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Dealing with Intermittent Fasting Fatigue: 5 Common Causes

author avatar Dr. Eric Berg 08/31/2023

You recently started intermittent fasting and now feel wiped out? Intermittent fasting fatigue is common and nothing to be concerned about—it can actually indicate that your body is adapting to ketosis.

Intermittent fasting primes your metabolism to use fat instead of carbs as fuel and changes how your cells utilize energy. As your body adjusts to periodic fasting, fasting-related fatigue will likely disappear on its own.

Let’s talk about the common causes of intermittent fasting fatigue and what steps you can take to minimize intermittent fasting side effects. 

Fatigue at the workplace

Five causes of intermittent fasting fatigue—and what to do

According to research published in Behavioral Science, feeling tired is one of the most common side effects after starting intermittent fasting.

“The good news is that intermittent fasting fatigue is temporary, and you can take plenty of steps to overcome this side effect of fasting,” says Dr. Berg.

Here are five common triggers of fasting-related fatigue.

1. Low blood sugar 

Low blood sugar is among the top reasons for feeling tired and drained after starting intermittent fasting. 

During fasting periods, your body relies on ketones for energy. During the early days of intermittent fasting, your brain and muscle cells aren’t very efficient in using this new fuel source which can quickly cause fatigue, brain fog, and exhaustion. 

Low blood sugar can also indicate that you are trying to achieve long fasting periods too quickly. 

Kathy McManus, registered dietitian and director of the Department of Nutrition at Harvard-affiliated Brigham and Women's Hospital, advises, “Easing into an intermittent fasting plan can help your body adjust. Slowly reduce the time window for eating over a period of several months."

If you feel exhausted, shorten your fasting period and add a meal. If you feel better, slowly prolong fasting periods, listen to your body, and adjust as necessary.      

Another common mistake is not eating enough calories during the eating window, leading to fatigue and lack of energy. That’s why it’s recommended to combine intermittent fasting with a high-fat ketogenic diet like Healthy Keto®. 

Although it sounds counterintuitive, a low-carb keto diet helps maintain healthy blood sugar levels and boosts your ketone production, which provides plenty of fuel for your cells to combat fatigue.   

Watch the video below to learn why fatigue is a common intermittent fasting side effect. 

2. Dehydration and electrolyte imbalance

Both intermittent fasting and keto cause water loss and electrolyte depletion in the early days of keto-adaptation. While this water loss explains the rapid weight loss most people experience after starting intermittent fasting, it can also cause dehydration symptoms, including fatigue and lethargy.  

As your body sheds increased amounts of fluids, your kidneys excrete essential electrolytes, including sodium, potassium, magnesium, and calcium. These electrolytes control fluid balance, muscle function, and heart rate, and electrolyte imbalances are common causes of fatigue.

Low sodium and potassium levels can significantly impact adrenal function and result in imbalanced levels of adrenal hormones, including aldosterone and cortisol. Cortisol plays a crucial role in regulating energy metabolism, and a lack of sufficient electrolytes, poor adrenal function, and fatigue are closely linked. 

Increase your water intake and replenish electrolytes to avoid dehydration and fatigue. Eat more potassium-rich foods like vegetables and salad, and increase your intake of mineral-rich sea salt

Alternatively, find a good quality electrolyte supplement to support balanced nutrient levels and avoid fasting-related fatigue. 

3. Lack of B vitamins 

B vitamins are essential for energy production and help your body to adapt to ketosis—so not including enough vitamin B-rich foods in your diet can worsen intermittent fasting fatigue. 

Because carbohydrate metabolism depletes vitamin B1, you may have developed a vitamin B1 deficiency if your diet is high in carbs and sugars. Without adequate vitamin B1, your cells cannot efficiently utilize ketones as an energy source.

Vitamin B5 supports healthy adrenal function and cortisol production, which is essential for blood sugar balance and utilizing fat as an energy source. This makes vitamin B5 an especially vital nutrient if you are doing intermittent fasting and keto.

To support healthy B vitamin status, follow a balanced ketogenic diet that focuses on nutrient-dense foods rich in vitamins and minerals. A Healthy Keto® diet supports your body’s nutritional stores and can help to keep intermittent fasting fatigue at bay. 

The best foods to combat fatigue are high-quality, nutrient-dense foods like green leafy veggies, seeds and nuts, avocados, seafood, meat, and full-fat dairy.

Liver health risk factors

4. Sluggish liver function 

The liver is your body’s main metabolic organ that plays an essential role in energy production, fat breakdown, and protein synthesis. The liver also stores essential vitamins and minerals and is crucial for the detoxification of hormones, drugs, alcohol, and environmental toxins.

Fatigue and lack of energy are hallmarks of poor liver function, which can result from a fatty liver, cirrhosis, or an unhealthy diet. Processed foods, sugars, and carbs can strain the liver's metabolic function and deplete nutrients that support energy-making processes.

Consuming plenty of phytonutrient-rich foods helps to protect liver cells from inflammation and oxidation and can prevent fat accumulations linked to fatty liver and other liver diseases. 

To support liver health and combat fatigue, eat plenty of cruciferous vegetables, dandelion greens, olive oil, garlic, and green tea

5. Low stomach acid 

Many people have insufficient stomach acid levels, resulting in sluggish digestion, nutrient deficiencies, and fatigue. 

Stomach acid is essential for absorbing vitamin B12, zinc, and iron, which are crucial nutrients for forming red blood cells and oxygenating the cells in your body. Without adequate oxygen, your cells cannot produce energy efficiently, explaining why iron, zinc, and vitamin b12 deficiency are directly linked to fatigue.  

Intermittent fasting allows your digestive system to rest. While this supports a healthy intestinal microflora and gut health, it may cause a temporary reduction in stomach acid. As your body adapts to longer periods of fasting, stomach acid production will adjust. 

Avoid eating on the go, chew slowly, and enjoy your meals. This allows your nervous system to send signals to the digestive tract that activate the release of stomach acid.

You can support low stomach acid with an apple cider vinegar drink at meal times. Apple cider vinegar is a rich source of acetic acid that promotes digestion and nutrient absorption and supports healthy blood sugar levels.

keto foods on a plate

What is intermittent fasting?

Intermittent fasting is a method of meal timing that’s characterized by cycling from fasting periods to periods of eating. 

There are several popular methods of intermittent fasting, including:

  • Time-restricted eating refers to eating only in a specific time frame each day, typically 8-10 hours, and fasting for the remainder of the day. 

  • Alternate-day fasting involves alternating between days of normal eating and days of restricting caloric intake to around 500 to 600 calories daily.

Intermittent fasting has many health benefits. Research published in Nutrition Research suggests that it has similar metabolic effects to caloric restriction—a consistently low-calorie intake without causing malnutrition–linked to longevity and a significantly lower risk of heart disease. 

Intermittent fasting shifts your metabolism to utilize fat instead of sugar for energy. As your body breaks down fat, the liver produces large quantities of ketones, a type of acid and a very efficient energy source. 

As ketones enter your circulation and are used as a fuel source, your body enters a state known as ketosis. 

Not only does intermittent fasting help with weight loss, but people that practice intermittent fasting also experience better sleep quality, improved cognition and mental health, diminished cravings, better blood sugar control, and improved insulin sensitivity. 

Tired woman holding cup

Potential intermittent fasting side effects

Although intermittent fasting has profound benefits for your health, it may cause some side effects, especially during the early days and weeks of adjusting to an intermittent fasting regimen. 

Depending on your metabolism, sensitivity to insulin, adrenal health, and previous diet, it can take your body a few weeks to adapt to the profound metabolic changes of intermittent fasting and ketosis. 

These are common side effects of adapting to intermittent fasting: 

  • Headaches 

  • Dehydration

  • Sleep disturbances

  • Hunger   

  • Difficulty concentrating 

  • Mood changes 

  • Keto flu

Most of these symptoms develop due to your body adjusting to ketosis and should disappear once your cells adapt to using ketones as an energy source. 

However, these symptoms may also be indicative of underlying medical issues, and you should consult a healthcare provider to determine the underlying causes of persistent fatigue and develop an appropriate treatment plan based on your needs. 

It’s important to note that intermittent fasting isn’t recommended for pregnant women, individuals with an eating disorder, and people taking certain medications to treat diabetes. 

Woman eating salad

Key takeaways

Fatigue is one of the most common intermittent fasting side effects, and it doesn't mean that intermittent fasting isn’t right for you.

As your body adapts to the powerful metabolic and hormonal changes of intermittent fasting, your energy levels will bounce back, and you will likely feel more energetic than ever.

You can help to avoid intermittent fasting fatigue by transitioning slowly into your new fasting regime and combining it with a nutrient-rich Healthy Keto diet. 

Staying hydrated, replenishing electrolytes, and supporting your stomach acid levels support energy metabolism and combat fatigue.     


1. How do you prevent intermittent fasting fatigue?

To avoid intermittent fasting fatigue, it’s best to transition slowly into your fasting regime, support healthy vitamin and mineral levels with a nutrient-rich diet like Healthy Keto, stay hydrated, and replenish electrolytes.


2. What are the side effects of 16:8 intermittent fasting?

Common side effects of 16:8 fasting are headaches, dehydration, hunger and carb cravings, brain fog, fatigue, and mood changes. 

3. How long does it take for your body to adjust to intermittent fasting?

That depends on many factors–your previous diet, body weight, and how metabolically flexible you are. Some people can transition into intermittent fasting within a few days, while others need a little longer.

In most cases, intermittent fasting fatigue disappears within a few weeks.  

4. Does intermittent fasting affect energy levels?

While fatigue can be a side effect during the early days and weeks of intermittent fasting, people practicing intermittent fasting typically feel more energized than ever once their body adapts to the metabolic changes as a result of fasting. 

5. What are the negative effects of intermittent fasting?

Intermittent fasting can, in some rare cases, increase the risk of gout and uric acid kidney stones. There is also an increased risk of fainting, but this is typically only caused by very long fasting periods.

6. What are the disadvantages of intermittent fasting?

Intermittent fasting is generally well tolerated and has many health benefits. However, it can cause symptoms similar to keto flu, including headaches, brain fog, cravings, and mood changes. 


7. Is intermittent fasting OK to do every day?

In fact, it’s recommended to do intermittent fasting every day. This helps your body stay in ketosis, boosts your metabolism, and amplifies the many health benefits of intermittent fasting.

8. What happens if I do intermittent fasting every day?

Intermittent fasting helps with rapid weight loss, improves sleep and cognition, and supports healthy blood sugar control and insulin sensitivity. 

Intermittent fasting has been shown to lower the risk of heart and neurodegenerative diseases, metabolic syndrome, and diabetes.

9. Who shouldn’t do intermittent fasting? 

Pregnant women and people with eating disorders or a history of disordered eating should avoid intermittent fasting. 

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