What Does Brain Fog Have to Do With Gut Health?

Introduction

Do you have difficulty concentrating on the task in front of you? Maybe you lose track of conversations halfway through or notice yourself forgetting deadlines. You feel a little slower than normal and all of a sudden, giving something your full attention feels way more tiring than it should be.

You aren’t alone. A lot of people call this “brain fog.”

Maybe you’ve heard that your gut health could be the reason why. I’ve heard it too, and that’s what got me interested in researching it for myself.

On the surface, the connection makes sense. The gut and food are already surrounded by emotion. We associate certain foods and eating experiences with emotions. We can react emotionally when we experience digestive symptoms. So is brain fog just another way that the gut affects the brain?

Are the connections real? How can we tell? Let’s look at how the gut and the brain are connected and what the research has to say about gut health and brain fog.

What Is Brain Fog?

Brain fog has commonly been described as forgetfulness, difficulty concentrating, difficulty communicating, feeling slowed down, or a kind of fuzziness. Someone with brain fog might struggle to focus on the task in front of them, follow a conversation, or remember an appointment.

But defining or measuring brain fog beyond that has proven difficult. There’s no standard definition, and no official criteria to diagnose it. It’s unclear how much brain fog varies among people who report having it.

It seems that brain fog can’t be pinned down to one area of thinking, like memory, attention, or processing speed. For now, it’s best described subjectively in the terms that everyday people use to describe it.

In fact, in a 2024 study of over 25,000 people, more than a quarter said they had brain fog but on cognitive tests they scored only slightly lower than everyone else. This could suggest that something other than measurable declines in thinking are at play.

Brain fog can show up across many conditions: menopause, chronic fatigue syndrome, celiac, lupus, long COVID.

So, where does the gut fit in?

Can Poor Gut Health Cause Brain Fog?

The gut has connections to the rest of the body, including the brain. So researchers have started asking what role the gut plays in how well our brain works.

One clinic saw that brain fog is more commonly reported among patients with irritable bowel syndrome (IBS) and gastroparesis (slow emptying).

Across 12 studies, people with inflammatory bowel diseases (IBD) scored lower than people without it on tests of attention, executive function, and working memory.

So people whose guts work abnormally or are inflamed seem to run into thinking problems more often.

This doesn’t prove that gut problems cause brain fog, though. It hints at a relationship between the two that other factors could also explain. Something else could be causing both the fog and the gut inflammation, or pain could simply pull someone’s attention away from everything else. To show cause and effect, researchers would have to create inflammation in healthy people and watch what happens to their thinking.

It turns out, researchers have done this. When healthy men were given a small injected dose of a bacterial toxin known to cause inflammation, their memory and mood dipped for several hours. But that study shows the effects of inflammation in general. We’d need more research on inflammation that starts in the gut.

Researchers think the link between gut inflammation and brain fog makes sense for a few reasons. Cognitive problems are also common in other inflammatory diseases, like rheumatoid arthritis and lupus. Also, nutrient deficiencies that are commonly seen with IBD are themselves related to brain fog.

There’s also evidence that the brains of people with IBS are wired differently. Researchers looked at brain scans in these patients and found differences in areas tied to pain and emotion when compared to healthy subjects.

It’s possible the microbiome can play a role in brain fog indirectly, through its influence on the gut. A study following over 100,000 adults examined the relationship between how often they had a bowel movement and how well they could perform on cognitive exams. People who went every three days or less had on average about three extra years of cognitive aging, compared to those who went once every day.

The researchers also found that the overall makeup of the microbiome was tied to how often people went. So the microbiome, gut function, and thinking may all be connected.

Brain fog and gut symptoms seem to appear together more often than chance would predict. No direct cause has been found, but researchers have biological reasons to look to the gut when searching for answers.

How the Gut Talks to the Brain

Your gut and brain talk to each other through several routes. Here are three of the main ones:

1. Vagus nerve
2. Gut Microbiome
3. Serotonin

Vagus Nerve

Cells in the gut lining connect to the vagus nerve, one of your cranial nerves. The vagus nerve is connected directly to your brainstem, and it influences many different functions in your body that you don’t consciously control (your heart rate, digestion, mood, and immune responses). This connection lets the brain sense what’s arriving in your gut after you eat. Gut cells connecting with the vagus nerve can send signals to the brain in milliseconds.

It’s possible that inflammation can weaken the vagus nerve’s signals to the brain and memory can suffer, although it’s only been seen in animal studies. When vagus nerve activity was restored, memory improved.

Gut Microbiome

There appears to be a relationship between the gut microbiome and bowel movement frequency, and bowel movement frequency has potential ties to cognitive function.

Some researchers believe that changes in the gut microbiome composition due to infection, antibiotics, or probiotics can influence cognitive function.

There’s a lot of buzz around probiotics and their role in influencing the gut-brain axis. It’s been proposed that probiotics can make the microbiome healthier, which could lower inflammation and help bacteria make chemical messengers that send signals to the brain.

Gut bacteria ferment fibers, creating molecules known as short-chain fatty acids (SCFAs). SCFAs act as signals too. They help keep immune cells in the brain in balance, which helps keep inflammation in check. They may even send signals to the brain through the vagus nerve. People who had less bacteria that produced SCFAs also tended to score worse on measures of cognitive function.

Serotonin

Serotonin is a chemical messenger. It’s best known for its prominence as a neurotransmitter in the brain where it’s targeted by many medications that aim to improve mental health.

Over 90% of serotonin is synthesized in the gut, where it helps run the digestive tract. In the gut, serotonin can activate gut cells and neurons which send signals to the brain through the vagus nerve.

Serotonin prompts the gut’s nervous system to contract the gut wall. Those contractions keep food moving along through your gut.

Altered serotonin production may play a role in gut-brain disorders like IBS.

Why Fogginess Can Occur After Eating

Some people experience brain fog after eating. This suggests that the food we eat can play a role in brain fog. But how does that relate to gut health?

What you eat could have something to do with it. Researchers conducting a small trial containing 51 women concluded that their attention capacity was worse five hours after eating a meal high in saturated fat (common in the Western diet), compared to when they ate a different meal.

The Western diet has been linked to a less healthy gut and inflammation. Ultra-processed foods, a big part of that diet, have also been linked to gut disease. Eating these foods could create negative responses in your gut that appear elsewhere.

Does Fixing Gut Health Improve Brain Fog?

If the gut truly plays a role in the experience of brain fog, then we would expect that improving gut health would help resolve brain fog.

Unfortunately, it’s hard for us to know for certain, because there’s very little research on it.

It helps to observe cases with a specific gut-related diagnosis.

A study on celiac disease patients is perhaps one of the clearest examples. This small study followed 11 people newly diagnosed with celiac disease for a year after they began a gluten-free diet. They observed the state of their gut lining as well as relevant celiac antibodies. Over the year, the gut lining healed, and their scores on tests of attention improved at the same time.

So, could this mean that healing the gut lining improves cognitive function?

If we assume the pattern is true, then maybe. There are some caveats: the sample size was small, and without a comparison group it’s hard to say how meaningful these differences actually are. I’d be excited to see future research on a similar subject with larger groups and different conditions.

On the other hand, a study of over 13,000 middle-aged women without celiac found that the amount of gluten they ate over two decades had no link to their scores on cognitive tests.

This could mean that there was already no room to improve. Without celiac, gluten wasn’t damaging their gut in the first place so there was nothing to repair. If we interpret these results alongside the celiac study, it suggests that the gut may only affect cognitive functions when something is wrong with it. That’s just my theory, though.

It also means there’s no sign that going gluten-free helps cognitive function in people without celiac. If you suspect your brain fog is related to your gut, it’s better to hold off on restrictive diets until you know more about what’s going on.

What about gut bacteria? Can they help improve brain fog?

A trial of 36 pairs aged 60+ put that to the test. One twin in each pair took a daily prebiotic, a supplement that feeds your beneficial gut bacteria. After 12 weeks, the prebiotic changed the microbiome in those who took it. The prebiotic group also improved on a cognition score compared to the control group.

Microbiome diversity (the variety of species within your microbiome) is commonly cited as an important marker of microbiome health, but in this case the twins on the prebiotic didn’t end up with a more diverse microbiome than their siblings. So something else might be at play. Perhaps it isn’t bacteria themselves that affect brain fog, but what they produce from the prebiotic fibers. Or it’s possible that prebiotics could be affecting cognition in other ways.

In a different study, after 8 weeks on a prebiotic diet, subjects didn’t improve in cognitive domains and researchers called the results inconclusive. What gives? If anything, it means this topic needs more research.

The effects of probiotics are also hard to pin down. Some reviews find higher scores on cognitive tests after daily probiotics in people who had cognitive impairments. Others find no clear effect. Part of the reason may be that a lot of probiotic studies have small sample sizes, work with different populations, and test many different probiotic strains. Different strains can have different effects, which leads to a lot of variation in the research.

Gut-focused changes seem to help most when something is clearly wrong, like a diagnosed gut condition. In healthy people, results have been small or unclear.

Most of these trials only measured thinking on tests. They didn’t ask people whether they felt foggy. So we don’t know whether the “healthy” people noticed any change in their own mental clarity, because that wasn’t what was tested.

If you have gut symptoms alongside your brain fog, improving your gut health might be worth a try.

Gut Health and Mood

What about mood? From the same bowel-habit study mentioned earlier, people who had less frequent bowel movements were also more likely to have depressive symptoms. And the SCFAs your bacteria make may play a role in mood too. Human evidence remains unclear.

A recent analysis of probiotic studies found moderate benefits for depressive symptoms in people diagnosed with depression. It found mixed effects on anxiety. Any study analyzing probiotic data is going to have the same strain problem from earlier. However, this does suggest that there are links between the gut and depression specifically.

Another review of over 3,000 healthy adults found probiotics can provide a small improvement in everyday symptoms of depression, anxiety, and stress, the kind that don’t qualify for a diagnosis. The researchers suggest probiotics may help build a healthy, diverse microbiome, which could soften the harmful effects of stress and help with emotional regulation.

Other Causes of Brain Fog

Brain fog isn’t only a gut problem. It has no clear definition and looks different from person to person, so it can come from many places outside the gut.

Sleep

Short term sleep deprivation has been shown to have effects in reducing performance in almost every cognitive domain tested. The biggest drops were in vigilance and simple attention, the kind of focus you need to stay on task. Most of us aren’t pulling all-nighters, but we lose an hour or two of sleep here and there. Consistent, low-quality nights of sleep over time could start to hurt you too. If you’re feeling foggy, take a look at how you’ve been sleeping.

Vitamin Deficiencies

The vitamin with the clearest link to brain fog is B12. B12 deficiency affects about 2-3% of US adults and can cause fatigue, brain fog, and depression. The risk increases with age, low B12 intake in your diet, poor absorption of the vitamin in your gut, and certain medications for diabetes and acid reflux.

Folate (B9) works with B12, and a deficiency in either one can cause overlapping problems with mood and thinking.

Studies that tracked people over time linked low vitamin D to a higher risk of cognitive impairment and dementia, although no one knows why. Studies of vitamin D supplements have had mixed results on cognitive effects. It’s possible that there is an unknown factor that might influence vitamin D levels and brain fog at the same time.

Iron Deficiency

Low iron is common, especially in women. About 38% of non-pregnant women of reproductive age are low in iron without being anemic. One of the symptoms is difficulty concentrating. That means a normal blood count doesn’t rule out an iron deficiency that’s related to brain fog. Low iron shows up in a different test, called ferritin.

Iron supplementation has been shown to improve short-term memory, fatigue, anxiety, and overall physical well-being in people who were low in iron but not anemic. The improvements were small to moderate.

The gut could play a role, since celiac disease and IBD are common causes of poor iron absorption.

Thyroid

Thyroid problems are a classic cause. One survey found 79% of people with hypothyroidism reported brain fog often or all the time. Fogginess can show up before a diagnosis and doesn’t always go away with treatment.

Medications

Some common medications can cloud your thinking. Benzodiazepines, first-generation antihistamines, and tricyclic antidepressants consistently impair attention, information processing, or memory. Make your doctor aware of any side effects or brain fog-like symptoms if you’re taking any of these.

Head Injury

A study of over 25,000 people revealed that people with a history of concussion had 2.4 times the odds of reporting brain fog. Feeling foggy or slowed down are common concussion symptoms. If you’ve hit your head recently and are experiencing brain fog, get checked by a clinician. Go to the emergency room right away if you notice a headache that gets worse and won’t go away, repeated vomiting, seizure, slurred speech, weakness or numbness, increasing confusion or agitation, or one pupil larger than the other.

Long COVID and Migraines

Long COVID has recently gained attention in the context of brain fog.

It describes symptoms that linger after a COVID infection, most commonly fatigue, feeling worse after exertion, and memory loss. The cause is still unknown, and there’s no established treatment. In the 25,000-person study, people with long COVID had 3.8 times the odds of reporting brain fog, the strongest link of any condition in the study.

People with brain fog also reported more disabling migraines.

Chronic Fatigue Syndrome and Fibromyalgia

Brain fog is also a hallmark of chronic fatigue syndrome and fibromyalgia. In a study of nearly 80,000 people, those who met the criteria for either condition reported far more trouble concentrating than people without them, even though cognitive test scores were only mildly lowered.

Pain and fatigue can pull attention inward, making everyday thinking take more effort than it normally should.

Gut health is best looked at alongside these other factors. If focus is only given to the gut, something important might be missed. Still, the gut deserves its fair share of attention.

Could Your Fog Be Gut-Related? What To Do

If you suspect your fogginess is related to your gut, start tracking your symptoms and when they happen. Signs that may point toward your gut include bloating and slow or irregular bowel movements. If you have a diagnosed gut condition like IBS or IBD, that also might point toward an explanation for your brain fog.

Track your own daily patterns: does your brain fog appear when your bloating does or after certain meals? How often does it show up and how long does it take for it to go away? Answering these questions can show you whether your fog follows your gut.

Rule out common non-gut causes with a clinician. Looking at your sleep habits and getting tested for vitamin deficiencies or ferritin levels are the easiest places to start.

If you suspect your gut is involved, the biggest change you can make for your gut is your diet. Adding fiber to your diet gradually, up to about 25g a day for women and about 38g a day for men, can be an easy way to feed your gut microbiome.

Don’t restrict your diet without information. Add the good stuff that you need and keep tracking your symptoms alongside what you eat. If you notice patterns that keep occurring, modifying your diet further might be necessary, but talk to a qualified medical professional before any big changes.

The goal is integration. Your brain fog might not be directly caused by your gut, but taking care of your gut can help your gut feel and work better. When your body is telling you something is wrong, your gut could be one piece of the puzzle.

Conclusion

Brain fog and gut symptoms appear together more often than chance alone would predict. The gut and the brain talk to each other through the vagus nerve, the microbiome, and other chemical messengers. But no direct causes have been clearly identified. Most of what we know comes from relationships we see in observational studies.

Fixing the gut seems to help thinking most when there’s something clearly wrong, like we saw with celiac disease patients.

Unfortunately, a lot of studies in healthy subjects don’t measure brain fog directly. So the honest answer is that we don’t know a lot.

Research is still emerging. But it suggests that taking a look at your gut and your diet could be worth your time, especially if your brain fog shows up alongside gut symptoms. The gut might not be the whole story, but it could be one step in your plan towards getting answers.