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Alex Manos | 22 Jan 2025 | Gut Health

What Causes Brain Fog: What You Need To Know

Have you ever found yourself struggling to focus, forgetting simple things, or feeling like your mind is wrapped in a thick cloud? If so, you’ve experienced what many people describe as “brain fog.” It’s not a medical diagnosis but a term used to capture that frustrating sense of mental fatigue, sluggishness, or lack of clarity that can make even simple tasks feel like climbing a mountain. Brain fog can affect anyone and is often linked to factors like stress, poor sleep, diet, hormonal changes, or underlying health conditions. But what exactly is happening in the brain to cause this mental haze?

More importantly, are there evidence-based ways to lift the fog and regain mental sharpness? In this blog, we’ll explore the science behind brain fog, uncover the common triggers, and dive into proven strategies—from nutrition and exercise to stress management and lifestyle tweaks—that can help you think clearly and feel your best.

What is Brain Fog?

Brain fog is a term that is used to describe symptoms including slow or cloudy thinking, difficulty focusing, reduced memory or forgetfulness, and difficulty completing cognitive tasks (source).

Brain fog has been described or reported in relation to several different chronic health conditions, including:

  • Chronic fatigue syndrome.
  • Crohn’s disease.
  • Fibromyalgia (sometimes referred to as “fibro-fog”).
  • “Post-COVID syndrome” and other chronic viral infections.
  • Small intestinal bacterial overgrowth (SIBO).
  • Celiac disease and non-celiac gluten sensitivity.
  • Hypothyroidism.
  • Postural tachycardia syndrome (POTS).
  • Mast cell disorders.
  • Menopause.

What Causes Brain Fog?

Common causes of brain fog include chronic inflammation, gut imbalances and poor digestion, hypothyroidism, vitamin B12 deficiency, hypothyroidism, chronic stress, head injuries, and blood sugar imbalances.

Gut Imbalances

It now appears that gut microbes represent direct mediators of psychopathology (source).

Brain fog can often be linked to imbalances in the gut, highlighting the crucial role of the gut-brain axis—a bidirectional communication network between the gut and the brain. The gut houses trillions of microorganisms, collectively known as the gut microbiome, which play a vital role in digestion, immunity, and even brain health. When the microbiome becomes imbalanced due to poor diet, stress, infections, or overuse of antibiotics, it can lead to a condition called dysbiosis.

Dysbiosis can trigger the release of pro-inflammatory cytokines and harmful metabolites that can cross the gut lining and enter the bloodstream, a phenomenon known as “leaky gut.” These inflammatory molecules can then travel to the brain, contributing to neuroinflammation (brain inflammation), oxidative stress, and disruptions in neurotransmitter production, all of which can cause cognitive dysfunction and mental fatigue (source).

SIBO and Liver Dysfunction

The gut-liver-brain connection might not seem obvious, but it may have a role to play in brain fog. Think of the connection this way: all toxins in your gut are processed by your liver. If your liver is overburdened for any reason, these toxins may build up in your blood and get into your brain by crossing the blood-brain barrier.

A type of cognitive impairment that sometimes occurs in liver disease, hepatic encephalopathy (HE), has been linked to small intestinal bacterial overgrowth (SIBO). While HE is fairly uncommon and is usually seen in those with chronic, severe liver disease, it does underline the connection between a taxed liver, a disrupted gut, and cognitive dysfunction.

Research has also shown that Rifaximin, an antibiotic that is often used to treat SIBO, can prevent and improve HE. In a 2020 study, treatment of SIBO significantly improved mild HE, while treatment of the stomach infection Helicobacter Pylori (H. pylori), did not. This further highlights the significance of a gut-liver-brain connection (source).

These findings may be relevant for brain fog and cognitive function in the general population, as they suggest that treating bacterial overgrowth in the gut can help to reduce mental fogginess and cognitive dysfunction.

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Infections

Perhaps the best example of this is long Covid. A systematic review and meta-analysis concluded that 20% of patients with long Covid have brain fog and mental health conditions (source). Disruptions in the blood-brain barrier along with a hyperactive immune system are the likely mechanisms behind “brain fog” in patients who are experiencing long COVID (source).

Inflammation

Inflammation could be the result of any of the above three causes. Systemic inflammation and brain inflammation (often called neuroinflammation) are closely linked to brain fog, as both can disrupt normal brain function and lead to cognitive impairment. Systemic inflammation occurs when the body’s immune system is activated, often in response to infection, chronic stress, poor diet, or autoimmune conditions. This process releases pro-inflammatory molecules called cytokines which can cross the blood-brain barrier and trigger inflammation in the brain. Neuroinflammation directly impacts brain cells, impairing the communication between brain regions that are critical for memory, attention, and processing speed.

For example, chronic activation of microglia—the brain’s immune cells—has been linked to oxidative stress and the release of neurotoxic substances. These can interfere with neural connectivity and cause mental fatigue. Research has also found that systemic inflammation is associated with disruptions in the brain’s default mode network. This area of the brain is a key system involved in focus and mental clarity (source).

Addressing inflammation through dietary changes, physical activity, and managing underlying conditions may help alleviate brain fog by reducing both systemic and neuroinflammation.

Mast Cells and Histamine Intolerance

Brain fog is a common symptom in individuals with histamine intolerance and mast cell disorders, as excessive histamine and mast cell activation can disrupt normal brain function. Mast cells, a type of immune cell, release histamine and other inflammatory substances in response to triggers such as stress, certain foods, or allergens. When histamine levels become too high—either due to overproduction, insufficient breakdown by the enzyme diamine oxidase (DAO), or both—it can cross the blood-brain barrier and contribute to neuroinflammation. This can impair communication between brain cells, leading to symptoms like difficulty concentrating, memory problems, and mental fatigue.

In conditions like mast cell activation syndrome (MCAS), where mast cells are overly reactive, brain fog is often reported as a primary complaint. Research suggests that histamine may also dysregulate neurotransmitters such as dopamine and serotonin, further contributing to cognitive dysfunction (source).

You can read our comprehensive article on histamine intolerance here.

Vitamin B12 Deficiency

New research suggests that addressing vitamin B12 deficiency may improve brain fog. Vitamin B12 is closely related to energy levels and brain function, and many people do not consume or make enough of this essential vitamin due to a compromised gut microbiome, low or no meat consumption, and certain medications.

A study was conducted to see if vitamin B12 supplementation could improve symptoms of minimal cognitive impairment (MCI). The selected patients reported symptoms such as memory loss, forgetfulness, poor concentration, tiredness, lethargy, poor balance, and feeling pins, needles, and tingling sensations.

A total of 202 patients with brain fog symptoms and low vitamin B12 levels (either mild, moderate, or severe deficiency) were included in the study and given vitamin B12 for three months. The results showed that 84% of participants reported symptom improvement and 78% had improved cognitive scores after three months of taking vitamin B12 (source).

Hypothyroidism

It has become common for many patients taking levothyroxine (a medication prescribed to those with an underactive thyroid) to complain of brain fog (source).

The complaint of brain fog in patients with hypothyroidism may signify a state of persistent hypothyroidism despite treatment. This may be due to under-treatment with thyroid hormone replacement, given that it has been observed that up to 35% of patients with hypothyroidism have elevated serum thyroid-stimulating hormone (TSH) levels.

In one study where researchers interviewed patients, it was found that getting more rest and more exercise were the most commonly reported lifestyle factors to improve symptoms (51.7% and 10.4%, respectively). You may therefore need to do a little trial and error to see which of these is right for you. Several dietary factors (less sugar, less alcohol, less gluten) were reported as well, but less frequently. A total of 28.3% of participants reported that adjustment of thyroid hormone helped their symptoms. (source).

Hormonal Changes (Menopause)

Hormonal changes related to menopause may contribute to brain fog (source). Basic and clinical studies show a role for estrogen in mediating menopause-related changes in cognition. In addition, menopause symptoms, including vasomotor symptoms (VMS), sleep disturbances and mood changes contribute to cognitive difficulties at midlife.

Overall, these findings suggest that the normal hormonal changes that occur during menopause are associated with brain fog. Also other issues such as sleep and mental health disturbances may also occur during this time of life.

But just because these hormonal shifts are normal, doesn’t mean you have to suffer through the side effects. You may like to read our blog on the Estrobolome to learn more about the connection between the gut and estrogen.

How To Treat Brain Fog

Diet

  • One of the most important things you can do to support brain health is to eat an anti-inflammatory diet that focuses on whole foods and anti-inflammatory fats, like omega 3. These can be found in oily fish like salmon, mackerel and sardines (source).
  • Limit sugar, and processed foods.
  • Consider a short trial of removing gluten from the diet to see if this leads to improvement in symptoms (some find a significant improvement, others don’t notice anything. In the latter case, you can bring gluten back in to the diet).
  • Support your gut microbiome by eating a diverse diet and, if tolerated, fermented foods like kefir and sauerkraut. Please note that those with histamine intolerance may need to avoid fermented foods until the intolerance is addressed.
  • If you’re in need of a helping hand then checkout our meal delivery service.

Exercise

  • A 2016 systematic review concluded that exercise may help to treat brain fog associated with cancer and chemotherapy (“chemo fog”) (source). While this doesn’t mean it will help everyone suffering from brain fog, it does suggest many will. Exercise has been shown to reduce inflammation and improve the microbiome, which may help explain the cognitive benefits.

Sleep

  • Getting enough sleep might be one of the most important things for your gut and your brain (source).
  • Things that might help with our sleep include:
    • Getting outside in the morning to support a healthy circadian rhythm.
    • Time restrictive feeding (only eating within a certain time frame such as between 07:00-19:00).
    • Keeping the bedroom cool and as dark as possible.
    • Avoid caffeine from midday.
    • Avoid alcohol.
    • Avoid exercising in the evening unless it is gentle – yoga, tai chi, or a walk as examples.
  • One of my favourite sleep supplements for those that need a little support is Cytonight.

Stress

  • Chronic stress is a common cause of inflammation and leaky gut, which can have a negative impact on cognitive function.
  • Practicing mindfulness meditation and reducing stress levels in other ways may help to calm inflammation and reduce brain fog.
  • The product l-theanine and lemon balm, or/and ashwagandha are two great options to support you when experiencing stress.

Probiotics

Recent studies have shown that probiotics can help to improve cognitive function. But note that there is also evidence demonstrating that probiotics can cause brain fog in some people (source).

  • One study demonstrated that probiotic supplementation improves cognitive function and mood in community-dwelling older adults (source).
  • A systematic review concluded there may be potential for probiotics to enhance cognitive function or attenuate cognitive decline. Particularly in clinically relevant adult populations for whom cognitive dysfunction may be present. The authors went on to say, however, that the limited number of studies available, and the quality of those studies makes it hard to interpret the data. Watch this space!

Other Treatments

There are a few other treatments that have been shown to provide some benefits for patients with brain fog:

  • Two studies showed that the use of the supplements PEA and Luteolin (PEA-LUT) improved cognitive impairment (source).
  • Another review found that fasting can help improve cognition and brain health for those with neurological diseases.
  • Noninvasive brain stimulation and hyperbaric oxygen therapy showed promising results in the treatment of brain fog caused by long-COVID (source).
  • For some, a cold shower can alleviate brain fog in the short term. A helpful ‘hack’ if you are preparing for a presentation or important conversation!

Conclusion

There are multiple causes of brain fog and so understanding which might be your underlying cause is essential. So ask yourself:

  • Do you have gut symptoms such as bloating, abdominal pain, changes to bowel movements? If so then perhaps start with the gut.
  • Do you struggle with your energy and either get tired after eating, or find you get hangry? If so then perhaps consider focusing on blood sugar balancing.
  • Are you always cold? Constipated? Do you have dry skin and hair? Are you loosing the pouter third of your eyebrow? These are symptoms of thyroid dysfunction.
  •  Are you under chronic stress and do you need support in managing this?
  • Are you sedentary and could you focus on building some physical activity in your day/week?
  • Do you struggle with sleep? Then start here.

This is why working with a Functional Medicine practitioner can be worth the investment. They will be able to support you find your root cause(s) and create a plan of action.

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