. Now that we have some background of AD pathology, how does this relate to diabetes?
Humans need insulin. When we eat a carbohydrate-rich food, insulin is released to help the cells take in glucose in the blood (sugar) for energy. This is what occurs in non-diabetics. But in those with type 2 diabetes, the story is much different. The cells can only hold so much glucose. When we repeatedly drink sugar-sweetened beverages and eat high-sugar processed foods, the cells become resistant to signals from insulin. In response, insulin levels elevate even higher to try to signal the cells to take in the glucose, and these high levels are bad for blood vessels and the brain. Glucose is the primary source of energy for our bodies and although most of our bodily processes can survive on fat and protein in dire circumstances, our brains need glucose to survive. When people experience insulin resistance, or the inability of the cells to properly utilize insulin and pick up glucose, it reduces brain functions such as memory.
There are many studies now showing this phenomenon which I would like to discuss here. At Brown University, Dr. de la Monte blocked the path of insulin to rats’ brains and observed their neurons deteriorate and their brains show all signs of Alzheimer’s. She also analyzed 45 postmortem brains and found that insulin expression declined in line with the Braak stages, the standard system of neurodegeneration classification. In the most advanced stage of AD, insulin receptors were nearly 80% lower than in a normal brain, meaning that these individuals severely lacked the capacity to uptake glucose and properly use it. Further, the researchers found that insulin is important in the brain to stimulate the expression of an enzyme responsible for making acetylcholine, a neurotransmitter. Deficiency of this neurotransmitter, due to lack of insulin, is a marker of AD.
Another study investigated the association between glucose tolerance status and the development of dementia in a cohort in Japan. The incidence of AD was significantly higher in subjects with diabetes, and thus insulin resistance, than those with normal glucose tolerance-- in fact those with diabetes had a 74% increased risk of being diagnosed with some type of dementia over the course of the study. Further, adults with diabetes had more than double the odds of developing AD compared to those who passed the glucose tolerance test! These results have stark implications for the development of AD.
A third study investigated macronutrient intake and the risk of cognitive impairment in elderly persons followed for about four years. The risk of dementia or mild cognitive impairment was elevated in subjects who consumed a high percentage of carbohydrates, but reduced in subjects who consumed high percentage of fat or protein diets.
Overall, it may not be that Type 2 diabetes is causing AD, per se, but that they both have a common cause of overconsumption of sugary foods that negatively affect insulin in the body. Those with type 2 diabetes or insulin resistance are at higher risk and this is a huge problem for the U.S. Approximately 29 million people in the U.S. have diabetes and it costs the US an estimated $245 billion a year! Further, 5.4 million Americans have AD and care for those with AD costs around $200 billion dollars a year! This is nearly half a trillion dollars spent on healthcare for these two diseases! If we can manage Type 2 and “Type 3” Diabetes by improving diet, specifically sugar intake, hopefully we could help decrease the rates of these debilitating diseases in the United States. So put down that soda, and grab a piece of fruit instead!