Office Hours: 5 Foods That Are Making Your Brain Fog Worse

Mark Hyman, MD

Dr. Mark Hyman is leading a health revolution—one focused on using food as medicine to support longevity, energy, mental clarity, happiness, and so much more. Mark Hyman, MD is a practicing family physician and an internationally recognized leader, speaker, educator, and advocate in the field of Functional Medicine. He is the founder and director of The UltraWellness Center, Founder and Senior Advisor for the Cleveland Clinic Center for Functional Medicine, a sixteen-time New York Times best-selling author, and Board President for Clinical Affairs for The Institute for Functional Medicine. He is the founder and chairman of the Food Fix Campaign, dedicated to transforming our food and agriculture system through policy.

He is a co-founder and the Chief Medical Officer of Function Health. He is the host of one of the leading health podcasts, The Dr. Hyman Show with 150+ million downloads. Dr. Hyman is a regular medical contributor to several television shows and networks, including CBS This Morning, Today, Good Morning America, The View, Fox, and CNN.

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Full Transcript

Automatically generated. Please forgive any typos or errors in the following transcript. It was generated by a third party and has not been subsequently reviewed by our team.

Dr. Mark Hyman
Why do some foods leave you mentally exhausted? What can you do about it? Well, if your brain feels like it's running in slow motion, you can't focus, you lose your train of thought, you walk in a room and forget why you're there, maybe you struggle to find the right words, you you might be thinking you're just getting older. Well, you're probably not. Well, you are getting older, and that's probably not the reason.

One of the biggest myths about brain fog is that it's a normal part of aging. It is not. In functional medicine, we don't think of brain fog as a diagnosis. We think of it as a symptom, a warning light on your body's dashboard telling you something deeper needs attention. And one of the biggest drivers may surprise you.

It isn't your brain, it's your gut. I know how frightening brain fog can be because I've experienced it myself. After suffering mercury poisoning, I developed severe brain fog. I felt I was trying to think through wet cement. My memory wasn't sharp.

I couldn't concentrate. I mean, even simple tasks suddenly really felt difficult. That experience completely changed the way I think about brain health. Because I learned this, your brain doesn't function in isolation. It's constantly communicating with your gut, with your immune system, with your metabolism, and even the trillions of bacteria living inside your digestive tract.

Today, I'm gonna show you the five foods I most commonly see contributing to brain fog in susceptible people and why they affect your gut and how that disruption can ripple all the way to your brain. And most importantly, what you can do to start thinking more clearly again. Because brain fog isn't something you simply have to live with. It's often your body asking you to look a little deeper. Brain fog isn't a disease.

It's a symptom. Just like a cough or a fever. You can have dozens of causes, poor sleep, stress, dehydration, nutrient deficiencies, hormonal changes, environmental toxins, blood sugar swings, even certain medication. This episode was brought to you by Function. Get access to over 160 lab tests annually at functionhealth.com/mark.

Today, we're focusing on one piece of that puzzle, one of the biggest and one of the most overlooked, the gut. Now at first, that might sound strange, but what does your digestive system have to do with your ability to think clearly? Well, the answer is almost everything. Your gut and your brain are in constant communication. The scientists often refer to this as the gut brain axis.

Think of it like a two way highway. Your brain is constantly sending messages to your gut, and your gut is constantly sending messages back to your brain. One of the main communication pathways is a remarkable nerve called the vagus nerve. It's the longest cranial nerve in your body running from your brain stem all the way down to your digestive tract. It's constantly carrying information in both directions.

In fact, much of the traffic travels from your gut to your brain, not the other way around, which means what's happening in your gut has enormous influence on how your brain feels and functions. Then there are trillions of bacteria living inside your digestive tract, what we call the gut microbiome. These microbes aren't just helping you digest food. They're producing vitamins. They're training your immune system.

They're helping regulate inflammation. Even producing or influencing many of the chemical messengers your brain relies on. For example, about 90% of your body's serotonin is produced in the gut. Now serotonin made in the gut doesn't simply travel into the brain because of the blood brain barrier, but it still plays an important role in the gut itself. It communicates with the nervous system in ways that can influence mood, digestion, and your overall health.

Your gut microbiome also helps produce compounds called short chain fatty acids. These help nourish the cells lining your colon. They help reduce inflammation. They support the integrity of your gut lining. They may even influence brain function through immune and nervous system signaling.

In other words, a healthy gut creates an environment that supports a healthier brain. But when the gut becomes unhealthy, which is a lot of the time, the opposite can happen. One problem we often see in functional medicine is something called intestinal permeability increase or leaky gut. Your intestinal lining is designed to act like a highly selective filter. It lets nutrients pass through into your bloodstream while keeping bacteria, toxins, and partially digested food where they belong.

But when that barrier becomes damaged, some of the substances can slip through than only would. Your immune system recognizes them as foreign, and then it mounts an inflammatory response. And that inflammation doesn't stay confined to your gut. It can affect your entire body, including your brain. When inflammation reaches the brain, people experience it as mental fatigue or trouble concentrating or poor memory or what we commonly call brain fog.

One of the most fascinating examples of this comes from lung COVID. Now many people with lung COVID describe persistent brain fog that lasts for months after their infection. For a long time, researchers assumed the problem was happening primarily in the brain. But newer research suggested in some people, lingering viral material in the gut may continue to activate the immune system. Now that ongoing immune activation can alter serotonin signaling.

It can contribute to fatigue, to brain fog, and lots of other lingering symptoms. Now the research is still evolving, but it's another powerful reminder that your brain doesn't function in isolation. It's deeply connected to what's happening in the rest of your body, especially your gut. So that naturally raises the next question. What damages the gut in the first place?

Well, there are lots of possibilities, chronic stress, poor sleep, certain medications, infections, environmental toxins, and and one of the biggest influences is the food that you eat every single day. Now before I start this list, I wanna be really clear. Functional medicine isn't about creating a list of good foods and bad foods. It's also not about putting everyone on the same diet. One person's superfood can be another person's trigger.

The goal isn't restriction. The goal is discovering what helps your body function at its best. And that said, after treating thousands of patients over years, I've noticed that some foods show up again and again in people struggling with brain fog. So let's count down the five foods I most commonly see contributing to brain fog in susceptible people. Number five.

Let's start with one that probably won't be a surprise to you, ultra processed foods. These foods are heavily altered from their natural state. They're often loaded with refined carbohydrates, sugars, industrial oils, artificial additives, emulsifiers, flavor enhancers, preservatives, the whole list of things. It's terrible. The problem isn't just one ingredient.

It's the combination. It's the ultra processing. These foods tend to feed the less beneficial gut bacteria while crowding out the microbes that keep your gut healthy. Research has linked diets high in ultra processed foods with lower microbial diversity, greater intestinal permeability, and higher levels of chronic inflammation, which essentially is what causes brain fog. They also tend to create rapid spikes and crash in blood sugar.

Your blood sugar is on a roller coaster, your brain often comes along for the ride. Now many people describe it as the afternoon crash, difficulty concentrating, or feeling mentally sluggish after meals. Next food is number four. Well, it's not really a food, but something you consume, alcohol. Many people assume that one or two drinks a day is harmless.

But let's be clear. An occasional drink isn't likely to cause brain fog in everyone, but alcohol can affect the brain in several different ways. One, it disrupts sleep quality. Even if it initially helps you fall asleep faster, it increases intestinal permeability or leaky gut. It alters the gut microbiome, and it promotes inflammation.

All of those factors can influence how your brain functions the next day. And many people blame feeling foggy and being busy when in reality, their brain may still be recovering from what they drank the night before. Alright. Coming in at number three, the thing you shouldn't eat, artificial sweeteners. Now many people switch from sugar to artificial sweeteners because they're trying to make a healthier choice.

Now you understand why. And the thinking is, well, I'm not eating sugar. I'm protecting my brain. My metabolism, my waistline. But biology is rarely that simple.

Some artificial sweeteners, particularly sucralose and saccharin, have been shown in the research to alter the composition of the gut microbiome. Some studies have also found that they may affect the way certain people regulate blood sugar, although the evidence is still evolving, and not all studies have reached the same conclusions. But remember, your gut microbes aren't just passive passengers. They're active participants in your health. They help regulate inflammation.

They support the gut barrier. They produce compounds that influence your immune system. They communicate with your brain. So when you disrupt that ecosystem, your brain can feel the effects. Now another thing I want you to think about is what these sweeteners are teaching your body.

You're experiencing intensely sweet taste, but very little or no actual nourishment. For some people, that may increase craving for sweet foods or make it harder to reset their palate. In functional medicine, our goal isn't simply to replace sugar with something that tastes just as sweet. It's to help people become less dependent on intensely sweet foods altogether. Let's talk about that 3PM crash.

You know the one where your energy drops, your focus disappears, and suddenly you're reaching for another coffee just to finish the day. Most people think that's normal. Here's why it's not. Steady energy depends on stable blood sugar. And when your blood sugar spikes and crashes, your energy often does the same thing.

The good news, this isn't a mystery. It's measurable when you test fasting glucose, h b a one c, and fasting insulin. Function gives you access to 160 plus lab tests annually, including these blood sugar markers, hormones, nutrients, and lots more. Because your body's trying to tell you something, check your health. Go to functionhealth.com/mark.

And if you're one of the first thousand people this week, use the code Mark twenty twenty six for a $50 credit toward your $365 annual membership. Does that mean everyone needs to avoid all artificial sweeteners? No. Not necessarily. Some people may use them occasionally without any noticeable problems.

And not all non nutritive sweeteners are created equal. But if you're drinking several diet sodas a day, adding artificial sweeteners to your coffee, eating protein bars, yogurt, snacks made with them, and you're struggling with brain fog, it's worth asking whether it's contributing to the problem. Sometimes a healthy swap isn't finding a different sweetener. It's gradually retraining your taste buds to enjoy foods that are less sweet or naturally sweet. Because when you change your palate, you often change your health.

Alright. Let's talk about dairy. Dairy is one of the most individualized foods I see in practice. Some people can enjoy yogurt, kefir, cheese without any problems at all. Others notice that they feel congested.

They feel bloated. They feel tired, maybe mentally foggy after eating it. So what's the difference? One possibility is lactose intolerance. Lactose is the natural sugar found in milk.

As we get older, many of us produce less of the enzyme needed to digest it. That can lead to symptoms like gas, bloating, diarrhea. But lactose intolerance usually doesn't explain brain fog. What's often more interesting from a functional medicine perspective is a protein in dairy, and especially this protein called casein. Now there are two main forms of casein, a one and a two casein.

And most conventional dairy from Holstein cows contains a mixture of a one and a two. While milk from certain breeds like Jersey and Guernsey cows, well as goat and sheep milk have predominantly A2 casein. Now here's why that matters. When A1 casein is digested, it can produce a peptide called beta caseomorphin seven or BCM seven. Now researchers have been studying this compound because it appears to interact with the body differently than A2 casein.

Some studies suggest that in susceptible individuals, A1 dairy may be associated with more digestive discomfort and inflammatory responses than A2 dairy. Now the research is still evolving, we certainly don't have evidence that everyone should avoid any one dairy. But clinically, I've seen patients who don't tolerate conventional milk very well and do just fine with dairy or dairy from goats or sheep. I'm one of those people. Another thing to consider is how dairy is produced.

A couple plain grass fed yogurt is very different from a highly processed sugar loaded flavored yogurt, or fermented foods like yogurt or kefir also contain lots of beneficial bacteria that can actually help support the gut microbiome in lots of people. So we can't talk about dairy as though it's one single food. The quality matters, the type of dairy matters, and most importantly, your body's response matters. The very time you eat dairy, feel bloated, congested, fatigued, or thinking becomes less clear, well, your body's probably telling you something. That's where an elimination and a reintroduction trial can be really helpful.

Remove dairy for about three or four weeks, pay attention to your energy, your digestion, your skin, your focus, and then slowly reintroduce and see how you feel. Because the goal isn't to avoid dairy forever. The goal is to understand whether dairy is working for you or working against you. And remember, sometimes your brain recognizes a food sensitivity before your digestive system does. And finally, the most important food to consider in number one is gluten.

This is probably the food I get asked about more than any other. It's also one of the most misunderstood. So let's start with what we know for sure. If you have celiac disease, avoiding gluten is not optional. It's an autoimmune disease where eating gluten triggers your immune system to attack the lining of your small intestine.

Over time, that damages the gut. It creates inflammation. It interferes with nutrient absorption, and it can contribute to symptoms far beyond the digestive tract, including fatigue, headaches, mood changes, and, yeah, brain fog. It's very well established. But celiac disease is the only part of the story.

Some people have a wheat allergy, which is classic allergic response involving the immune system. That's different than celiac. There's a third group, people who don't have celiac disease but still have a wheat sensitivity. They don't really feel well when they eat gluten containing foods. It's often called non celiac gluten sensitivity.

Science is evolving around this. Researchers are working to better understand exactly why it happens. But for some people, it may be the gluten itself. For others, it may be proteins found in wheat or certain fermentable carbohydrates called FODMAPs. For others, it may be a combination of factors that increase inflammation or affect the gut barrier.

We don't have all the answers yet, but here's what I can tell you from decades of practicing functional medicine. I've seen thousands of patients struggling with brain fog, with fatigue, with digestive issues, with joint pain, skin problems, even autoimmune conditions, who experience remarkable improvements after removing gluten for a period of time and then carefully reintroducing it. That's why gluten is always one of the first foods I investigate when someone comes to my office with unexplained brain fog. Not because I believe everyone should avoid gluten. I don't.

Many people tolerate gluten just fine. But if your immune system is reacting to it, it can create chronic inflammation in the gut throughout your whole body. It can disrupt the integrity of your gut lining. It can interfere with absorption of nutrients your brain depends on, like b vitamins, iron, magnesium, zinc. And remember what we talked about earlier.

Your brain and your gut are constantly communicating. So when the gut's inflamed, the brain often pays the price. Functional medicine isn't about fear. It's about curiosity. It's about asking, is this food helping me thrive, or could it be getting in the way?

If you've been living with brain fog for months or even years, a simple elimination and reintroduction experiment may give you more information than you ever expect. Sometimes the answer isn't another supplement or another medication. Sometimes it's discovering that your body has been trying to tell you something every time you sat down to eat. Alright. Now here's the thing you should notice.

None of these foods are directly causing brain fog. They're changing the environment inside your gut. They're increasing inflammation. They're disrupting the microbiome. They're affecting the gut barrier or making it harder for your body to function the way it was designed to.

And that raises an important question. How do you know if one of these foods is affecting you? Let's talk about how to find your own personal triggers. Most people don't need to eliminate everything forever. In fact, I don't recommend that.

Instead, think of your body as a laboratory. Every time you eat, your body's running an experiment. The question is, are you paying attention to the results? Now if you suspect different foods that are contributing to brain fog, you should remove those foods. In fact, you should remove all the things that potentially could cause brain fog for about three or four weeks and see how you're doing.

Why should you do it that long? Well, because if that food is contributing to inflammation or disrupting your gut, it can take time for your body to calm down for you to notice a meaningful difference. But I also know that for a lot of people, that can feel overwhelming. You may be wondering, where do I start? Well, that's actually one of the reasons I created the ten day detox diet.

It's a structured whole food reset designed to temporarily remove most of the common inflammatory triggers, ultra processed foods, and junk while helping stabilize your blood sugar, support your gut microbiome, nourish your body with nutrient dense foods. Now despite the name, it's not about drinking juices or doing an extreme cleanse. Your liver and kidneys already do a great job of detoxifying your body. The idea is to give your body a break from many of the foods that commonly disrupt metabolism and contribute to inflammation while also flooding it with real whole foods to support healing. And for many people, ten days is enough to notice really meaningful changes.

They tell me they have more energy. They're more stable in their mood. They have better focus and better fewer fewer cravings. In many cases, less brain fog. In most cases, in fact, think of the ten day detox as a reset.

If your symptoms improve, then you figure something out, something you're eating. And if you wanna learn more about how it works, about the ten day detox, click the link below in the show notes. And whether you choose a structured program like the ten day detox or foods over several weeks and do it on your own, the principle is exactly the same. You're gathering information. You're learning how your body responds.

And during those few weeks, don't just ask yourself, do I have brain fog today? Pay attention to the whole picture. How's your energy? How's your focus? Are you remembering things more clearly and easily?

Is your mood more stable? Are you sleeping better? How's your digestion? Has your skin changed? Your joints feel better?

Because when food is creating inflammation, the effects often show up in more than one place. I also encourage people to keep a journal. Just simple journal. Nothing complicated. Just write down what you eat and how you feel.

And any changes you notice, patterns that seem invisible day to day often become obvious when you start to look back over a few weeks. And then comes the most important step, the reintroduction of each food. So eat a normal serving, pay attention over the next day or two, Do only one food at a time, so introduce gluten and then do that for three days. And then dairy, do that for three days. And then maybe soy for three days.

Whatever you've eliminated, slowly introduce those things, not all at the same time because you won't know what's going on. And notice do you feel better? Does your brain fog come back if you eat gluten? Do you feel more tired? For example, if you dared your digestive system to return.

Do you notice headaches, joint pain, or change in your mood? Now if nothing changes, that's good information. That food is probably not an issue for you. But if your symptoms come back, well, you'll learn something really important. Not because it means you'll never eat that food again, but because you now understand how your body responds, and that's what functional medicine is all about.

So instead of guessing, instead of following the latest trend of food or copying someone else's diet, gather information for yourself. You test yourself. You observe. You personalize your nutrition based on what your body tells you. There's no single perfect diet for everybody.

There's only the diet that helps you think clearly, feel your best, and thrive. Okay. So here's what I hope you remember. Your brain isn't separate from the rest of your body. Every bite of food, every night of sleep, every interaction with your microbiome, every change in your metabolism is sending signals to your brain.

That's why functional medicine asks a different set of questions. Instead of asking what drugs treats brain fog, we ask why is the brain struggling in the first place? Because brain fog isn't the problem, it's the messenger. And when you listen to that messenger, when you begin healing your gut, stabilize your blood sugar, reducing inflammation, nourishing your microbiome, you're not just helping your digestion. You're helping your brain do what it was assigned to do.

Think clearly, learn, remember, focus, create, connect. Your brain has an extraordinary capacity to heal when you remove what's getting in the way. So don't settle for feeling like you're walking around through mental quicksand. Start asking better questions because a clear mind often begins with a healthier body, and that's one of the most empowering things I know. If you love that last video, you're gonna love the next one.

Check it out here.