Fasting Ketosis Symptoms: Common Side Effects

If you’ve decided to combine fasting and ketosis, you’re likely aware of the many potential benefits, such as increased energy, mental clarity and decreased fat mass[*]. But what you might not be familiar with are the signs of fasting ketosis. How do you know when you’ve entered fasting ketosis and what are the side effects?

What Is Fasting Ketosis?

In addition to eating a high-fat diet and limiting carbohydrates, the metabolic, fat-burning state of ketosis can be induced by fasting. Unlike keto, fasting is not a diet, but rather a method. It doesn’t determine what you eat but rather when you eat. There are a few different types of intermittent fasting, which will vary on your routine and schedule.

Eating ketogenic foods can make fasting easier because the high fat content makes you feel full with less food. Fasting can help you get into ketosis quicker, which helps you see the benefits such as weight loss and mental clarity much sooner[*].

Fasting Ketosis Symptoms

Before you begin to see the incredible effects of fasting ketosis, you may experience at least one of several common symptoms. While you may initially think the diet isn’t working for you, this is a natural reaction, and these symptoms are only temporary as your body is adapting to being in ketosis.

In this article you’ll learn about:

Common Side Effects of Fasting Ketosis

Combined with a high fat, low carb diet, fasting can help your body burn fatty acids instead of glucose faster and start producing ketones. After a week or two you’ll train your hunger hormones to adapt, but in the beginning you may experience at least one of the following temporary symptoms.

Water Flushing

As food is eliminated while fasting and your body burns through its glucose, glycogen stores in the muscles are reduced. Glycogen is responsible for muscle retention and holds a large amount of water, so as its levels fall a lot of water is released. For each gram of glycogen, your body holds onto three grams of water in storage, which can add up in extra pounds[*].

Your kidneys will also excrete more sodium as insulin drops. This is why people who start a low carb diet or a fast often experience a big initial loss of water weight and reduced bloating, before it comes to a plateau once glycogen stores have been fully depleted. This weight will return when high levels of carbs are consumed again and glycogen stores are refilled, which is why transitioning to a keto diet from fasting helps your body stay in ketosis.


With the loss of excess water, your body also flushes out electrolytes like sodium, potassium and magnesium. Your body needs to produce sugar for energy when fasting, so it begins a process called gluconeogenesis, during which your liver converts non-carbohydrate materials like lactate, amino acids, and fats into glucose. As this occurs, your basal metabolic rate (BMR) uses less energy and your blood pressure and heart rate are lowered. Think of this as your body going into “power saving mode”[*].

This can cause you to feel unusually tired and weak until things balance out more. This is often a symptom that causes people to quit fasting before they get into ketosis and see the benefits. Remember that this transition doesn’t occur immediately and you need to give your body time to adjust, since it’s not used to going long stretches of time without food. Don’t expect to be full of energy as you begin intermittent fasting.

Hunger and Sugar Cravings

As your body adjusts to fasting and ketosis, you might experience hunger or sugar cravings for a couple days. This is a normal reaction to a reduction in calorie intake, but it also has to do with your hormones. Researchers have found cells in the stomach, which regulate the release of a hormone associated with appetite called ghrelin, are controlled by a circadian clock that is set by mealtime patterns[*]. This means that following a set number of meals at specific times each day is a trained behavior.

Once you realize this, and you’re running on ketone bodies, fasting feels much easier, and fasting can balance your hunger hormones and reduce cravings for unhealthy foods, especially sugar and carbs. Hunger symptoms can also be triggered by dehydration, so be sure to stay hydrated[*]. Upon transitioning to eating keto after fasting, following a meal plan can be helpful in avoiding cravings and sticking to your diet.

Keto Breath

You may notice bad breath that takes on a fruity smell in the initial stages of fasting. This is a result of elevated ketone levels, in particular the ketone acetone, which is released through the breath[*]. Acetone is created spontaneously from the breakdown of acetoacetate and is the simplest and most volatile ketone. It diffuses into the lungs and exits the body when you exhale.

This wears off within the first week or so, but in the meantime you could brush your teeth more often or try some sugar-free gum. Just be sure to check the carbs in the gum, as they could raise your blood sugar levels and reduce ketone levels. The bad breath may also be accompanied by a dry mouth feeling.

Digestive Concerns

Some people may notice a change in bowel movements during the first phases of ketosis. Diarrhea or constipation may occur as changes happen in your gut’s microbiome.

Stool is formed from the food that is consumed, and depending on your individual body it can take up to three days for food to be digested. From there the remaining material will form stool, after which bowel motility helps push it out during defecation. When you’re not eating food, there is not enough material to form stool: your bowels are empty[*]. This, in addition to a lack of fiber intake, is why many people experience constipation while fasting[*].

Other reasons could include not staying hydrated enough or sensitivity to certain keto foods. This will very from person to person as each body is different.

Take psyllium husk or activated charcoal during this phase to help with diarrhea, and be sure to always drink lots of water.

Muscle Cramps

Lack of hydration can cause minor, temporary muscle cramps in some people when in fasting ketosis. Pains and aches may also be caused by malnourishment and insufficient levels of minerals nutrients. In particular, magnesium, potassium and calcium deficiencies can cause muscle pains due to an imbalance of electrolytes.

You may choose to replenish your magnesium levels with supplements to remedy this[*]. Try to increase the amount of water you’re drinking as a preventative caution, especially in the first week as dehydration can be the main cause of muscle cramps[*]. You can also avoid coffee, a diuretic, which can further dehydrate you.

Less Energy

Steady energy is a perk of both a ketogenic diet and intermittent fasting, but in the beginning stages, you might experience a dip in energy levels as your body adapts to using ketones for fuel. Carbohydrates are usually the main source of energy and the sudden restriction of them will naturally result in feeling drained.

Decreased physical performance at first is not uncommon, but it’s only temporary[*]. Your body is trying to conserve as much energy as possible, so try to slow down if you can. In the long run you’ll discover greater, steadier energy levels[*].

Heart Palpitations

An increase in heart rate may happen within the first few weeks of fasting or being in ketosis. If you normally have lower blood pressure, this is more likely. Heart palpitations can occur from lack of water and salt, making it important to increase your intake of both.

Since a ketogenic diet can help improve blood pressure, those on medications that affect blood pressure may need to speak with their doctor about dosage[*].

Ketosis Flu

When beginning the keto diet, you may experience what’s commonly known as the “keto flu” as your body adapts to fat burning. But when fasting, you can actually prevent or lessen the chances of keto flu symptoms, since it kick starts the process! Everytime you eat, there will be some glucose released into the bloodstream (even if it’s a very small amount) and an insulin response will follow. This response can last up to 20 hours, so when you fast for more than 20 hours, ketones will quickly begin to replace glucose.

While the symptoms will likely be less dramatic when fasting than eating a ketogenic diet, you still may experience the keto flu in the first few days of ketosis. Here are the side effects to be aware of.

Managing Fasting Ketosis: Common Signs of Ketosis From Fasting

Tiredness or Feeling Unmotivated

If your body isn’t keto adapted or used to fasting, you’ll likely feel short term fatigue at the start. This is due to the electrolyte loss and dehydration, combined with a withdrawal from carbs and sugar as your body adapts. Keep in mind that it’s temporary, and aim to rest during and avoid strenuous physical activity.


Since your cells haven’t adapted to running on ketones yet when you start fasting, headaches can be fairly common. This is a hypoglycemic issue related to your blood sugar, as your insulin is lowered. Until your body to ketones, this may be unavoidable but is always temporary.


While you’ll soon notice elevated mood levels, you might find yourself snapping at people more than usual while your body adjusts. Moodiness is not uncommon during a period of withdrawal from carbs and sugar, which is only enhanced by the other keto flu symptoms. A cup of black coffee can provide energy and liven your mood if you feel like you need something to tame your temporary grumpiness.

Brain Fog or Feeling Confused

Ketosis can lead to more mental clarity, but at first you may find yourself feeling confused, forgetful or foggy. If your body is used to a blood sugar roller coaster, then lowering levels can trigger brain fog. It can show up no matter how much sleep you get or whether or not you’ve had coffee. Try to go into the first day or two of fasting being aware of the potential brain fog, as it takes some time to transition.


A sense of sluggishness is possible in the first few days of fasting. You may feel weary and inactive, as your blood sugar levels are lowering while your body adapts. Be sure to avoid any possible stressors such as mental strains or a lack of sleep.


Not unlike the regular flu, you may feel feverish when entering ketosis. It’s possible that  if you are burning fat, that it could cause an increase in heat which leads to an elevated temperature, however there is not enough conclusive evidence to determine this.

Now that we’ve covered some of the main concerns when fasting and entering ketosis, let’s discuss how to best deal with them as your body adjusts.

How to Deal With Fasting Ketosis Symptoms

While most fasting ketosis side effects are temporary, there are several things you can do to reduce the symptoms and make the process more pleasant. Try to get enough sleep and work on reducing stress as much as possible. When combining intermittent fasting with a ketogenic diet, make sure you’re still staying within your macros and eating enough calories during your eating periods.

Managing Fasting Ketosis: Signs of Ketosis From Fasting

Stay Hydrated

You can expect a big flush in water weight, which can leave you dehydrated and exacerbate the above symptoms. Drink plenty of water and consume bone broths that are rich in minerals.

Increase Salt Intake

As your body flushes out water from lack of carbs, sodium is excreted as well. For best results, use high-quality sea salt, such as Celtic sea salt or Himalayan sea salt, each time you eat.

Eat Mineral-Rich Foods

If you’re doing intermittent fasting, be sure to eat high-nutrient meals during your eating windows to replace any lost electrolytes. Eat plenty of leafy greens, celery, seaweed, cucumber, meat, poultry, fish, avocados and high fat, quality dairy products (if dairy agrees with you). You’ll also want to eat plenty of fats such as coconut oil and MCT oil, as they’ll keep you satiated without spiking your insulin level.

Supplement With Magnesium

Take magnesium citrate to help balance hydration and electrolyte levels. Be careful with this if you experience diarrhea, and if you have kidney problems, check with your doctor before taking magnesium supplements.

Take Exogenous Ketones

One way to avoid keto flu symptoms is by taking exogenous ketones. They provide your body with extra ketones to use for fuel during the transition, getting you into ketosis faster. Speeding up the process can help reduce all of the common symptoms mentioned above.

General Self Care

Make sure you’re taking care of yourself. Whether you’re starting a new ketogenic diet, trying intermittent fasting or both, ketosis is an adjustment for your body. Know you’re doing something good for yourself, and keep your long-term health in mind if your symptoms frustrate you. Be sure to get plenty of sleep. Sleep deprivation can lower testosterone, which cause insulin resistance and make your blood sugar levels rise.

Positive Fasting Ketosis Symptoms

If these symptoms have you feeling nervous about your new keto diet, don’t forget that there are many positive symptoms of fasting in ketosis. Once you get through the first couple of weeks you’ll be reaping the amazing benefits.

Weight Loss and Appetite Suppression

Not only does fasting in ketosis naturally reduce food intake and increase fat loss, eating a high-fat, low carb diet also increases satiety and naturally reduces appetite.

Boosted Energy

After you get through the initial phases of ketosis, a great benefit of switching from a carb-heavy diet to a low-carb diet is a more steady stream of energy. Fasting in ketosis reduces spikes in blood sugar even more, leaving you with more stable energy levels throughout the day.

Higher Mental Clarity and Cognition

The ketones produced when burning fat, whether you’re fasting or eating a ketogenic diet, are able to cross the blood-brain barrier, providing energy for your brain and even providing neuroprotective benefits[*].

Long-Term Preventative Health

Besides the immediate positive symptoms of fasting in ketosis, there are amazing long-term health benefits of running on fat instead of glucose, including decreasing the risk of diseases like diabetes, Alzheimer’s and cancer[*].

Avoiding Symptoms

Eating a ketogenic diet and fasting regularly can increase your chances of a long, healthy life. Being aware of the common symptoms and using the preventative measures above can help you avoid any initially uncomfortable side effects, and get the most out intermittent fasting.

Dr. Anthony Gustin is a board-certified sports chiropractor, functional medicine practitioner, entrepreneur, podcast host, and founder of Perfect Keto.

After growing his sports rehab and functional medicine clinics to six locations in San Francisco, he shifted his mission to help as many people as possible achieve optimal health and well-being.


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Responses (16)

  1. Hi Kim! Try putting Cacao butter/nibs in your coffee or tea. It taste just like chocolate…because it is the base for candy bars. Make chocolate fat bombs. I eat a little one after dinner or when I’m doing intermittent fasting. I put sea salt in my recipe for the potassium and magnesium we need to stay alert.

  2. I’ve been using your Chocolate Sea Salt Exogenous Keytones during my 3 day fast (blended in decaf coffee) and I can’t help but feel that it’s “cheating”. Is this breaking my fast? I typically stick to straight water during fasts.

  3. Anyone experiencing facial flushing? Back on the Keto wagon and here it is again. Haven’t noticed if for the last few months of being off Keto.

  4. Yes, I experience that as well. Since I started my Keto lifestyle I’ve been experiencing facial flushing especially on my cheeks and nose and a general feeling of my skin being warmer. I also sweat a lot more than usual. I can’t find much info on this online but have read about others experiencing the same.
    I would like to know what causes it though – perhaps increased metabolism…?

  5. Are you taking a multivitamin that has a lot of niacin or a B vitamin complex? Sounds like niacin flush to me. Cut back on your supplementing and get your nutrition from vegetables and meat, maybe the occasional piece of fruit. Just because fruit has carbs doesn’t make it evil. Nature is really good at allocations.

  6. You are sweating because on Keto lifestyle, you are always loosing water because you are losing carbs and carbs in the body as glycogen in muscles are attached to water molecules. Also it depends on how many of grams of carbs you are taking. You need to eat greens, veggies and fruit. These are very important especially if you are continously doing less in low tens of carbs.

  7. I went off Keto for one day and had a beer and 2 slices of pizza. I got right back on the Keto diet the next day however I have been experiencing nausea off and on now for 6 days and feel so tired. Is this normal and has anyone ever experienced this before? P.S. the beer and pizza both had an aftertaste of sugar.

  8. I was diagnosed with a 127 fasting glucose even though I work out every day! Maybe 10 lbs overweight. Yikes! Ben IF for 19 days-20/4 plus keto. My fasting blood sugar down 30+ points! Doc can’t believe it! I have history of eating a lot of carbs. I am sold on this and recommend to anybody prediabetic!

  9. Hi Kyle, there are several factors that can affect strip readings. These include hydration, food consumption or becoming keto-adapted when you’ve been in ketosis for an extended period of time.

    It’s good to use the strips when you first start keto, as you can test at different times to gauge what kind of meals and activities affect ketone levels, however, as your body adapts to using fat as a fuel source, fewer ketones will “spill over” into urine because they are actually being used by the body.

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