Your brain is a finely tuned organ. But it’s sensitive to the amount of sugar, or glucose, it receives as fuel. Whether you have type 1 or type 2, both the high blood glucose of uncontrolled diabetes and the low blood glucose that sometimes comes with diabetes treatment can affect your brain.
The Dangers of High Blood Glucose
Some of diabetes’ effects on the brain aren’t obvious right away, especially when they are related to high blood sugar.
“With diabetes, you have an increased risk of damage to blood vessels over time, including damage to the small blood vessels in the brain. This damage affects the brain’s white matter,” says Joseph C. Masdeu, MD, PhD, of the Houston Methodist Neurological Institute.
White matter is the part of the brain where nerves talk to one another. When the nerves in the brain are damaged, you can have changes in thinking called vascular cognitive impairment or vascular dementia.
Vascular cognitive impairment can happen with either type 1 or type 2 diabetes, but there are some differences in risk, says Joel Zonszein, MD, director of the Clinical Diabetes Center at Montefiore Medical Center in New York.
“The longer you have diabetes [in general], the more of a chance there is of developing dementia, but we see much less of it in people with type 1 whose diabetes is well-controlled,” he says.
People with type 2 may face a double-whammy because they tend to have other problems that also can cause blood vessel damage.
“These patients tend to be less metabolically fit overall, with low HDL [“good”] cholesterol, high triglycerides, and high blood pressure, and they are more likely to be obese,” Zonszein says.
Diabetes can combine with these other problems to create inflammation that damages blood vessels, so good diabetes control is all the more important, he says.
“Sometimes people want to try a lot of different things before they go to insulin or other injectable diabetes drugs. The last thing they want is to take shots,” says Zonszein. But it’s important to bring down your blood glucose early in diabetes and not chase it for 5 years.”
Low Blood Glucose Can Cause Sudden Trouble
If you tightly control your diabetes, it can be easier to accidentally slip into low blood sugar. And low blood sugar has more immediate, obvious effects on your brain than high blood sugar.
The symptoms of low blood sugar get worse the lower your blood sugar goes. It can affect your mood and make it difficult for you to think. You might get a headache, feel dizzy, have poor coordination, or have trouble walking or talking. Severely low blood sugar can give you seizures or convulsions, make you pass out, or put you in a coma.
Repeated bouts of low blood sugar can become problematic, says Gail Musen, PhD, assistant professor of psychiatry at Harvard Medical School in Boston.
“Going too low once in a while probably doesn’t have a huge long-term effect on the brain,” she says. But if you have frequent low blood sugar, you can become unaware of it, and it’s dangerous to live like that.”
This condition, called hypoglycemia unawareness, happens when your brain has trouble noticing low blood sugar levels. When that happens, you won’t get the usual early symptoms of low blood sugar, such as nausea, hunger, shakiness, cold or clammy skin, or a pounding heart.
Usually these symptoms are enough to wake someone with diabetes up from sleep, but if you have hypoglycemia unawareness, you might not wake up, and your blood sugar can continue to drop until it’s an emergency.
Hypoglycemia unawareness can also catch you off-guard while you are driving and lead to an accident, or cause you to fall.
The jury is still out on whether repeated bouts of low blood sugar can cause long-term memory problems or raise the risk of dementia. One large study, called the Diabetes Control and Complications Trial, showed that low blood sugar does not have a long-term impact on memory or the ability to think in people with type 1. But another study suggests a link between a history of severe low blood sugar and a higher risk of dementia in older people who have type 2.
The bottom line is that careful diabetes control is important, Zonszein says. “Low blood glucose might not make you get dementia, but it feels terrible. High blood glucose may not feel terrible, but it might cause problems with dementia.”
A Possible Alzheimer’s Connection?
Research does suggest an association between diabetes and Alzheimer’s. People with type 2 diabetes are twice as likely to develop Alzheimer’s as people without diabetes. But researchers are still trying to figure out whether diabetes actually causes Alzheimer’s.
“Alzheimer’s is characterized by local deposits of beta-amyloid, an [abnormal] protein accumulating in the brain,” says Peter Butler, MD, director of the Larry Hillblom Islet Research Center at the University of California, Los Angeles.
In some people with Alzheimer’s, beta-amyloid forms clumps that interfere with nerve cells’ ability to communicate with each other.
In the pancreas, where insulin is made, "there are similar proteins that lead to damage and the death of cells,” Butler says.” It’s likely to be a shared risk for developing either Alzheimer’s or type 2 because the thing that goes wrong [in both of them] is very similar.”
But Butler adds that vascular cognitive impairment (a possible side effect of diabetes) is another cause of Alzheimer’s. That makes the matter more confusing. It seems there is a lot more to learn about which happens first, whether one is causing the other, and exactly how they are linked.
“It’s hard to separate out in chronic disease processes where cells don’t function as they should anymore,” Butler says. “It would be naive to say that 100% of one person’s Alzheimer’s is from amyloid plaques while someone else’s Alzheimer’s is vascular.”