How long should you fast? You’ve perhaps heard some people say that 12 hours is enough, whereas others swear by 16 or even 24 hours. You’ve read about these different fasting periods and wondered whether there’s a magic number that works for everyone. The truth is, there isn’t.
Any fast that lasts for longer than 12 hours is considered an extended fast, and this is sometimes suggested for people with specific health goals.
However, if you’re looking to improve your overall health and well-being, you may want to consider working your way toward an 18-hour fast. We know that going three-quarters of a day without a meal sounds intimidating, but it’s possible. Better yet, it may even be beneficial.
Read on to understand why this may be just what your body needs for optimal health.
What Is an 18-Hour Fast?
An 18-hour fast is a form of intermittent fasting where you eat all your daily calories within a six-hour window and fast for the remaining 18 hours. For example, you may choose to eat between 12 pm and 6 pm and fast from 6 pm to 12 pm the next day. A large part of this fast is spent asleep, so you only need to skip one meal.
During the fasting period, you are only allowed to consume zero-calorie drinks, including water, black coffee, or tea. This means no snacking or even adding a dash of creamer to your morning coffee.
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Is Fasting for 18 Hours Good for You?
Fasting for 18 hours is good for you, particularly when it is done right. By doing it right, we mean:
- You’ve taken time to understand exactly what fasting means and how it affects your body
- You’ve consulted a doctor or health care professional to ensure you’re physically able to handle an extended fast
- You’ve slowly worked your way toward an 18-hour fast, starting with shorter periods such as 12 or 14 hours first
- You’ve researched what breaks a fast and you’re careful not to consume any foods or drinks that would affect the benefits of your fast
- You listen to your body and make adjustments if necessary
Once you have taken these steps, an 18-hour fast may be incredibly beneficial for your body. We’ll explain why in the next section and share solid, science-backed reasons.
What Will Fasting for 18 Hours Do?
Fasting 18 hours a day for a month is likely to have visible results regarding your energy, mood, and weight. Keeping it up for longer may benefit your overall health in many ways, as it may:
Super-Charge Your Weight Loss
An 18-hour fast naturally results in fewer hours of eating during the day, making it easier to achieve this deficit without feeling deprived.
When fasting for an extended period, your body starts to increase lipolysis, the process of breaking down stored fats for energy. This occurs because, in the absence of readily available glucose from food, your body must tap into its reserve energy—your stored fat (1). The result? Theoretically, more fat burned and gradual weight loss.
Fasting also enhances your metabolic flexibility, which is the ability of your body to switch between burning carbs and fats for fuel. Improved metabolic flexibility means that your body becomes more efficient at utilizing whatever fuel source is available, whether from the meals you eat or your fat stores (14).
You may worry that fasting can slow down your metabolism, but interestingly, research has suggested the opposite.
Short-term fasting can actually boost your metabolic rate by approximately 3.6%, which helps you burn even more calories (6) (16). So 18:6 fasting for one month, or even up to six, may work for this purpose.
Finally, fasting is believed to enhance mitochondrial biogenesis, the process by which new and more efficient mitochondria (the powerhouses of your cells) are formed (19). This means your cells become better at producing and using energy.
Essentially, an 18-hour fast is thought of as being like hitting a reset button on your metabolism, which prompts your body to become more efficient and flexible in the way it uses energy. This is why it could be an effective strategy for weight loss.
Give Your Digestive System a Break
An 18-hour fast provides your digestive system with a well-deserved break, which allows it to reset and restore optimal function.
Firstly, let’s talk about something called the migrating motor complex (MMC). This is a series of electrical waves that sweep through your intestines in between meals, approximately every 90 to 120 minutes (13).
You should think of MMC as the cleaning crew that comes in when the production line (your food processing line, in this case) is on a break. It helps clear out undigested material from your stomach and small intestine. It is a crucial process, but it only kicks in when you aren’t actively digesting food.
Therefore, an 18-hour fast gives MMC ample time to do its job, which may help mitigate issues such as bloating, gas, and irregular bowel movements.
So essentially, an 18-hour fast allows your body to tap into its natural mechanisms (such as MMC) to optimize digestion. This may make your system more efficient and help alleviate common digestive issues.
It may seem counterintuitive—less food, better digestion? But once you understand the science behind it, it all makes sense.
Support Your Immune System
Restricting your eating window is believed to boost your immune system through various mechanisms.
As previously mentioned, an 18-hour fast may help reduce inflammation in the body (23). This is important because chronic inflammation has been linked to many diseases, including obesity and autoimmune disorders (3).
In addition, fasting may help regulate the production of immune cells and cytokines (messenger proteins that are involved in immune response), which may enhance your body’s ability to fight off infections and diseases (12).
A study on mice found fasting to be protective against intestinal salmonella infection, at least partially due to changes in the gut microbiome (8).
At the same time, your immune system requires nutrients to function optimally, so it is likely that there is a point where fasting will become detrimental to the immune system.
Super-Charge Your Cognitive Function
Your brain may experience both short- and long-term benefits from an 18:6 fast. In the short term, an interesting phenomenon occurs in your brain when you fast—it starts producing more of a protein called brain-derived neurotrophic factor (BDNF) (22).
This protein helps promote the growth and maintenance of new neurons, which is essential for learning and memory. It also protects existing neurons from damage (2).
In addition, fasting has been linked to increased levels of human growth hormone (HGH), a hormone that plays a role in brain health (7).
HGH can help improve cognitive function, learning, and memory. It also has neuroprotective effects, which means that it can help protect your brain against age-related decline (11).
It has been suggested that fasting has potential long-term benefits for conditions such as Alzheimer’s disease and Parkinson’s disease (22).
What to Eat After an 18-Hour Fast
Earlier, we recommended properly understanding what your body needs and finding a meal plan that works for you. And we stand by that 100%. Here are some general guidelines to get you started:
- Break your fast gently to prevent any potential digestive discomfort. We recommend bone broth, which is nutrient-dense and easy to digest. You can also eat some cooked vegetables or a small portion of protein, such as eggs or fish.
- After your initial meal, you should eat nutrient-dense whole foods such as lean proteins (chicken, fish, lentils), complex carbohydrates (vegetables, fruits, whole grains), and healthy fats (avocado, olive oil).
- Eat until you feel satisfied, not stuffed. Remember, it’s not about restriction or deprivation, it’s about nourishing your body with foods that support its natural functions.
18-Hour Fast and Exercise
Now, you may be wondering how fasting for 18 hours will affect your exercise routine. Our suggestion is that you experiment with one of these strategies:
- Perform light exercise during your fasting window: Some people find that movement while they are fasting helps them feel more energized and focused. This may be because their bodies are already using fat as fuel, which makes the workout more efficient. However, this probably isn’t the time for an intense workout.
- Exercise after breaking your fast: If you prefer to work out with some fuel in your system, break your fast with a small meal or snack before you exercise. This will provide your body with the energy it needs to power you through your workout.
- Adjust your exercise intensity: Fasting for 18 hours may make you feel more fatigued than usual, so you may want to adjust your exercise routine accordingly. You may need to lower the intensity or shorten the duration of your workout.
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Frequently Asked Questions
Do you burn fat on an 18-hour fast?
Yes, when you fast for 18 hours, your body enters a state of ketosis where it starts burning fat for energy. This is because during the fasting window, your body depletes its stores of glucose (sugar) and starts using stored fat as fuel instead (9).
How many days a week should you do an 18-hour fast?
The idea behind 18/6 intermittent fasting is to make it a regular part of your lifestyle, rather than a short-term diet.
As it falls under the umbrella of time-restricted eating, it’s most effective when it is done daily. You can vary the length of your fasts throughout the week – they don’t all need to be 18 hours. That being said, there are also versions of intermittent fasting where you fast every other day or a few times a week.
Why is 16 hours the magic number for fasting?
The 16-hour mark is often mentioned as it is believed to be around that time that the body enters ketosis and starts burning fat for fuel. This duration is also manageable for most people and fits well into a daily schedule with eight hours of eating time.
Fasting for 18 hours vs 16 hours – which is better?
Fasting for longer may increase the benefits, such as autophagy and reduction of inflammation (20). However, some people may find it more challenging to go for 18 hours without eating.
Starting with shorter fasts, such as 12-16 hours, and working your way up to the 18-hour mark is recommended. It is also recommended to increase the frequency of your fasts gradually. For example, you could start with an 18-hour fast once a week before increasing it to multiple days.
What is the best time for an 18-hour fast?
Some people prefer to skip breakfast and start eating in the afternoon, whereas others prefer to skip dinner. To ensure you make the right decision, you should consider your daily schedule and the time of day you feel most comfortable fasting. Experiment with different times to see what works best for you.
What should I eat after an 18-hour fast?
After an 18-hour fast for weight loss, it’s best to break your fast gently. You should start with something light and easy to digest, such as bone broth or cooked vegetables. Then, you should eat nutrient-dense whole foods, such as lean proteins, complex carbohydrates, and healthy fats.
Is it better to fast for 18 or 24 hours?
Both have their benefits and the best choice is dependent on your goals and how your body reacts to fasting. Fasting for 24 hours may offer more benefits in terms of weight loss and autophagy, but it can also be far more challenging.
You should talk to a healthcare professional before you start longer fasts.
The Bottom Line
An 18-hour fast may initially seem daunting, but the potential benefits for your body and mind are worth considering. From improved digestion to enhanced immune protection, cognitive function, and weight loss, fasting can have a positive impact on a variety of aspects of your health.
To get the most out of your fast, make sure you have a proper understanding of the science behind it and consult a healthcare professional before you start.
This article is intended for general informational purposes only and does not address individual circumstances. It is not a substitute for professional advice or help and should not be relied on to make decisions of any kind. Any action you take upon the information presented in this article is strictly at your own risk and responsibility!
- Biochemistry, Lipolysis (2023, ncbi.nlm.nih.gov)
- Brain-derived neurotrophic factor and its clinical implications (2015, ncbi.nlm.nih.gov)
- Chronic inflammation in the etiology of disease across the life span (2020, ncbi.nlm.nih.gov)
- Cyclic AMP (2008, ncbi.nlm.nih.gov)
- Effects of Ketone Bodies on Brain Metabolism and Function in Neurodegenerative Diseases (2020, mdpi.com)
- Enhanced thermogenic response to epinephrine after 48-h starvation in humans (1990, pubmed.ncbi.nlm.nih.gov)
- Fasting enhances growth hormone secretion and amplifies the complex rhythms of growth hormone secretion in man. (1988, ncbi.nlm.nih.gov)
- Fasting increases microbiome-based colonization resistance and reduces host inflammatory responses during an enteric bacterial infection (2022, journals.plos.org)
- Fasting: Molecular Mechanisms and Clinical Applications (2014, ncbi.nlm.nih.gov)
- Fat loss depends on energy deficit only, independently of the method for weight loss (2007, pubmed.ncbi.nlm.nih.gov)
- Growth hormone and cognitive function (2013, nature.com)
- Innate immune remodeling by short‐term intensive fasting (2021, onlinelibrary.wiley.com)
- Interdigestive migrating motor complex -its mechanism and clinical importance (2014, ncbi.nlm.nih.gov)
- Metabolic Flexibility as an Adaptation to Energy Resources and Requirements in Health and Disease (2018, academic.oup.com)
- Physiology, Fasting (2023, ncbi.nlm.nih.gov)
- Resting energy expenditure in short-term starvation is increased as a result of an increase in serum norepinephrine (2000, sciencedirect.com)
- The Beneficial and Adverse Effects of Autophagic Response to Caloric Restriction and Fasting (2023, sciencedirect.com)
- The effect of fasting or calorie restriction on autophagy induction: A review of the literature (2018, sciencedirect.com)
- The effect of fasting or calorie restriction on mitophagy induction: a literature review (2020, onlinelibrary.wiley.com)
- The effect of prolonged intermittent fasting on autophagy, inflammasome and senescence genes expressions: An exploratory study in healthy young males (2023, sciencedirect.com)
- The Effects of Fasting During Ramadan on the Concentration of Serotonin, Dopamine, Brain-Derived Neurotrophic Factor and Nerve Growth Factor (2017, ncbi.nlm.nih.gov)
- The Effects of Intermittent Fasting on Brain and Cognitive Function (2021, mdpi.com)
- The influence of fasting and caloric restriction on inflammation levels in humans (2021, journals.lww.com)