How Blood Sugar Impacts Your Brain Health

Key points

• Your brain uses 20 to 25 percent of your body’s glucose; balance is vital for brain health and function.
• High and low blood sugar levels can harm brain health and memory and increase dementia risk.
• Prediabetes affects 40 percent of Americans and significantly raises the risk for dementia and depression.
• Regular exercise, balanced diets, stress management, and quality sleep improve blood sugar and brain health.

To think, act, and feel in any way, your brain requires access to large amounts of consistent energy. For most of us, that energy comes in the form of blood sugar (glucose), which is transported into the brain and used by our brain cells. While it’s been well known that very high and very low blood sugar levels can have acute and significant negative effects on brain function, research is now showing us that long-term blood sugar issues may also play a significant role in our mental and especially our cognitive state. In this post, we'll explore how blood sugar influences brain health and practical strategies that speak to this science.

Despite a weight of only a few pounds, your brain uses up around 20 to 25 percent of your body’s blood sugar (double what a chimpanzee uses). The brain can use other forms of fuel as energy, including ketones and lactate, but for most people, this makes up a relatively small percentage of total brain energy. Our blood sugar levels go up and down throughout the day, typically rising after meals and then reaching a lower level after longer periods between meals. Blood sugar levels are often measured in milligrams per deciliter of blood (mg/dL). Blood sugar levels are typically measured in labs after fasting; a usual range for this metric is around the 70 to 100 mg/dL mark. When blood sugar is very high (e.g., over 300 mg/dL) or very low (e.g., less than 55 mg/dL), there is a much higher risk of having acute alterations in mental status and, in the case of extremes, even coma.

A rapid development of very high or very low blood sugar levels and the resultant alterations in brain state is far more likely to occur in people with diabetes, although there are other causes. However, we’re now learning that long-term, more subtle changes in blood sugar may also have a significant effect on brain health and brain state. For example, simply having Type 2 diabetes (which affects about 1 in 10 Americans) may translate into an over 50 percent increase in risk for developing Alzheimer’s disease, and a doubling of risk for being diagnosed with a mental health condition.

What about people who haven’t yet developed diabetes? In a telling 2023 publication, prediabetes (which impacts about 40 percent of Americans and 50 percent after age 65) was found to significantly increase the chances of getting any type of dementia. Having prediabetes may also increase the risk of depression, especially in younger people (based on the results of a 2024 meta-analysis). Shockingly, over 80 percent of people with prediabetes don’t know they have the condition.

How do blood sugar levels influence brain health?

There are many proposed pathways linking blood sugar alterations to brain health issues. At a very basic level, we can think about our brain’s constant need for regular access to glucose as key to our brain health, and anything that compromises that flow can lead to brain health issues. Hypoglycemia (abnormally low blood sugar) leads to an explicit lack of energy to power our neurons, and in animal data is shown to impair neurotransmitter signaling. Other pathways include generation of oxidative stress and damage to mitochondria.

On the opposite end of the spectrum, high blood sugar is linked to brain dysfunction through the direct toxicity of elevated glucose levels, as well as through problems with insulin signaling (this tends to accompany diabetes). Additionally, high blood sugar may lead to brain cell growth suppression and even death in the hippocampus (a part of the brain key to long-term memory) as well as elevated inflammation, vascular damage, impairment in the blood-brain barrier, mitochondrial issues, increased oxidative stress, and problems with synapses.

Beyond these direct effects of high and low blood sugar on the brain, recent research suggests that too much variability in blood sugar may also act on many of the same pathways that contribute to brain issues, including inflammation and elevated oxidative stress. This means the brain risks related to blood sugar may not be all about peaks and valleys but rather how often and how quickly blood sugar goes up and down.