Glucose is the primary energy source that fuels the body’s cells. The brain is a voracious consumer of energy, hogging half of available glucose. So blood sugar (the concentration of glucose in the blood) obviously impacts brain function—but only in recent years have we discovered just how great that impact is.
When blood sugar falls too low (hypoglycemia), we feel unfocused, muddled, and irritable. On a more subtle level, the brain doesn’t have enough energy for routine maintenance and repair.
High blood sugar (hyperglycemia) also does a number on the brain, and diabetes significantly raises risk of memory problems and Alzheimer’s disease.
But it’s not just diabetics who need to be concerned. Even modestly elevated blood sugar—at levels not high enough to be classified as diabetes—are linked with increased risk of cognitive impairment and dementia. Eighty-four million Americans—one in three adults—have this condition (prediabetes), and if you are obese, odds are you’re one of them.
Effects of High Blood Sugar on the Brain
The harmful effects of high blood sugar on the brain are well known. Diabetes is a leading cause of vision loss, neuropathy, kidney failure, amputations, atherosclerosis, heart attacks, and strokes—and there’s a culprit common to them all. Chronically high blood sugar damages the blood vessels, which impairs circulation and delivery of oxygen- and nutrient-rich blood to tissues throughout the body.
The brain is no exception. A decline in blood flow deprives neurons and other brain cells of life-sustaining oxygen and glucose. Serious interruptions such as strokes, which completely cut off blood supply to affected areas, have devastating effects on brain function. But poor circulation and “silent strokes,” which often go unnoticed, also cause neurodegenerative changes that over time can contribute to vascular dementia and Alzheimer’s.
The Insidious Effects of Insulin Resistance
Insulin resistance plays a central role as well. To review the basics, as glucose enters the bloodstream and blood sugar rises, the pancreas secretes insulin, which signals the cells to let glucose in. Sometimes, however, the cells don’t get the message and blood sugar remains high. So the pancreas churns out more and more insulin in an effort to clear glucose out of the blood and into the cells, resulting in high levels of both glucose and insulin.
This is the essence of insulin resistance, and it is the underlying problem in type 2 diabetes and prediabetes.
In addition to overworking the pancreas, insulin resistance promotes fat storage, high blood pressure, lipid abnormalities, and inflammation—all of which have adverse effects on the brain. Hypertension is a major risk factor for strokes and other brain pathology.
Neuroinflammation is a common feature of all neurodegenerative diseases. And obesity ups the odds of developing dementia. But there’s more.
Insulin receptors are present and active throughout the brain. Researchers have discovered that insulin signaling encourages the growth and repair of neurons and the formation of synapses. It also helps regulate beta-amyloid, the primary component of the plaques that litter the brains of individuals with Alzheimer’s. So it’s no surprise that insulin resistance in the brain, which disrupts insulin signaling and promotes degenerative changes, is bad news for cognitive function.
Effects of Low Blood Sugar on the Brain
The effects of low blood sugar on the brain can be equally problematic because low blood sugar deprives neurons of energy. When this occurs in individuals who do not have diabetes, it could be due to an underlying disease, but it’s most likely reactive hypoglycemia.
Caused by overproduction of insulin that drives blood sugar too low, it generally comes on an hour or two after meals and can make you feel tired, shaky, sweaty, lightheaded, and hungry. Reactive hypoglycemia, which is often triggered by simple carbohydrates that spike blood sugar, may be avoided by eating more protein and fiber-rich plant foods. Frequent bouts of reactive hypoglycemia could be indicative of diabetes or prediabetes and should be checked out.
Very low blood sugar is most common in people with diabetes, and it’s usually a side effect of insulin or other drugs. As you can imagine, hypoglycemia poses real problems when episodes are frequent or severe and can even lead to seizures, coma, or death.
In a 2018 study, Johns Hopkins researchers found that older people with diabetes who had a history of severe hypoglycemia had smaller brain volumes and a significantly greater incidence of dementia. And even mild episodes increase risk of falls and fractures.
Hypoglycemia sends at least 100,000 people to emergency rooms every year, and older people are particularly vulnerable. This is one of the biggest problems with aggressive use of diabetes drugs. What good is maintaining target A1C and blood sugar levels if it increases your risk of dementia and death?
Not Diabetic? Not Off the Hook
This is a complex topic, and research is still unfolding. But the central role of high and low blood sugar in brain dysfunction is increasingly clear—and you don’t have to have diabetes to suffer the consequences.
A 2018 study published in Diabetologia involved over 5,000 people, average age of 65, whose cognitive function and blood sugar levels were tested periodically over an eight-year period. The researchers found that the higher the A1C, the faster the rate of cognitive decline. And although some degree of brain shrinkage is considered to be a part of normal aging, atrophy was more significant in people with diabetes or prediabetes.
More than 100 million people in this country have diabetes or prediabetes, which puts them at greater risk of mild cognitive impairment and dementia. Sure, age and genetics also contribute to memory loss, but insulin resistance is one risk factor you can tackle.
Eating lots of sugar and fast-burning carbohydrates wreaks metabolic havoc. You may not notice problems today—younger bodies are incredibly resilient. But studies suggest that people who are insulin resistant and/or obese, especially in the abdominal area, during midlife are more likely to suffer with memory problems in old age.
Along with improving your diet, here are a few steps you can take to protect yourself from the effects of high (and low) blood sugar on the brain:
- Exercise. Physical activity is associated with a lower risk of cognitive decline and diabetes. Exercise helps to clear beta-amyloid proteins and increases brain-derived neurotrophic factor, both of which help to reduce Alzheimer’s risk.
- Drink coffee. In a 21-year-long Scandinavian study, people who drank three or more cups of coffee a day had a 65 percent reduced risk of dementia, compared to people who drank two or fewer cups. Another study showed that every cup of regular coffee you drink lowers your risk of type 2 diabetes by 7 percent.
- Get adequate sleep. Good, solid slumber is necessary for consolidating memories and refreshing the brain’s capacity for learning. It’s no coincidence that you feel mentally slow and foggy after a poor night’s sleep.
Degenerative changes in the brain begin years—even decades—before symptoms become evident. Among the most important things you can do to preserve cognitive function throughout life is to adopt a healthy lifestyle and keep your blood sugar in check. After all, our memories make us who we are.