What causes brain fog? The metabolic health connection

Understanding brain fog, its causes, and relation to blood sugar can help reveal why you feel more cloudy than clear-headed—and offer solutions.


Article highlights

  • “Brain fog” refers to symptoms such as lack of focus, clarity, recall, sharpness, and more, as well as mental fatigue. COVID-19, a lack of sleep or exercise, depression, hormonal changes, medications, and poor metabolic health may cause brain fog.
  • Glucose is essential for the brain to function, but too much can be harmful. The blood-brain barrier controls the flow of nutrients into and out of the brain and takes in glucose through a glucose transporter molecule, GluT1.
  • When blood glucose levels are high, the level of GluT1 decreases, reducing uptake. This can have acute and chronic effects, including decreased energy production in the brain, which can lead to immediate brain fog and increased oxidative stress, contributing to eventual cognitive decline.
  • Poor metabolic health has been linked to diminished executive function, poor word recall, worse verbal memory, and accelerated cognitive decline with age.
  • Steps to lessen your risk for brain fog include keeping glucose (blood sugar) levels stable, avoiding alcohol and drugs, trying time-restricted eating, and addressing other potential causes.
  • TAKEAWAY: Poor metabolic health is one known cause of brain fog. The brain needs glucose to function, but high blood sugar levels can be problematic for cognition. But you can take steps to lessen your risk for brain fog and other forms of cognitive impairment.


Do you ever have days where you feel a little foggy? We’ve all been there—say, after drinking too much alcohol or getting a terrible night of sleep. If you’re feeling more cloudy than clear-headed and can’t pinpoint a likely culprit, you may have what’s commonly called brain fog, and poor metabolic health could be part of the problem.

“High blood sugar—both in people with and without diabetes—is often coupled with symptoms such as memory lapses, poor attention, reduced productivity, and an inability to think clearly—all common complaints that fall under “brain fog.””

Research shows that metabolic dysfunction and elevated glucose impact cognition and may even alter the brain’s anatomy, including key areas that control memory, attention, and thinking. Moreover, high blood sugar—both in people with and without diabetes—is often coupled with symptoms such as memory lapses, poor attention, reduced productivity, and an inability to think clearly—all common complaints that fall under “brain fog.” What’s the connection?

What is Brain Fog?

While it’s not technically a medical diagnosis or even fully understood, many clinicians use the term “brain fog” when they hear patients complain of absentmindedness or not feeling as sharp as usual.

“It’s a common problem of mildly degraded mental ability that includes diverse symptoms, such as lack of focus, clarity, recall, and acuity, as well as mental fatigue,” says Sara Gottfried, MD, a clinical assistant professor in the Department of Integrative Medicine and Nutritional Sciences at Thomas Jefferson University and the author of Brain Body Diet: 40 Days to a Lean, Calm, Energized, and Happy Self. “Some doctors describe it as subacute cognitive decline, meaning it’s not sudden and not enough to warrant a diagnosis.”

But, she adds, “Brain fog can be a precursor to more serious brain issues or an early sign of neurodegeneration, and it’s underdiagnosed by clinicians and patients alike. Don’t fall into the trap of thinking you’re ‘just getting older.’” In other words, brain fog may be a temporary condition from an acute cause like lack of sleep or medications [see below], or it could be the beginning of more serious cognitive decline, with other more chronic causes.

According to Gottfried, brain fog may manifest in people’s lives in any of the following ways:

  • Loss of mental sharpness or mental fatigue—like you’re not fully present—particularly as the day rolls on.
  • Low energy or fatigue, as if you’re walking in molasses.
  • Struggle to exercise or fatigue after exercise.
  • Caffeine doesn’t provide the perk it once did, and you’re consuming more than usual.
  • Difficulty with prioritization, managing tasks or multitasking, or completing to-do lists, including trouble with numbers.
  • Slow thinking or mental processing speed, or haziness in the thought process, like your thoughts are scattered or muddled.
  • Problems with verbal fluency, like the words are there in your brain, but you can’t get to them.
  • Irritability, moodiness, or mild depression. Low motivation or feeling hopelessness, anxiety, fear, or confusion.
  • Forgetfulness and trouble retaining information.
  • Worsening of brain fog under stress, which some call “stress dementia.”

What Causes Brain Fog?

Just like the symptoms of brain fog can show up in a variety of ways and look different from person to person, there are several reasons why it might emerge, including:

  • Hormonal changes: Our bodies go through significant changes many times during our lives. For some women, the large hormonal changes that occur during pregnancy, postpartum, perimenopause, and menopause can affect the brain’s functions for some time. For example, the dramatic decrease in estrogen and progesterone during the years leading up to menopause (a.k.a. perimenopause) can temporarily cause forgetfulness, poor concentration, and cloudy thinking. A growing body of research shows estrogen plays a vital role in memory processing in two areas of the brain—the prefrontal cortex and hippocampus.
  • Medication: Brain fog can sometimes be a side effect of medications (sleep aids and pain medications are top culprits) or even an interaction between two medications you may be taking. If you’re dealing with brain fog, talk to your healthcare provider or a pharmacist about all of the medications you take to determine if one of your medications may be causing your brain fog.
  • Medical conditions: Cancer, lupus, chronic fatigue syndrome, and neurological disorders such as multiple sclerosis, for example, can prompt brain fog. Head injuries and thyroid problems have also been associated with brain fog. While the link isn’t clearly understood, some experts theorize that inflammation in the brain due to certain medical conditions could lead to brain fog symptoms.
  • Gut dysbiosis: The human body hosts an enormous number and diversity of microbes that live in the gut, which perform various functions beyond aiding in digestion. The disruption of these microbes (a.k.a. dysbiosis) in the gut can cause cognitive issues. That’s because of something called the gut-brain axis (GBA). The GBA can be thought of as a bidirectional highway between the central and gut nervous systems, which links the emotional and cognitive centers of the brain with peripheral intestinal functions. When gut microbiota are “off,” it can negatively influence this communication pathway. Factors contributing to gut dysbiosis include poor dietary choices, recent antibiotic treatment, illness, and psychological or physical stress.

The Link Between Metabolic Health and Brain Fog

In addition to the conditions above, poor metabolic health may cause brain fog. Glucose plays an essential role in your brain’s day-to-day function and has longer-term effects on cognitive health.

Glucose is essential for your brain to do its job. The brain consumes about 20% of the body’s glucose to support high-energy activities involved in cognition and regulating every process in the body. The blood-brain barrier, which controls the flow of nutrients into and out of the brain, is designed to take in glucose through a glucose transporter isoform, GluT1. GluT1 enables constant glucose uptake even when levels in the body are low. Because the brain can’t synthesize or store energy the way organs like the liver or muscles do, food intake is its primary energy source. “If you don’t eat well and regularly, your brain suffers, and the result can be brain fog,” says Dr. Gottfried.

While glucose is vital to brain function, too much blood glucose can be problematic. Research demonstrates that laboratory-induced hyperglycemia leads to a decreased ability for brain cells to take in glucose (though the study authors note that this premise is “controversial”). When blood glucose levels are high, the level of GluT1 decreases, thereby reducing uptake. This can have both acute and chronic effects, including decreased energy production in the brain, which can lead to immediate brain fog, and increased oxidative stress, which contributes over time to cell damage or cell death and eventual cognitive decline. Why hyperglycemia decreases GluT1 is still unknown, as are the long-term consequences of this dysregulation.

Glucose may also affect critical parts of the brain’s anatomy. For example, impaired glucose tolerance is associated with hippocampal shrinkage in people without diabetes. Changes to the hippocampus may lead to impairments in episodic memory, a common attribute of brain fog. “The hippocampus is in charge of emotional regulation and memory consolidation,” says Dr. Gottfried. “When it’s affected by too much sugar, brain fog, confusion, and poor emotional control can result.”

The cognitive effects of poor metabolic health have been observed in numerous studies on people with and without diabetes.

  • Diminished executive function: Those with evidence of insulin resistance—but who weren’t yet diagnosed with diabetes—performed worse on cognitive function tests, according to one 2009 study that looked at using higher-level cognitive skills to coordinate behavior and resources to reach a goal.
  • Poor word recall: A 2018 study assessed the correlation between cognitive function and intraindividual fasting glucose levels for men and women without diabetes at ages 25 and 30. Participants were given a list of 15 words and asked to recall these words after a period of distraction (10 min). This research found that people with higher fasting glucose had poorer word recall; in fact, people with more elevated blood sugar who didn’t have diabetes scored worse than people with diabetes.
  • Worse verbal memoryOne research review of 19 controlled studies examining cognitive function found that in 13 of those studies, people with Type 2 diabetes performed more poorly than people without diabetes in at least one aspect of cognitive function. The most commonly affected ability was verbal memory. Examples of verbal memory include repeating a short story or attempting to recall a list of words. A clear majority of studies showed diminished function in this area.
  • Accelerated cognitive decline with ageThe effects on cognitive function seem to exacerbate over time. One study examined cognitive performance in more than 9,000 white women age 65 and older with Type 2 diabetes over six years. It found they had lower baseline scores on cognition than those without diabetes—and the cognitive decline worsened with age. Women who had diabetes for more than 15 years had a 57% to 114% greater risk of significant cognitive decline than women without diabetes.

Longitudinal studies suggest that diabetes can be a root cause of long-term cognitive problems. The study authors note that underlying mechanisms may include factors intrinsic to diabetes, such as hyperglycemia and hyperinsulinemia, and may be worsened by diabetes-related complications, such as stroke. High insulin and glucose levels may influence cognitive function directly or through changes in blood vessels or other undiagnosed vascular diseases. For example, in the case of vascular dementia, the brain does not receive enough oxygen through blood vessels, causing neurons to die.

Inflammation is another likely factor. “Excess glucose in the bloodstream can create inflammation and have a toxic effect on the brain by clumping proteins, damaging blood vessels to vital organs—which can decrease blood flow to the brain, harm nerve cells, increase risk of dementia and Alzheimer’s disease, and contribute to brain fog,” says Dr. Gottfried.

Neuroinflammation is what happens in the brain when the central nervous system’s immune system is triggered. This often occurs in response to injury, neurodegenerative disease, aging, or exposure to toxins, such as too much sugar, pesticides, food additives, and other endocrine disruptors. In a healthy brain, the majority of the immune response is almost exclusively restricted to a population of cells that are brain-specific, called microglia. The blood-brain barrier prevents the passage of peripheral immune cells into the brain. However, inflammation elsewhere in the body caused by hyperglycemia can interfere with blood-brain barrier integrity, so immune cells enter the brain and trigger neuroinflammation.

How to Avoid Brain Fog

While there’s still a lot to learn about brain fog and its many causes, there are several steps you can take to combat the processes that lead to cognitive impairment:

  • Eat for stable blood sugar. Research suggests that high glycemic variability has a poor outcome for cognition in people with and without diabetes, including youngmiddle-aged, and older adults. The good news is that the flip side is also true: More stable glucose may improve cognitive function—even in people with diabetes. The basic tenets of eating for stable blood sugar: Avoid foods high in carbs and sugar and choose whole foods rich in micronutrients, including lots of vegetables, healthy fats, and lean protein.
  • Avoid alcohol and drugs. These substances impair cognitive function, and a growing body of research explains how. One recent study indicates that even moderate drinking is associated with shrinkage in the hippocampus, the area of the brain involved in cognition and learning. Similar findings show brain volume shrinks in proportion to alcohol consumed—and atrophy (read: shrinkage) was greater even in light and moderate drinkers than in teetotalers. Research also shows that cannabis may impair cognitive functions on several levels, from executive function tasks (such as the ability to plan, organize, solve problems, make decisions, and remember things) to basic motor coordination. If you’re in the habit of using these or other substances and are experiencing brain fog, try taking a break for a week or more to see how you feel.
  • Try intermittent fasting. Limiting the number of hours you spend eating each day has many proven health benefits, and it may help ease brain fog by improving metabolic flexibility. Research suggests that both time-restricted eating and extended fasts can help your cells take in glucose more efficiently due to improved insulin sensitivity after fasting, which has cognitive benefits such as better learning, thinking, reasoning, remembering, problem-solving, decision making, and attention. If your brain fog tends to set in after you eat lunch, you’re not alone: Task performance decreases following a lunch meal, whereas lack of a meal doesn’t elicit the same effect. Aim for meals that promote stable glucose to avoid a blood sugar spike and crash.
  • Address other possible causes. Take a look at other factors that may be contributing to brain fog, such as insufficient sleep or lack of exercise, and consider reviewing your medication list with your doctor.