Brain Fog After Eating? These Are the Worst Culprits

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Last updated March 20, 2024.
Edited and medically reviewed by Patrick Alban, DC. Written by Deane Alban.

If you experience brain fog after you eat, you need to look at your diet. Follow these steps to uncover the offending foods and clear your fuzzy thinking.

Has this ever happened to you? …

You just finished eating a meal.

But instead of feeling energized as you should, you feel exhausted and mentally fuzzy, and you just can’t think.

When you experience brain fog after eating, your brain is telling you that there’s a problem.

Certain foods, additives, an imbalance of macronutrients, or the amount you eat can all be to blame.

Finding the answer requires some detective work, but it’s worth it.

How Food Allergies Can Cause Brain Fog

If you have brain fog after eating a certain food, you may be allergic to it.

Here’s a list of the top allergenic foods known as “The Big 8”: 

  • dairy
  • eggs
  • fish
  • peanuts
  • shellfish
  • soy
  • tree nuts
  • wheat

It can be challenging to determine if any of these are causing your brain fog since these foods are commonly hidden in processed foods.

FoodAllergy.org has compiled an extensive list of hidden sources of these and other top food allergens.

Here’s how to access it.

Go to their food allergens page and select the food in question.

There you’ll find comprehensive information on foods to avoid, plus lists of unexpected sources of those foods.

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And while the foods in the top 8 list account for 90% of food allergic reactions, it’s possible to be allergic to almost any food. 

Some people are allergic to just one specific meat, fruit, vegetable, seed, or even spice.

Food Sensitivities and Intolerances Commonly Cause Brain Fog

You don’t have to be truly allergic to a food for it to cause brain fog.

A food sensitivity or food intolerance can also cause mental fuzziness.

These are not the same thing as a true food allergy.

Food allergies are due to an overreaction of the immune system

Intolerances, on the other hand, work by other mechanisms and are much more common than true food allergies. 

They can occur when enzymes needed to digest a specific food are lacking, as happens with lactose intolerance.

Intolerances can also be due to compounds present in food such as additives, histamines, or salicylates. 

Some people are intolerant of a group of short-chain carbohydrates collectively known as FODMAPs.

It’s estimated that 15% of the population can’t properly digest foods high in FODMAPs, which can lead to intestinal distress and, often, brain fog.

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The foods most likely to cause intolerance reactions include: 

  • dairy products
  • chocolate
  • eggs, particularly egg whites
  • artificial sweeteners
  • food additives, including colorings, flavorings, and MSG
  • strawberries, citrus fruits, and tomatoes
  • histamine-rich foods such as fermented foods, vinegar, red wine, cured meats, and aged cheese

How Wheat and Gluten Contribute to Brain Fog

For thousands of years, bread has been called the staff of life.

But modern wheat bears little resemblance to wild wheat or the earliest domesticated wheat.

It has been hybridized and crossbred to increase yields, with little consideration as to whether these new strains of wheat are healthy to eat. 

Wheat now contains significantly fewer nutrients than it once did. 

Just as important, other compounds in wheat can trigger negative reactions (such as fructose and fructans, both of which are FODMAPs) but, for most people, gluten is the main culprit

The Problem With Gluten

Until recently, it was not understood how ubiquitous problems with gluten have become.

Roughly 1% of the population has celiac disease — up five-fold from 50 years ago — while another 6% has gluten sensitivity

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Celiac disease is quite serious and, if left undiagnosed, is linked to a nearly four-fold increased risk of death

Research suggests that modern wheat uniquely triggers chronic inflammation and an autoimmune reaction in the gut. 

This is thought to be the main reason that celiac disease and gluten sensitivity are on the rise.

It’s been known for decades that gluten can cause a long list of neurological problems in those who are gluten-sensitive, including: 

  • dementia
  • memory loss

A review of Mayo Clinic medical records covering 35 years discovered a strong link between celiac disease and dementia and other forms of cognitive decline.

Cognitive decline could often be stabilized or even reversed when patients were faithful to a gluten-free diet.

Gluten is most commonly associated with wheat, but can also be found in other grains like rye, oats, and barley, prepared foods of all kinds, beer, medications, and nutritional supplements. 

Why Processed “Gluten-Free” Foods Are Not the Answer

If you experience brain fog after eating, eliminating wheat and gluten is definitely something to consider.

However, avoiding gluten should not have you running to the gluten-free aisle of your grocery food store.

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Gluten-free food has become a big business and you can easily find highly processed gluten-free foods such as cake mix, pizza crust, bread, and crackers.

" After decades in practice, neurologist David Perlmutter, MD, concluded that “nothing is worse for your brain than a low-fat diet.”

These aren’t “health foods” — they are junk foods in disguise — so don’t expect that switching to them will make your thinking clearer.

To eliminate gluten from your diet, focus on unprocessed foods that are naturally gluten-free.

It will be only a few weeks until you know if wheat or gluten is the root cause of your lack of mental clarity.

How the Blood Sugar Roller Coaster Causes Brain Fog

The brain’s main fuel source is glucose.

Complex carbohydrates, the kind of carbs found in unprocessed fruits, vegetables, whole grains, and legumes, provide the brain with a steady supply of the energy it needs.

Simple or refined carbohydrates like sugar, high fructose corn syrup, and flour products send blood sugar levels on a roller coaster ride, first up and then down.

When your brain glucose level becomes low, you may experience some of these mental symptoms:

  • brain fog
  • mood swings
  • irritability
  • tiredness
  • mental confusion
  • dizziness
  • shaking
  • anxiety
  • impaired judgment

Besides reactivity to gluten, here’s another good reason to avoid wheat.

Wheat (even whole wheat) has a higher glycemic index than a Snickers candy bar

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Regular consumption of white sugar and other refined carbohydrates can lead to insulin resistance and diabetes.

This can lead to memory loss and even Alzheimer’s, a disease that many experts believe is a form of diabetes that selectively targets the brain. 

If you fall into a mental fog several hours of eating, you may be experiencing a condition known as reactive hypoglycemia.

When this occurs, your blood sugar dips too low two to five hours after a meal. 

How Too Many Carbs & Too Little Fat Lead to Brain Fog

David Perlmutter, MD, is uniquely qualified to speak about the effects of food on the brain.

He is a neurologist, a Fellow of the American College of Nutrition, and author of several bestselling books, including Grain Brain: The Surprising Truth about Wheat, Carbs, and Sugar–Your Brain’s Silent Killers.

He recently retired from an active practice at the Perlmutter Health Center where he used a variety of complementary nutrition techniques to treat neurological problems, including dementia, stroke, and Alzheimer’s.

After decades in practice, Dr. Perlmutter concluded that “nothing is worse for your brain than a low-fat diet.”

The brain is 60% fat by weight and, according to Perlmutter, the ideal brain-healthy diet contains 50-60% good fats (by calories).

In his practice, he has had excellent results putting his patients on a diet rich in healthy fats like olive oil, coconut oil, butter, grass-fed meat, wild salmon, nuts, and eggs.

You, too, can get off the blood sugar roller coaster by removing simple carbohydrates from your diet and having some complex carbohydrates, protein, and healthy fats at each meal.

Give your brain an added boost by including plenty of proven brain-healthy foods such as salmon, blueberries, kale, nuts, coconut oil, olive oil, and green tea.

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And finally, don’t overeat.

Too much food at one sitting, regardless of what it is, can make you lethargic and unable to focus.

Food Additives Linked to Brain Fog

The US Food and Drug Administration (FDA) allows the use of over 10,000 additives in the US supply. 

Whenever you eat processed food, fast food, or restaurant food, you really don’t know what you are consuming in the way of hidden additives.  

This is another good reason to make unprocessed foods the major part of your diet.

According to the FDA’s website, some ingredients can be lumped together and listed as “flavors,” “spices,” “artificial flavoring,” or “artificial colors” without naming each ingredient.

Two common food additives, monosodium glutamate (MSG) and artificial sweeteners, are notorious for causing brain fog.

Monosodium Glutamate (MSG)

Not everyone is bothered by this ubiquitous flavor enhancer, but in sensitive individuals, it can cause myriad problems.

Reported side effects from MSG ingestion include brain fog, dizziness, lightheadedness, headache, sweating, abdominal pain, hives, muscle tightness, numbness, tingling, general weakness, heart palpitations, and flushing. 

This chemical food additive gets broken down into glutamate, a known neural excitotoxin that, in excess, can stimulate brain cells to the point of damage and, ultimately, death.

Avoiding MSG can be tricky since innocuous-sounding ingredients like seasoning, spices, natural flavors, and hydrolyzed soy protein can contain hidden MSG.

Related on Be Brain Fit —
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The FDA states that naturally occurring MSG doesn’t need to be included on food labels which is no consolation if it’s a trigger for you. 

Foods high in naturally occurring MSG include seaweed, soy sauce, parmesan cheese, tomatoes, hydrolyzed vegetable protein, autolyzed yeast, soy extract, and protein isolate. 

Some of the worst sources of added MSG are foods high in sodium — carryout food, canned soups, salty snacks, ramen noodles, and veggie burgers.

In fact, a good rule of thumb is that the saltier the food, the more likely it contains MSG.

If you feel MSG-related symptoms coming on, immediately take vitamin C, taurine, or ginger

These natural supplements can minimize the side effects of an MSG attack.

Artificial Sweeteners

Artificial sweeteners like aspartame and sucralose have been controversial since they came on the market, and they are bad news for your brain.

Aspartame is comprised of three brain-damaging chemicals — aspartic acid, phenylalanine, and methanol.

According to the Epilepsy Foundation, over 10,000 complaints of adverse reactions involving aspartame have been reported to the FDA. 

The most commonly reported side effects include brain fog, migraines, dizziness, mood swings, and even serious memory loss. 

One study found that participants who consumed aspartame experienced irritability and depression and performed worse on some cognitive tests. 

Sucralose starts with sugar bonded to chlorine (a highly toxic chemical) via phosgene, a toxic gas used to make pesticides. 

It also has a long list of common neurological side effects, including brain fog, headaches, migraines, dizziness, anxiety, depression, tinnitus, and, ironically, weight gain.

To learn more about the hazards of artificial sweeteners, MedicineNet.com has compiled a comprehensive article that includes lists of suspected and reported side effects for these and other non-nutritive sweeteners.

Determine the Source of Your Brain Fog With an Elimination Diet

You may be able to figure out which foods are troublesome for you by keeping a food diary — making note of everything you eat and how you feel afterward.

If that doesn’t provide the answer, you can largely rule out food intolerances with an elimination diet.

This involves avoiding a food for two weeks, then reintroducing it gradually and noting your reactions.

For details, download the elimination diet patient handout created by the University of Wisconsin School of Medicine and Public Health.

This includes a chart that you can use as a food diary.

Interestingly, doing an elimination diet is usually more helpful than having food allergy tests done. 

Such tests often don’t reveal food sensitivities even though they are much more common than true allergies.

And if you can’t figure this out on your own, seek professional help.

You can find a healthcare professional who understands how food impacts the brain in our Mental Health Resources Guide.

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