How Much Fasting Is Too Much?

Fasting for more than 24 hours without supervision can be too much for some people. The longer the fasting period, the greater the chance of side effects.

Therefore, if you decide to engage in a multi-day fast, you should do so under a doctor’s supervision.

During my coaching career, I’ve witnessed several instances where people deliberately starved themselves for several days to lose more weight in the name of fasting.

Extended fasts, while often seen as a means to detox, lose weight, and improve cognitive function, can be harmful. When taken to extremes, prolonged fasts can lead to nausea, insomnia, fatigue, irritability, muscle loss, metabolic disruptions, and hormonal imbalances. (1)

So, where do you draw the line? How much fasting is too much? Let’s find out.

How Much Fasting Is Too Much?

As a general rule of thumb, healthy people can safely fast for up to 24 hours without supervision. If you decide to engage in a multi-day fast, you should do so under a doctor’s supervision.

Adhering to these general guidelines can be a great way to avoid fasting side effects. However, the problem with generic guidelines is that they’re often quite conservative and don’t cover all the bases. Fasting for 24 hours rarely causes any dire side effects. People usually just feel hungry and may struggle to fall asleep.

In fact, most healthy adults should be able to fast for up to 72 hours with no significant adverse effects. Usually, folks experience mild to moderate side effects between the 36th and 48th hour. However, they usually get better after this point.

However, the risk exists, and it is best to err on the side of caution and listen to your body. From my experience, you’ve taken your fast too far when you start experiencing any of the following symptoms:

• Dizziness and lightheadedness
• Insomnia
• Excessive fatigue
• Frequent and painful muscle cramps

You should stop your fast and contact your doctor as soon as these side effects occur.

Which Fasting Side Effects Shouldn’t Worry You?

You are guaranteed to experience some side effects during an extended fast, but not all of them mean you should stop. Missing a meal can make us moody and irritable, so not every side effect you encounter is a cause for concern.

The common fasting side effects you don’t need to worry about are:

1. Hunger and Cravings

An increase in hunger and food cravings are natural responses to reduced food intake and usually diminish as the body adapts to the new eating pattern.

It might take several hours for your body to get used to the lack of food, but some hunger pangs and junk food fantasies are perfectly normal.

2. Mood Swings

Being “hangry” is a thing, so don’t worry if you get snappy or agitated. In fact, a recent study has shown a direct correlation between hunger and irritability, even without fasting. (2)

Minor mood swings aren’t anything to worry about. However, if they lead to outbursts of anger or aggression towards others, you should seek help.

3. Weakness and Tiredness

After your glucose stores are depleted and your body starts burning fat stores for energy, you will likely feel tired. You might feel weak, your motivation might drop, and you might feel like you desperately need a nap, and that’s fine. This happens to many people and usually goes away after a good night’s sleep.

4. Headaches

Some folks experience headaches early into their fast. Headaches could result from dehydration or changes in blood sugar levels, so they shouldn’t last long. Stay adequately hydrated to prevent headaches.

5. Loss of Focus

Many people have trouble concentrating while fasting. Once again, this is likely the result of low blood sugar levels disrupting optimal brain function and causing mild brain fog.

Neuroscientists’ Take on Cognitive Impairment During Fasting

Dr. Andrew Huberman, associate professor of neurobiology and ophthalmology at Stanford University School of Medicine, has discussed fasting on numerous occasions.

In one of his podcast episodes, Dr. Huberman touches on fasting, low blood glucose levels, and their effects on focus and concentration.

“Your ability to focus, and in fact, your ability of neurons to encode specific information in your environment, that is to represent what’s out there in the world, is actually related to your blood glucose level.

There’s a beautiful study published in Neuron not long ago that showed that the tuning, that is, the precision with which neurons in the brain will represent things in our environment, is actually much greater when there is sufficient glucose in the brain.

Translated into English, this means that when we are fasted or when our blood glucose is very low, we aren’t able to perceive and think about things as clearly.” — Dr. Andrew Huberman

What are the Potential Drawbacks of Prolonged Fasting?

Excessive fasting, especially for more than a few days at a time, can lead to an array of side effects, such as:

1. Excessive Fatigue

When you go without food for a long time, your body’s energy reserves become depleted, leading to chronic fatigue. You become exhausted, weak (and weak-willed), and sleepy. You might even start experiencing physical pain.

This is all due to your body’s attempts to conserve energy for vital functions. Instead of giving you the energy to move around and do things, it wants to use its reserves to keep the organs running.

2. Nutrient Deficiencies

Prolonged fasting limits food intake, which could lead to nutrient deficiencies. You will miss out on vitamins, minerals, and every other micro or macronutrient needed to maintain optimal health and immune system function.

However, many studies do not support this notion. It generally takes a very long time for your body to become nutrient-deficient. In fact, a study shows that even a 44-day voluntary starvation did not lead to nutrient deficiencies. (3)

3. Metabolic Slowdown

When you don’t eat for a while, your body adjusts to the lack of calories and reduces the basal metabolic rate (BMR) to prevent starvation. Therefore, if you fast for several days, your body will rely on this survival mechanism to suppress energy expenditure and significantly lower your BMR. (4)

This can disrupt your metabolic health, causing you to gain weight more rapidly after you end the fast, and make the weight loss journey more challenging.

4. Electrolyte Imbalance

Electrolyte imbalance can be caused by reduced food intake and dehydration and can lead to various issues.

One of the first issues you’ll notice is frequent muscle cramping and excessive fatigue, both of which indicate you’ve taken your fast too far.

If things get really out of hand, electrolyte imbalance can lead to severe consequences, such as arrhythmias, seizures, and hormonal imbalances.

5. Insomnia

Insomnia is a very common problem among people who fast, as it is easily triggered by the lack of food in their system. Food restriction, as well as the stress response to not eating, can affect your circadian rhythm, which will not only prevent you from falling asleep quickly but also wreck the quality of your sleep.

The biggest problem with insomnia is the slowball effect and the cascade of other issues it can lead to.

Not getting enough sleep during the night can lead to increased cortisol production, anxiety, heart palpitations, and so much more. Combined, those create an environment not conducive to restful sleep. This intensifies the problem, trapping you in a vicious cycle of feeling horrible, anxious, and sleep-deprived.

6. Dizziness

Dizziness can occur when blood sugar levels drop significantly, which happens when one fasts for an extended period.

As a result, your balance and coordination may be affected, and in more severe cases, you may lose consciousness, fall, and injure yourself.

7. Muscle Loss

Besides relying on the stored fatty tissue for energy, your body will also turn to your muscles. This process is known as muscle catabolism, and it happens when your body’s glucose and fat reserves are insufficient to meet the energy demands.

The 44-day voluntary starvation study I referenced earlier also showed that three-quarters of total body mass lost during the experiment was fat-free mass, meaning it was predominantly muscle.

Are There Any Benefits To Prolonged Fasting?

Of course, prolonged fasting, when done correctly, has its advantages. Otherwise, no one would do it. Some of the benefits you can expect are:

1. Enhanced Autophagy

Autophagy is the process of cellular “cleanup,” and it is one of the most common reasons people experiment with prolonged fasting.

One of the primary benefits of autophagy is the removal of damaged cells and organelles and the creation of new ones, leading to improved health, better metabolic function, a decrease in inflammation, and improved neurological function.

There has also been much talk about autophagy and its role in cancer prevention, but based on research data, there are two sides to this coin. Autophagy can prevent tumor formation in the initial stages, but it could also exacerbate the issue in many cases — especially at the later stages.

2. Weight Loss

Not eating for a few consecutive days, whether that’s two or three days without food, will lead to weight loss. However, if you couple this with a structured training and diet program after the fast, you should be able to maintain a large portion of your lean muscle mass.

3. Improved Metabolic Health

While prolonged fasting can reduce your basal metabolic rate, it can also improve insulin sensitivity, which can be highly beneficial for those with type 2 diabetes or those at risk of it.

Essentially, when your blood sugar is low, the insulin won’t spike as high. There will be a balancing act between the two when either gets too high or too low.

Additionally, fasting stimulates autophagy, which can clean up insulin receptors, further optimizing glucose metabolism and decreasing the risk of metabolic syndrome.

4. Better Cognitive Function

While there’s not a lot of concrete evidence, prolonged fasting has been linked to improved brain function.

Many believe fasting can stimulate the production of brain-derived neurotrophic factor (BDNF), a protein that supports brain health, enhances memory and learning ability, and protects against cognitive decline.

How to Fast for a Long Time With No Consequences

The only way to achieve your fitness goals while fasting, whether fat loss or muscle gain, is through a carefully planned and personalized approach that accounts for all the variables. Here’s how you do that:

1. Get your bloodwork done: Before you begin a fast, go to a doctor’s office and get your bloodwork done — especially if you suffer from a pre-existing condition. This will allow your doctor to identify potentially problematic areas and advise you on the best course of action.
2. Start slowly: Don’t just rush into prolonged fasting. Start slow and work your way up. If you wish to fast for 72 hours, start with a 24-hour fast first. Then, you can work your way up in 12-hour increments every three weeks until you reach your goal. Also, don’t fast for more than 72 hours without medical supervision.
3. Stay hydrated: I can’t stress the importance of hydration while fasting. Drinking water, herbal teas, and electrolyte-rich drinks can help you stay hydrated and keep your electrolytes in balance.
4. Drink Bone Broth: Drinking bone broth can technically break your fast. However, the 40–50 calories you get from bone broth aren’t enough to cause a massive glucose or insulin spike, disrupt ketosis, or significantly affect autophagy. But it can help curb hunger and maintain electrolyte balance.

It’s as simple as that. Of course, you will have to listen to your body and react to the signals it gives you, but what I’ve said above is essentially all you need to do.

Conclusion

Fasting is an excellent way to lose weight and detox the body, but overdoing it is not a good idea. Going without food for several days at a time can be detrimental to your health and well-being if you don’t know what you’re doing.

Thankfully, fasting has been all the rage in recent years, and there are many resources and literature you can turn to. So it’s safe to say the only thing between you and a side-effect-free fast is a little bit of reading.

Ultimately, the only thing I want to say to you is — don’t push it. Break the fast when you feel you’ve had enough. Pushing yourself beyond your limits and sacrificing your health for a fast is not worth it.

References:

1. Shalabi H, Hassan AS 4th, Al-Zahrani FA, Alarbeidi AH, Mesawa M, Rizk H, Aljubayri AA. Intermittent Fasting: Benefits, Side Effects, Quality of Life, and Knowledge of the Saudi Population. Cureus. 2023 Feb 7;15(2):e34722. doi: 10.7759/cureus.34722. PMID: 36909028; PMCID: PMC9998115.
2. Swami V, Hochstöger S, Kargl E, Stieger S. Hangry in the field: An experience sampling study on the impact of hunger on anger, irritability, and affect. PLoS One. 2022 Jul 6;17(7):e0269629. doi: 10.1371/journal.pone.0269629. PMID: 35793289; PMCID: PMC9258883.
3. Jackson JM, Blaine D, Powell-Tuck J, Korbonits M, Carey A, Elia M. Macro- and micronutrient losses and nutritional status resulting from 44 days of total fasting in a non-obese man. Nutrition. 2006 Sep;22(9):889-97. doi: 10.1016/j.nut.2006.06.001. PMID: 16928474.
4. Müller MJ, Enderle J, Pourhassan M, Braun W, Eggeling B, Lagerpusch M, Glüer CC, Kehayias JJ, Kiosz D, Bosy-Westphal A. Metabolic adaptation to caloric restriction and subsequent refeeding: the Minnesota Starvation Experiment revisited. Am J Clin Nutr. 2015 Oct;102(4):807-19. doi: 10.3945/ajcn.115.109173. Epub 2015 Sep 23. PMID: 26399868.

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