The mind is a terrible thing to waste. If you’re having a hard time thinking, trouble focusing, or just not feeling like yourself, you might be experiencing a condition known as brain fog.

It’s not a headache, not depression, but a general feeling that clouds your mind and prevents you from being you. Not only that, if you’re a high-achiever with lofty goals, brain fog could be preventing you from accomplishing them.

If brain fog sounds like something you might be experiencing, the good news is that there’s hope. Brain fog isn’t a terminal illness or a personality flaw, but rather, a fixable condition. All it takes is a little understanding of how the brain works, and how interventions like diet can work to improve its function.

Lost in the Clouds: What is Brain Fog?

Brain fog is a vague term. This means that the way each individual experiences the symptoms will be highly variable.

However, some common descriptions of brain fog include “forgetful,” “cloudy,” and “difficulty thinking, focusing, and communicating.”Ross2013 Low energy, physical and mental fatigue, irritability, headaches, anxiety, confusion, low motivation, hopelessness, memory loss, memory problems, and trouble sleeping—these can be more serious and life altering consequences of brain fog.

If you are suffering from brain fog, it could not only lower your mood and thinking ability, but alter your productivity in other areas of life.

Quality of work might suffer, no matter your profession. A poor ability to communicate or lack of interest in conversation might hamper relationships. Having brain fog may make it difficult to exercise and perform daily tasks that once were easy and enjoyable.

The negative life consequences of brain fog could be the result of a serious underlying condition, which we will cover shortly. Sometimes, it’s due to diseases like chronic fatigue syndrome, depression, Alzheimer’s disease, multiple sclerosis, or other debilitating medical condition. In these cases, medical intervention is sometimes necessary. Brain fog may also be due to some other lifestyle factor that can be easily changed—poor diet, environment, or stress.

Alternatively, brain fog could be a temporary side effect of the ketogenic diet. While this might seem paradoxical (keto is supposed to improve mental clarity, right?), the reason why this happens makes sense. And the good news is that once brain fog is “cured,” you’ll be a lot less likely to experience it again.

What Causes Brain Fog?

There are several potential causes of brain fog. These may take the form of a more serious clinical disease, or a less serious lifestyle-related factor.

One reason may be related to hormonal changes, especially those involving estrogen.

The transition to menopause (perimenopause) has been linked with difficulties in difficulty paying attention, memory complaints, and symptoms of depression, anxiety, and sleep disturbance—which all sound a lot like brain fog.Jacobs2016

Brain fog seems to afflict people with a condition known as fibromyalgia, a disorder characterized by widespread pain throughout the body, particularly in the muscles and the skeletal system. This disease is caused by alterations in the immune system, and may have other effects related to the nervous system. Along with the pain comes fatigue and impairments in memory and sleep.

People with fibromyalgia report difficulty thinking and remembering things, something that has been termed “fibro fog.” Patients with fibromyalgia rank “fibro fog” high on the list of how their disease impacts well-being.Kravitz2015 In one study, over 82% of patients with fibromyalgia reported cognitive dysfunction, and were more likely to report memory decline, mental confusion, and speech difficulty compared with a group of patients without fibromyalgia.Katz2004

There are other clinical conditions and treatments which can lead to symptoms of brain fog; these include chemotherapy,Kovalchuk2017 brain injuries, diabetes,Moheet2015 neurodegenerative disorders,Hussain2018,Trojsi2018 allergies,Trikojat2017 and any condition that might impair gut health or cause “leaky gut.” Even certain medications like statins and sleeping pills have been reported to cause brain-fog like symptoms such as cognitive impairment, difficulty in decision making, and poor executive function.Wagstaff2003,Chen2012

Lifestyle factors may play a role too. Ultra low-fat diets, food additives like MSG, artificial sweeteners, industrial seed and vegetable oils, improper hydration, and nutritional deficiencies are all colloquially proposed to be a cause of brain fog. In addition, it is likely that and sleep deprivation or sleep disorders—which are widespread among Americans and throughout the world, play a causal role in brain fog. However, research is currently unavailable to conclude a causative role of any of these variables in brain fog.

Being physically inactive and a lack of sleep are known to lead to poor cognitive performance, worse memory, and less mental clarity, and may even contribute to cognitive decline throughout life.Blair2009,Alhola2007 What’s more—physical activity and proper sleep are two ways to reverse brain fog and improve cognitive function and wellbeing!Mandolesi2018 It’s something we all intrinsically know. Who doesn’t feel better after a great workout or a sound night of sleep?

Another condition where brain fog is reported has nothing to do with underlying disease, and isn’t necessarily due to some poor lifestyle factor. Sometimes, brain fog or related symptoms are reported in people soon after initiating a very-low carb or ketogenic diet. Before going into the specifics of why keto can lead to short-term reversible brain fog, let’s talk about some basics of brain energy metabolism.

Brain Food: How Our Cranium Gets Energy

Our brain is over 60% fat, and while it makes up only 2% of our entire body weight (give or take a little in some people) it consumes a disproportionate amount of the total energy our body uses on any typical day.Belanger2011

Our brain is an energy-hungry organ. Let’s take a look at how it gets fed.

The Brain and Glucose

In terms of energy consumption, the brain uses about 25% of the total glucose and 20% of the oxygen made available to our body.Belanger2011

Glucose is the obligatory energy substrate of the brain, with a few exceptions.

The process by which energy is harvested from glucose starts by glucose getting taken up by the brain through a transporter known at GLUT1 (and a few others). This transporter is necessary because the brain is protected by something known as the blood-brain barrier (BBB).

The BBB is a protective layer surrounding the brain, tightly controlling what goes in and out. This is important, since our body undergoes biochemical shifts and is exposed to a variety of environmental toxins and chemicals throughout the day—any one of which could harm the brain if they were allowed entry. Lucky for us, the BBB is a watchful sentry, maintaining our brain health throughout life.

GLUT1 sits on the surface of the BBB. When brain activity increases due to increased cognitive or physical work, the activated neurons in the brain also increase their work rate because energy requirements increase. This increases the amount of blood flow to the brain, carrying glucose along with it.Latera1999

Glucose binds to the transporter and is then able to cross the membrane and enter brain regions. Here, brain mitochondria break down glucose in two processes: glycolysis and the Krebs Cycle (a.k.a TCA cycle). ATP produced through these pathways is then used as an energy source for neurons.Falkowska2015

In addition to providing ATP, glucose also seems necessary for the brain to produce neurotransmitters including glutamate, GABA, and acetylcholine (AcH)—without which we could not function properly.

Glucose isn’t the only molecule that is allowed entry into the brain through the BBB.

Turns out that the monocarboxylates lactate and pyruvate (two breakdown products of glucose) are able to cross the BBB and be used for energy.Falkowska2015 This pathway may predominate during situations like moderate to vigorous exercise (when we are producing more lactate and pyruvate through glycolysis).

Ketone Bodies and the Brain

What happens when glucose is no longer available—like during conditions of prolonged fasting or starvation?

If no other route was available, the only option would be for the brain to shut down. This wouldn’t exactly be evolution’s greatest achievement now, would it? Luckily, our brain (and body) has developed adaptive mechanisms that allow utilization of other substrates besides glucose for energy. The adaptive mechanism in humans (and animals) is the utilization of ketone bodies in the brain. In particular, the ketone bodies acetoacetate (AcAc) and beta-hydroxybutyrate (BHB) are energy-rich substrates.

Studies have shown that during “starvation” (3.5 days of fasting), the influx of BHB into the brain increases more than 10x normal levels.Hasselbalch1995 This corresponds to the amount of ketones present in the blood.

The increase in brain ketones during food deprivation may be due to the fact that the BBB becomes more permeable to ketones during fasting to allow more ketones inside the brain.

Any condition where ketonemia (increased blood ketones) occurs also leads to increased ketone influx to the brain.Hasselblach1996

The adaptive mechanism of ketone utilization by the brain ensures humans and animals can survive and function under conditions of low-energy availability. During times of glucose shortage (i.e. lack of carbohydrate), ketone bodies become the “preferred” energy source.

But what about fat? We know that the beta-oxidation of fatty acids is one pathway by which we burn fat to produce energy. Interestingly, unlike other body tissues, the brain does not oxidize fatty acids. While it’s known fatty acids can cross the BBB using a specific carrier—and that the brain even contains the enzymes for beta-oxidation—it appears the brain seems to avoid producing energy from fatty acids in order to protect neurons from oxidative stress and a hypoxic environment, which could result from beta-oxidation. Beta-oxidation is a “slow” process and the brain having to do this under situations where rapid neuronal firing is needed could cause impairment in cognitive abilities. So it looks like the brain avoids producing energy through this pathway in order to protect us and our ability to think.Schonfeld2013

We are left with the knowledge that the brain can use two different substrates as an energy source. Under conditions of high-glucose availability, the brain preferentially oxidizes glucose to produce ATP and maintain function. When glucose becomes less available, it is the production and utilization of ketone bodies that dominate the brain-energy production pathways.

Ketone utilization by the brain may exert profound benefits above and beyond glucose—at least when it is adapted to do so.Veech2004

Of course, natural ketone production takes time. Whether through fasting or ketogenic dieting, it can take days or even weeks for the body to begin producing its own ketones. And when you cycle in and out of keto, you may experience brain fog more frequently.

“Getting back into ketosis with ketone esters makes a big difference. It helps get me back on track and powers me through my day.”—Lidice F.

Exogenous ketone supplements increases ketone levels rapidly,Stubbs2017 providing the body with keto energy as it ramps up to natural ketone production. With the evidence illustrating the power of ketones for the brain, this could be an excellent solution to help reduce symptoms of brain fog and bridge the gap between carb-depletion and ketone production.

Do Low-Carb Diets Cause Brain Fog?

One of the purported benefits of going on a low-carbohydrate or ketogenic diet is the improved mental clarity.

For this reason, it might seem like a paradox that brain fog is a common symptom of something known as the “keto flu”—the term given to the panoply of symptoms experienced by people who begin a ketogenic diet. Along with brain fog, some of these symptoms include physical fatigue, cramps, irritability, dizziness, food cravings, and sleep problems. Like brain fog, most if not all of these symptoms are transient and resolve after a short adaptation period.

If keto is so good, why does brain fog happen? During the “transition period” from your habitual diet to a keto diet (also known as the “adaptation phase”), several changes can occur in the body and brain that can cause brain fog.

Falling Blood Glucose

Starting a ketogenic diet means you’ll have to cut out a majority (if not all) of your dietary carbohydrates; no external glucose will be coming in through food.

As a result, blood glucose levels will fall. While this is one of the long-term benefits of keto, it might have some short-term consequences. Short term reductions in glucose provision to the brain can lead to brain fog and related symptoms.

We already discussed how, in someone eating a high-carbohydrate diet, energy for the brain is provided mainly by glucose. Taking away this glucose by going low-carb means the brain is left without its main source of ATP and energy. Short term “hypoglycemia” deprives the brain of glucose.

In the initial days or weeks of keto, low glucose can cause brain fog.

What about glycogen? Can’t the brain use this for energy?

We know that stored glucose can be accessed in times when blood glucose is low to provide energy. Lactate and pyruvate from glycogen breakdown can maintain the function of neurons under conditions of hypoglycemia.Belanger2011 However, glycogen stores in the brain are limited, and are consumed within minutes in the absence of exogenous glucose. What’s more, the process of creating new glucose—gluconeogenesis (GNG)—is negligible in the brain.

It’s important to remember that this period of low glucose availability is likely transient. One study reported that endogenous glucose production declines after one day on a carb-restricted diet and remains low for at least one week.Harber2005 This was accompanied by a 20% reduction in plasma glucose concentrations after two days on the low-carb diet. However, levels normalized after three days.

It is interesting to note that the authors of this study concluded that if adequate calories were provided in the form of protein and fat, hypoglycemia could be prevented. This is likely a result of increased GNG and the sparing of glycogen breakdown. These results were apparent at the whole-body level, however, and cannot be said to apply to the brain, per se.

It seems reasonable to conclude that, before sufficient glycogen sparing and GNG begin to occur, the brain may experience a short-term state of energy deprivation. This may be compounded by the fact that even though ketosis might be present, the brain may not be suited to use ketones quite yet. Luckily, this is a transient period and not a form of chronic stress your body will be put through.

A sufficient adaptation phase is probably required before everything starts running smoothly.

Insufficient Ketone Body Production and Utilization

In the absence of glucose, the brain can use alternate routes of energy production like ketolysis (the oxidation of ketones).

It has been shown that during starvation conditions, ketone bodies can provide as much as two-thirds of the total energy required by the brain.Laffel1999

While a prompt increase in ketone production (ketogenesis) occurs when someone starts a ketogenic diet, this doesn’t necessarily mean that their use of ketones for energy increases at the same rate.

Adaptation must first occur before body tissues and the brain can take full advantage of ketones. Progressive adaptations in the brain need to occur.

A lifetime of a high-carb diet has left many enzymes and pathways of ketosis / ketolysis down-regulated, and we need time to build up these pathways.

The evidence for a downregulation of enzymes specific to ketosis can be seen in how we evolved to eat. The activities of ketone body metabolizing enzymes rise during the weaning period (to allow energy utilization from the high fat diet of breast milk) and fall in adulthood, presumably when diet transitions to contain more carbohydrates.Morris2005

For instance, in mice, eight weeks of a ketogenic diet was shown to increase enzymes related to the breakdown of ketones and enhance fatty acid oxidation capacity.Ma2018 This suggests these “fat-adapted” rodents were more able to both produce ketones from fat breakdown and use these ketones in metabolism.

Along with enzymatic changes, a high-fat diet and prolonged fasting can both increase the permeability of the BBB to ketone bodies. Shorter periods of fasting, interestingly, don’t have the same effect.Morris2005 Changes may take some time to occur, providing evidence as to why brain fog and other symptoms may only resolve after a few days to weeks of keto.

Prolonged, high concentrations of ketone bodies may be necessary to upregulate the transporters (specifically, the MCT1 transporter) needed to effectively get ketone bodies past the BBB and into the brain along with other peripheral tissues.

Thus, enzyme and transporter activity may be a factor limiting ketone body metabolism in the initial stages of a keto diet, before adaptation occurs.


One of the major changes that occurs when you start a low-carb, ketogenic diet is a drastic difference in the amount of water you have in your body—this is one of the main reasons that quick and perhaps drastic weight loss occurs in the initial phases of a ketogenic diet.

This happens because of two reasons. First, your body will start using its glycogen reserves for fuel. Because glycogen is stored with water, when you lose glycogen, you also lose water, which can contribute to dehydration and rapid weight loss.Kreitzman1992

Second, insulin, while normally thought of as regulating blood glucose, also regulates how much sodium our kidney retains. When insulin is high, the kidney holds on to more sodium, and water along with it.

However, insulin falls when you go low-carb, and this leads to more sodium excretion by the kidney.Rubenstein1975 To maintain osmotic balance, an increase in sodium excretion means that water excretion will increase as well.

If you aren’t able to make hydration a priority by increasing your water intake a bit, this could lead to dehydration and electrolyte imbalances. Along with other keto flu symptoms, brain fog is a likely outcome of dehydration, imbalanced electrolytes, or both.

Studies have indicated that dehydration of only 2% can cause a deterioration of mental functions.Gopinathan1988 Some studies indicate that dehydration of even 1% of body weight can adversely affect cognitive performance, short-term memory, and attention.Lieberman2007 Dehydration can also cause your blood pressure to drop too low—this may also contribute to impaired cognitive performance and brain fog-like symptoms.

Important to note is that most of these studies used experimental procedures like exercise and heat stress to induce dehydration, which adds a variable other than just dehydration to the mix.

Ketones Prevent Brain Fog

Current research is lacking to support the claim that ketosis has mind-altering capabilities that may lead to improved clarity, awareness, and creativity above and beyond your baseline function.

However, this “mental edge” is commonly reported among hard-core keto people. This feeling makes sense if you think about it in terms of evolutionary advantages. Our ancestors went into ketosis when they were starving, so having the advantage of mental clarity and focus would be beneficial for finding sources of food.

It isn’t uncommon to hear someone complain of brain fog when eating a high-carbohydrate diet full of processed foods and refined sugar. Upon transitioning to keto, symptoms of brain fog clear, making people feel better than they ever have before. This could be due to the elimination of processed foods in the diet or the presence of ketones. Most likely, it’s a combination of both.

Improving cognitive function suggests that ketosis—whether dietary or exogenous—has the potential to reduce brain fog.

While enhanced cognitive function in healthy people lacks scientific support, many studies on ketogenic diets have indicated their ability to improve cognition, mental clarity, and brain function in adults with impaired brain energy metabolism and neurological disorders.Krikorian2012,Vanitallie2005 Research consistently indicates that ketogenic diets have a strong neuroprotective effect in addition to their ability to improve social behavior and cognition.Hallbook2012

Exogenous ketones show similar benefits.

Provision of ketone esters like the BHB monoesters and ketone salts provides several benefits for the brain including improved behavior and brain plasticityCiarlone2016, cognitive performanceMurray2016, and reduced neurological impairment in diseases of metabolism.Vanhove2003

What about healthy individuals without cognitive or neurological impairments? Again, the benefits of ketosis here are largely speculative. We have to base the purported benefits on theoretical mechanisms of action, which are actually quite sound.

A fat-fueled brain may be more energy efficient than one that relies on glucose. This might have several benefits for brain fog and mental function, including the reduction of damaging free radicals and oxidative stress in the brain.Maalouf2007

Oxidation of ketone bodies generates fewer free radicals than does the oxidation of glucose.Sullivan2004 In addition, the metabolism of ketones generates more ATP per gram than does glucose—meaning your brain is getting more “bang for its buck” when using ketones instead of other fuel sources.Board2017

Ketones may allow the brain to process more of the neurotransmitter glutamate to GABA. A proper balance of these two neurotransmitters is necessary to avoid over (and under) stimulation. Too much of either can result in brain fog. By increasing conversion of glutamate to GABA, ketones may improve focus.Daikhin1998

In addition, ketogenic diets might lead to a more robust brain that can produce and utilize more energy. This is because keto has been shown to increase brain mitochondria (known as mitochondrial biogenesis) and mitochondrial energy dynamics.HasanOlive2019 The same increase was shown when the ketone body BHB was provided, suggesting this effect is due to ketones and not a different aspect of the low-carb diets.

Brain fog is no match for keto.

Whether you’re suffering from poor energy, reduced mental clarity, or trouble focusing, ketosis may improve symptoms by reducing oxidative stress in the brain, giving your neurons more energy, and boosting neurotransmitters that benefit mental health and function.

If you’re already on a ketogenic diet (or starting one) and want to supercharge it, taking ketone supplements might help. An easy, everyday-use supplement is the use of medium-chain triglycerides (MCTs).

MCTs aren’t ketones, but are actually highly ketogenic fats that readily convert into ketones.

Other Reasons Brain Fog Improves on Keto

We’ve just discussed how the addition of ketones through a ketogenic diet or exogenous ketones can fight brain fog. However, the elimination of many things when you go keto could also explain improved brain fog.

One might be dietary clean up. Going keto means eliminating many food groups, several of which may have been harming your health. These might include food containing sugar, gluten (for some), artificial sweeteners, and other processed ingredients. Refined food and a “western diet” categorized by a high intake of saturated fat and refined carbohydrates has been associated with impaired cognitive function and neurodegenerative conditions.Francis2013 The modern way of eating isn’t particularly great for anyone.

Furthermore, stable daily blood sugar could explain the improved mental clarity and rock-solid focus that come with keto.

Rather than riding the highs and lows of blood sugar spikes and crashes, keto can lead to fewer and less severe blood glucose fluctuations. When you once may have had a post-lunch brain fog, you now have steady energy and focus until the end of the day.

Fight the Fog

If you are experiencing brain fog, it’s time to end it once and for all.

First, figure out what may be causing brain fog—diet, sedentary time, something in your environment. Next, look at all of the potential strategies that are known to help prevent brain fog, and experiment.

Maybe a ketogenic diet is your first-line strategy. If this is the case, be sure to eat enough fat and calories to provide your brain with all the energy it needs.

Maybe exogenous ketones are the solution. Taking something like an exogenous BHB supplement or MCT oil could be the super supplement you need to power through the fog.

Other smaller changes—drinking enough water, getting adequate sodium, exercising more, getting out into the sun—can have a huge impact on your mental function.

Your health is your own responsibility, your own experiment. Take control, and don’t let brain fog impede your journey toward self-optimization.

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