“Magic pill,” “miracle cure,” “diabetes reversal diet”—you may have heard some of these terms used to describe the popular ketogenic diet.
While a few of these claims may be over the top, the research continues to indicate that ketogenic diets are beneficial for a wide variety of health conditions, weight loss, and in some cases, athletic performance. No matter the condition or goal, people are talking about the amazing results they’re achieving using keto’s low-carb, high-fat approach.
Many of the benefits of a ketogenic diet arise from the physiological changes that occur when someone adopts a low-carb, high-fat diet.
In particular, carbohydrate restriction seems to be a potent inducer of metabolic shifts that favor longevity, fat burning, and reduced inflammation. It’s rare to encounter downsides to the diet, but there’s one recurring theme encountered by some (but not all) keto dieters.
For those new to the keto diet, certain symptoms may occur in the first few days to weeks, a condition which has been termed the “ketogenic flu” or “keto flu” for short.
This is your body going through a fundamental metabolic shift.
What is Keto Flu?
The “keto flu” isn’t life-threatening and, despite the name, isn’t an illness akin to the actual flu. And no, you can’t go to your doctor to get a vaccination against the keto flu.
Rather, the keto flu is the colloquial name given to the constellation of symptoms (some “flu-like”) that some people experience when they embark on a keto diet and try to enter ketosis for the first time.
Included in the list of symptoms are: stomach pain, “brain fog,” dizziness or confusion, nausea, irritability, diarrhea or constipation, muscle cramping or muscle soreness, lack of focus, sleep problems, and sugar cravings (particularly for high-carb foods).
While the time of symptom appearance may vary, studies suggest that the symptoms of keto flu may be greatest within the first four days of starting a ketogenic diet or fast.
In addition to the unwanted physical side effects, the keto flu may be detrimental to keto diet adherence, as some studies report.
Feeling ill during the first few days on keto might lead some to abandon the diet for good. This is a mistake, since the benefits of the ketogenic diet start to manifest once the initial symptoms go away—which can occur rather quickly in some people. Patience is sometimes a virtue with the ketogenic diet. Rome wasn’t built in a day; nor is keto adaptation.
The good news is that keto flu symptoms are short term and will likely resolve within a few days to a few weeks. Some people may not even experience any symptoms at all.
Furthermore, the duration and severity of symptoms are highly variable from person to person. If you can stick it out long enough, you’ll eventually begin to reap the benefits of being adapted to utilizing fat and ketones as your main source of energy.
What Causes Keto Flu?
Adopting a ketogenic diet means that your body will need to transition from a state of using carbohydrates as an energy source to one in which it uses stored fat that it turns into ketone bodies.
This requires a “shift” in metabolism—an activation of certain pathways and enzymes that are specialized in the process of beta-oxidation and ketogenesis / ketolysis. In particular, the ketogenic / ketolytic pathways
A newly adopted low-carb, high-fat diet can lead to transient low blood glucose, changes in water and electrolyte balance, and some micronutrient deficiencies that all may play a part in the keto flu.
Falling Blood Glucose
On keto, carbs are drastically reduced to below 50g per day (and sometimes to around 20g - 30g on more restrictive keto diets). With little-to-no carbohydrates being consumed, blood sugar (and insulin) begin to fall.
While this is one of the goals of a ketogenic diet and a benefit in the long term, this can actually have short-term side effects. Why? Well, glucose is the preferred (sometimes required) energy source for some organs, including the brain. If you take away glucose and don’t provide an alternative source of energy, this leads to an energy deficit.
In the first few weeks of a keto diet, this is exactly what happens.
Without the metabolic machinery to utilize ketones, the brain and other tissues may be left low on energy.
Brain cells are able to utilize glucose and ketones as a fuel source, but interestingly, fatty acids are not a favored fuel source by the brain.
Another reason for a reduction in mood or feelings of fatigue may be carbohydrate withdrawal.
This isn’t to say anyone is truly “addicted” to carbohydrates. Rather, carbohydrates have been shown to stimulate several reward pathways in the brain, including those involving the neurotransmitter serotonin.
Electrolyte Imbalances and Dehydration
Electrolytes are crucial for the proper functioning of myriad physiological processes including heart rate regulation, blood pressure control, and muscle contraction, among others.
As the name suggests, electrolytes are minerals that have an electric charge—including calcium, potassium, magnesium, chloride, and sodium.
While we usually think of insulin as the blood-glucose regulating hormone, it also has a role in regulating the amount of sodium that is reabsorbed by the kidney. When insulin is high, the kidney reabsorbs (i.e. stores) more sodium (and water).
Insulin levels fall when you adopt a ketogenic diet, and the kidneys begin to excrete more water and electrolytes.
As a result, at least initially, ketogenic diets can lead to a low level of or imbalance in certain electrolytes along with dehydration.
In fact, this is one reason a sharp drop in body weight occurs when you start keto—most of this is water weight.
In addition to electrolyte and water loss through increased kidney excretion, carb-restricted diets also reduce total water in other areas of the body.
This is because the stored form of glucose— glycogen—is “bound” to about 2 - 3g of water per gram of glycogen. Thus, a reduction in stored glycogen (in the short term) on a ketogenic diet means you’ll lose water weight with any electrolytes that come with it.
A crucial side note. In the long term, research shows that keto-adapted individuals don’t have any less stored glycogen then their carbohydrate-eating peers.
While the keto diet is all about stressing food quality, sometimes it may not be adequately formulated with the right combination of macro and micronutrients. This might be especially true for someone new to the diet who has yet to experiment with different foods and nutrients.
A ketogenic diet can even be calorie-deficient if not planned correctly. Any or all of these could potentially lead to undernutrition, initiating a low-key “starvation response” in the body. In this case, stress hormones like cortisol are increased in order to mobilize glucose stores to provide energy for the brain.
This stress could be compounded by exercise. A study in resistance-trained men who adopted a ketogenic diet for a few weeks showed that cortisol rose on day five of the diet, but eventually returned to baseline after 10 days. Interestingly, the rise in cortisol occurred without a change in the subjects’ blood glucose.
Another hormone perhaps involved in keto flu-like symptoms is part of the thyroid family and is known as T3 (or euthyroid). In response to carb restriction, T3 has been shown to decrease (although this response is highly variable).
T3 is also known to play a role in energy metabolism. During the keto-adaptation period, a sharp drop in T3 or too low thyroid may lead to a loss of focus and fatigue in some people.
In the long term, however, it appears that as long as adequate protein and micronutrients are consumed, a low-carb and even a calorie-restricted diet may not result in hypothyroidism. However, consult your doctor before starting a ketogenic diet if you have been diagnosed with hypothyroidism or other thyroid disorders.
Getting enough micronutrients may be a hurdle for new keto dieters. What are micronutrients in the first place?
This broad range of substances includes minerals, vitamins, antioxidants, phytochemicals, and trace elements. While “small,” they contribute enormously to health maintenance and performance; some, like vitamins and minerals, are necessary for human health. Many of us fail to think much about “micros” from day to day—instead focusing solely on the “macros.” This is generally easy to do, since most micronutrients (aside from some minerals) don’t even appear on food labels. For this reason, they can be hard to track.
As long as you’re consuming a variety of high-quality whole foods, the risk of micronutrient deficiency is likely rare, but it’s still something to be aware of.
One reason for the risk of micronutrient deficiencies on keto is that many carbohydrate-containing foods like fruits, vegetables, and grains are also great sources of micronutrients. Keto diets generally restrict the intake of most of these. While eggs and cheese are keto favorites, they’re unfortunately lacking in micronutrients, comparatively speaking.
A simple strategy to ensure adequate intake of micronutrients is to eat plenty of micronutrient-rich “keto-approved” foods, including organ meats, grass-fed meat products, nuts and seeds, fish, and veggies like broccoli, cabbage, brussels sprouts, and dark leafy greens.
What to Do (and What Not to Do) if You Have the Keto Flu
We applaud robustness and determination around here, usually advocating the hard daily grind and a great work attitude.
But, if you’re feeling symptoms of the keto flu, the best thing may be to take it a bit easier on your body to lessen symptom severity. In fact, you might not even feel like doing your usual training or work regimen, and that might be O.K for a while.
Do: Low-Intensity Exercise. Don’t: Strenuous Physical Activity
If you have one or any keto flu symptoms, you might not be feeling 100%, mentally or physically. While exercise is awesome, continuing a killer cardio or strength routine while in the keto-adaptation phase may not be the best advice.
For one, it is known that performance can decline in the first few weeks on keto, which might be a drain on your confidence.
While many athletes go keto in order to fat adapt and improve performance in the long run, before keto adaptation happens, a short-term dip in performance may occur.
For example, one study in obese individuals indicated that exercise capacity declined in the first week of a low-carb high-fat diet, while performance got a boost in the fourth week,
Staying away from high-intensity physical exercise during the transient period of keto flu is recommended. Your body doesn’t have its usual fuel sources, and hasn’t yet built the capabilities needed to utilize ketones.
The good news is that over time, your body will adapt to burning fat and using ketones for energy. For some, this may lead to enhanced performance. The time taken to adapt will vary by person.
The best recommendation is to incorporate lower intensity cardio and strength activities into your routine for a few weeks—something that will keep you active but not something that will overwork your body or mind.
Physical exercise can increase cortisol, which might exacerbate some of the stress-response systems already happening from your dietary change up. However, low-intensity exercise might expedite the keto-induction process by jump starting fat burning. As a matter of fact, low-intensity exercise is one instance where the body is used to burning primarily fat already.
Do: Get Proper Sleep. Don’t: Stay out Late
If you had a real illness, you’d probably stay in bed and try to “sleep it off.” While the keto flu isn’t an illness per se, proper sleep may still help to mitigate some of the symptoms—in particular any fatigue you may be feeling.
Proper rest should be a primary goal all of the time, not just when you’re feeling a bit down.
We talked before about how one of the reasons for keto flu may be a rise in cortisol. Sleep loss won’t help, and will only add to this adverse reaction. That’s because sleep deprivation leads to increased levels of cortisol.
To improve sleep quality, practicing proper “sleep hygiene” is a must.
This means limiting screen time prior to bed, keeping you sleep space extra dark and cool, and structuring meals and exercise around your bedtime. A better night of sleep can also be achieved using nutritional supplements or nootropics for sleep. In particular, supplements that contain ingredients like melatonin, magnesium glycinate, and L-glycine are the best as each of these ingredients has been shown separately to induce quicker sleep and lead to improved sleep efficiency.
Do: Eat Enough Fat. Don’t: Count Calories
One of the main reasons people adopt a ketogenic diet is to lose weight, which means that ultimately, energy intake will have to be lower than energy output.
This concept is far more complicated than the “calories in calories out” (CICO) model proposes, but an energetic deficit is ultimately what determines weight gain or weight loss.
Obsessing over calories in the initial phase of starting a ketogenic diet could have consequences. If you’re simultaneously cutting your carb intake while drastically cutting calories, this could cause an extreme energy deficit and leave you feeling awful. Some symptoms of the keto flu arise because the body just doesn’t have enough usable energy for the time being.
“Under eating” may be a bit "easier" for some on keto, because ketogenic diets have been shown to have appetite-suppressing effects.
Prioritize your intake of adequate macros (mainly fat) and calories especially in the first few weeks of keto.
Continue this until you get a good idea of how much you need to eat in order to achieve your goal weight and stabilize your energy.
So, instead of trying to drastically cut calories and carbs all at once, prioritize cutting out carbohydrates first while making sure to integrate plenty of sources of healthy fats into your meals and snacks. Keto-friendly food like cheese, fatty fish, eggs, red meat, avocados, nuts and seeds, and non-starchy vegetables—these should become staples of your keto diet. They’re filling, and will provide the fat and other nutrients necessary to have an effective ketogenic diet experience.
Symptoms of the Keto Flu, and How to Cure Them
There is currently a lack of research into what actually causes the keto flu and its effects on the body.
But, there are some commonly-reported symptoms experienced by those new to the keto diet that we can explore.
Why are headaches common? This likely has something to do with electrolyte or mineral imbalances.
As stated earlier, low insulin is a signal for the kidneys to excrete sodium, which leads to more water excretion as well. In addition, you’ll lose some water that is stored with glycogen. If you’re not drinking a bit more water than usual to compensate for this (or making sure to bump up your consumption of minerals like sodium and potassium), this could lead to electrolyte- or dehydration-related headaches.
Electrolyte imbalances may also occur if mineral-rich foods are not prioritized or cut out of a ketogenic diet. Many processed foods that are often fortified with minerals and high in sodium are also limited on keto. While too much sodium may not be good for some people, it has several critical roles in the body and is especially important for those on a keto diet.
Another source of headaches is the fact that you’ve drastically cut off your brain’s preferred source of energy: glucose. Not only that, but you aren’t yet keto adapted and therefore aren’t providing another usable substrate for your cranium.
Keto flu-related headaches can be prevented if you drink plenty of water and are consuming enough salt. Increased sodium consumption could come in the form of adding salt to meals or drinking bouillon or bone broth. An alternative may be to consume an electrolyte supplement.
Eating more healthy fats, exercising, and supplementing with exogenous ketones are three steps that can help get the body more used to using fats as energy, which may lessen the severity of the keto flu or speed up symptom resolution. Supplementing with minerals may be another option in the adaptation phase of keto. For instance, you may choose to use an electrolyte supplement or supplement with magnesium and potassium, two electrolytes critical for muscle function, protein synthesis, cardiovascular health, and digestion.
One exogenous ketone supplement to try is a ketone ester which is a source of pure ketones that can provide the body with a fuel source while you ramp up to natural ketone product (thus lessening keto flu symptoms).
“Getting back into ketosis with [a] Ketone Ester makes a big difference. It helps get me back on track and powers me through my day.” — Lidice F.
In the long-term, headaches may become a thing of the past. Ketogenic diets have been shown to reduce the instance of migraines.
Another potential cause of muscle cramps is electrolyte imbalance or deficiency. Magnesium and potassium, two electrolytes, both help to regulate muscle contraction.
To combat muscle cramps, make sure to consume enough magnesium, potassium, and sodium-rich foods like bone broth (for sodium), dark leafy greens (for magnesium), and avocados (for potassium). Since cramps may also be the result of dehydration, drinking enough water is a must.
Some people may find that their bowel habits change or become more “difficult” after transitioning to a ketogenic diet, at least initially. There may be a few reasons for this.
The first reason may have to do with the gut microbiome. Low-carb, high-fat diets can change the gut microbiome (any diet can, really).
If keto is a drastic change from your normal dietary pattern, a huge shift in microbiome composition can impact bathroom habits, digestion, even the process of nutrient absorption.
While there haven’t been many studies to specifically show exactly how a keto diet influences “regularity,” some studies have shown that a low-carb diet leads to reduced frequency and volume of bowel movements as well as a reduction in the concentrations of short-chain fatty acids in the participants stool.
Another contributor to constipation may be dehydration and an overall reduction in how much food (particularly fibrous vegetables) you’re consuming.
Interestingly, some of the most important factors known to promote constipation are reduced physical activity, inadequate dietary intake of fibers and carbohydrates, and fluid loss or dehydration.
Maintaining regular bowel habits might be as easy as staying hydrated, consuming adequate minerals, and fitting in some fibrous but low-carb veggies like leafy greens, and cruciferous vegetables. In addition, maintaining some level of physical activity can help keep you regular.
A common symptom among those who have just started a ketogenic diet is what’s known as “keto breath.”
This is often described as breath with a “fruity” smell or that akin to nail polish remover. There’s an easy explanation for this. When the body produces the ketone body acetoacetate (AcAc), some of it gets broken down into another ketone known as acetone, a molecule that is also a common ingredient in nail polish remover. A majority of acetone is excreted in the breath.
While this may be mildly unpleasant, it’s a sure sign of ketosis and also why ketosis can be measured using a ketone breath meter. These devices measure the concentration of acetone in the breath to provide a qualitative level of ketosis.
Keto breath may go away within a week or two, once the body is more adapted to ketosis and begins to utilize more and excrete fewer ketones. While there may be no way to “prevent” keto breath, maintaining good oral hygiene and using gum or breath mints can help to mask the smell of keto breath.
Fatigue and Low Mood
Keto adaptation takes time, during which your body is upregulating the necessary enzymes and cell machinery to be able to use ketones as a source of energy. Unfortunately, before this happens, you may go through a period where energy and vigor, particularly during exercise, suffers.
Bodily fatigue can be attributed to a few things. First and foremost, could be the fact that you’re depriving muscle tissue of one source of energy—carbohydrates. Another reason may be a change in thyroid hormones and cortisol.
High cortisol can lead to irritability and poor sleep quality,
Decreased mood or trouble concentrating, sometimes referred to as “brain fog,” can occur due to reduced glucose availability for the brain. Since it can’t use fat, the brain is left without a preferred source of energy, at least before ketones come along. One other reason for perceived low mood could be the aforementioned carb withdrawal symptoms that the sharp removal of carbohydrates might bring.
The solution? Be sure to eat enough calories, and make sure most of these are coming from quality sources of fat.
Since fat is the main energy source on keto, increasing fat consumption needs to be a priority to properly fuel your body and mind.
As always, ensuring adequate hydration and mineral intake can prevent many fatigue-like symptoms as well.
Keto Adaptation and the Keto Flu
Let us first talk about the primary cause of the keto flu, which has to do with something called keto adaptation.
Keto adaptation refers to the process through which the body adjusts to burning fat (and producing ketones) for energy instead of carbohydrates.
On a diet containing moderate- to- high carbohydrate intake, the body will use a process called glycolysis to produce ATP from glucose molecules and provide the necessary energy for cells and tissues throughout the body. However, it’s important to know that at rest and even during low-intensity exercise, fat oxidation does contribute to the energy-production process even in non-keto adapted folks.
While glycolysis is one energy pathway, beta-oxidation is another. Beta-oxidation is the pathway by which fat is broken down into metabolic byproducts and, eventually, ATP.
During keto-adaptation, the body begins to become efficient at using ketones as an energy source (through a process called ketolysis: the breakdown of ketone bodies). Interestingly, keto adaptation doesn’t just mean you have elevated ketones. Just because you have elevated levels of blood ketones doesn’t mean you are able to access all of the metabolic benefits of ketone bodies.
While levels of ketone bodies can increase to levels >0.5mM after a day or two of fasting or carbohydrate restriction, full keto adaptation takes a bit longer.
There is insufficient data to make a definitive timeline for keto adaptation, but studies suggest that it may take anywhere from a few weeks to a few months to fully keto adapt.
Exogenous Ketone Supplements: A Cure for Keto Flu?
Whether you want to prevent the keto flu from happening or lessen the severity of symptoms, using an outside source of ketones may be an effective strategy.
The transient period where keto flu might occur is characterized by a fairly low level of ketone production by the liver and utilization by body tissues. Using exogenous ketone supplements—providing the body with ketones rather than relying on internal production only—may expedite the keto adaptation process and help you to start your keto diet already in ketosis.
Exogenous ketones, which include ketone salts, ketone esters, and medium-chain triglycerides (MCTs) can provide molecularly identical forms of ketone bodies to the circulation, rapidly elevating blood ketones and inducing high levels of ketosis.
In the case of MCTs, they provide a fat source that is readily converted into ketone bodies by the liver.
MCTs are a unique kind of fat. These 6 - 12 carbon-length fatty acids travel directly into the liver from the gut, where they are preferentially converted into ketone bodies.
For this reason, MCTs are sometimes referred to as being “highly ketogenic.” Dietary MCTs promote ketogenesis and ketonemia in animals and humans with and without health conditions.
Can MCTs reduce the time taken to get into ketosis? In one study, it was shown that ketosis was achieved within 30 - 60 minutes in children who were administered 30g of MCTs following an overnight fast, with blood ketone levels reaching 1.05mM within 180 minutes post-ingestion
Another study investigated whether MCT supplementation could reduce the time to ketosis and improve symptoms of the keto flu in healthy adults placed on a ketogenic diet. The keto diet group who also consumed MCTs experienced higher levels of blood BHB throughout the study, experienced ketosis faster (though a “non-significant’ finding), and experienced significantly fewer symptoms of keto induction compared to the control group who consumed sunflower oil instead of MCTs.
If you’re looking for a high-quality source of MCTs, look for a MCT Oil Powder made with pure C8 (the world’s most ketogenic fat) to help your body spur ketone production, while kicking your body into fat-burning mode and reducing brain fog (by helping your body produce ketones to fuel your brain).
In addition to the known benefits of exogenous ketones on endurance performance,
Numerous studies have shown that ingestion of the BHB monoester ketone ester rapidly induces high levels of ketosis in as little as one hour, with blood BHB shown to increase up to as much as 3mM in some studies.
This could come in handy for someone new to a ketogenic diet.
Exogenous ketones might allow one to increase levels of blood ketones quickly and get a head start on endogenous production as you transition to a ketogenic diet.
The idea is sound. At this time, however, it is unknown whether or not exogenous ketones can effectively reduce the keto flu or symptoms of keto induction. Results in other areas of research are promising, but more investigation needs to be done in this realm.
Nevertheless, if you’re transitioning to keto and experiencing any symptoms related to the keto flu (or hoping to avoid them), adding a ketone ester to the initial phase of your diet might be one workaround to this problem.
Your Keto Flu Prescription
It’s important to recognize that keto flu is neither a universal phenomenon nor a true diagnosable illness. Rather, it’s more an “observe and report” phenomenon—you’re more likely to hear about it on social media than a peer-reviewed research paper.
Nevertheless, the keto flu is something that people are experiencing enough to have given it a name. The keto flu and the associated symptoms are worth investing some time to learn about and recognize.
The severity of symptoms, length of symptoms, and even the symptoms themselves may be different for everyone due to genetics, environment, physical activity, and diet habits. People who transiently go on ketogenic diets claim that they only experience this flu or that it is more severe during the first time they enter this diet. Given what we have explained, this makes sense because their body is equipped with the machinery to use fats to make ketones, making it an easier transition. As with every n=1 experiment, it’s up to you to find out for yourself.
Maybe you’re already on a ketogenic diet and past the stage where keto flu is most prominent—great for you. If you’re thinking about or have recently started keto, use the knowledge of your own body and the experiences of others to ensure that your keto diet is tailor-made to be as symptom free as possible.
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