How Blood Sugar Fluctuations Slow Down Your Brain

Medically reviewed by Anna Goldman, MD.

You probably already know your brain struggles to think clearly during low or high blood glucose levels, but the impact of diabetes on your brain can be more significant than momentary fogginess.

A new study has confirmed the link: Blood glucose fluctuations in people with type 1 diabetes (T1D) affect how quickly the brain functions and processes information. 

In other words, when your blood glucose levels are high or low, your brain isn’t going to operate as quickly. But there are long-term concerns, too. Over time, both high and low blood glucose levels can decrease how well your brain works.

Let’s take a closer look.

Diabetes and Brain Function & Speed

The new study combined continuous glucose monitoring (CGM) data and cognitive ecological momentary assessments (EMA) to analyze the relationship between a person’s blood glucose levels and cognitive abilities in real-world circumstances.

The EMA test is pretty straightforward: The researchers tested the participants’ behaviors and cognitive skills in their natural environments — at home, at work, at school, etc. 

The study included 200 adults with type 1 diabetes, focusing on the participants’ “processing speed” and “sustained attention” in relation to the blood glucose levels, writes lead study author Zoe Hawks, PhD, a researcher and instructor with Harvard Medical School.

The results confirmed that study participants had reduced cognitive performance when their blood sugar was high or low. Processing speed was both slower and less accurate, and the larger the glucose fluctuation, the worse the effect.

Beyond real-time blood glucose levels, researchers also determined the following factors also played a significant role in a person’s brain function and speed: 

Older age
More time spent in hypoglycemia
More severe hypoglycemia events
The general presence of diabetes complications
More overall glucose fluctuations
More self-reported fatigue/tiredness
Larger neck circumference

The last factor, neck circumference, calls for further explanation. The study determined that the relationship of a larger neck measurement also increased the likelihood of a person’s risk of sleep apnea and obesity — both were strongly associated with slower cognitive function during low or high blood glucose levels.

The results showed no significant difference based on gender.

Researchers also found that some people are more affected by blood glucose fluctuations than others.The participants whose cognitive skills were most affected by fluctuating blood glucose levels also tended to have poor “diabetes control” overall, explained the study authors:

“[This included] greater CGM time in hypoglycemia, greater number of lifetime severe hypoglycemic events, greater CGM glucose variability, and microvascular complication.”

The authors suggested that anyone with diabetes should consider their blood glucose levels before approaching a complicated task.

In other words, if your blood glucose is 275 mg/dL, that might not be the ideal time to write your history essay, put together that crib from IKEA, or write a complex proposal at work. 

Why Glucose Levels Affect Your Brain

Your brain relies on a second-by-second delivery of glucose to function.

When your blood glucose levels are low, there simply isn’t enough glucose to supply neurons in your brain cells the energy they require to thrive.

You’ve probably been there and felt that lack of fuel during hypoglycemia. When your blood glucose is low, it can be hard even to speak clear sentences. Your brain does not have enough fuel, like a car trying to run on the last few drops of gasoline.

Over time, frequent hypoglycemia can lead to “hypoglycemia unawareness,” which means you no longer feel the symptoms of low blood glucose levels.

When your blood glucose levels are high, the glucose is present but your cells cannot access and use that glucose without enough insulin. Further, the excess glucose from high blood glucose levels can accumulate over time, which damages the nerve endings and blood vessels throughout your entire body, including your brain — increasing your risk of dementia. Severe high blood glucose levels can also lead to seizures, coma, or death

People with diabetes are often warned about the long-term damage of high blood glucose levels to their kidneys and eyes, but the brain suffers, too.

What We Know About Blood Sugar and Brain Health

This recent research adds to ongoing research on the effects of diabetes on your brain health. 

Diabetes & dementia risk: There is a strong association between blood glucose levels and dementia in people with type 2 diabetes (T2D). Researchers found that people with T2D and a generally limited social life had a significantly higher risk of dementia compared to those with a busier social network. With rising rates of T2D across the globe, some experts fear that dementia cases will increase at equally alarming rates.
High blood glucose levels add up: Persistently high blood glucose levels negatively impact brain function in people of all ages with T1D — from children to teens to senior citizens. Alarmingly, 48 percent of older adults with diabetes showed “clinically significant cognitive impairment.”
DKA & long-term brain injury: Multiple studies have found a strong association between diabetic ketoacidosis (DKA) at diagnosis and a child’s performance in school. Researchers concluded that DKA permanently damages the brain at diagnosis, leading to lower IQ levels and generally poorer performance in school. Children diagnosed with T1D before age 5 seemed most vulnerable to brain damage.

Protecting Your Brain

It’s easier said than done, but working with your healthcare team to improve your overall blood glucose levels is critical for protecting your long-term cognitive health.

You can break this down into simpler priorities:

Preventing frequent hypoglycemia: If you’re experiencing daily low blood glucose, this is a likely sign that you’re getting too much insulin. Work with your healthcare team to fine-tune your insulin doses to prevent frequent lows, even if it means letting your levels run a bit higher than ideal. 
Preventing significant hyperglycemia: Very high blood glucose levels wreak havoc on your entire body over time. Work with your healthcare team to get your blood sugars back into range.
Get a CGM: Using a continuous glucose monitor (CGM) can significantly help you catch fluctuating blood glucose levels and improve your overall time in your target blood glucose range!

While there’s tremendous pressure to achieve near-normal blood glucose levels, it’s important to work with your clinician to establish a target range that feels reasonable for you as an individual. 

The Takeaway

When your blood sugar is out of range — whether too high or too low — your brain operates in slow motion. Getting your glucose back into safer territory will help you perform better at work, school, and home, and it will help guard your long-term cognitive health, too.

Hawks ZW et al. Dynamic Associations Between Glucose and Ecological Momentary Cognition in Type 1 Diabetes. NPJ Digital Medicine. March 18, 2024.

Mergenthaler P et al. Sugar for the Brain: The Role of Glucose in Physiological and Pathological Brain Function. Trends in Neurosciences. October 2013.

How Hypoglycemia Unawareness Affects People with Diabetes. National Institute of Diabetes and Digestive and Kidney Diseases. April 5, 2023.

Luna R et al. A Comprehensive Review of Neuronal Changes in Diabetics. Cureus. October 2021.

Stehouwer CDA. Microvascular Dysfunction and Hyperglycemia: A Vicious Cycle With Widespread Consequences. Diabetes. August 15, 2018.

Nakhleh A et al. Hypoglycemia in Diabetes: An Update on Pathophysiology, Treatment, and Prevention. World Journal of Diabetes. December 15, 2021.

Marseglia A et al. Participating in Mental, Social, and Physical Leisure Activities and Having a Rich Social Network Reduce the Incidence of Diabetes-Related Dementia in a Cohort of Swedish Older Adults. Diabetes Care. February 2019.

Chaytor NS et al. Clinically Significant Cognitive Impairment in Older Adults With Type 1 Diabetes. Journal of Diabetes and Its Complications. January 2019.

Ghetti S et al. Cognitive Function Following Diabetic Ketoacidosis in Children With New-Onset or Previously Diagnosed Type 1 Diabetes. Diabetes Care. November 2020.

Aye T et al. Impact of Early Diabetic Ketoacidosis on the Developing Brain. Diabetes Care. March 2019.

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