This Is What Happens To Your Body During a 72 Hour Fast (2023)

Breakfast is the most important meal of the day. At least that’s what many of us were brought up to believe. But a growing body of research is seriously undermining that idea. Fasting, in one form or another, is all the rage as evidenced by the volume of ripped bros on YouTube who are itching to share the fasting secrets that have finally gotten them over that thing that happened in high school.


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For most people, a fast amounts to missing breakfast. They break their fast later in the day. Others chose to skip dinner instead. Either tactic will will result in a 16/8 fast. This means that in every 24-hour period, you fast for 16 hours and do all of your eating in an eight-hour window. Another popular variant is alternate day fasting, in which adherents typically eat no calories one day and whatever they want the next.


Some of the reported benefits of fasting regimens include a reduction in inflammation, decreased blood sugar levels and even a prolonged life span—although that last one has only been proven in rats so far. It wasn’t long before people started wondering if longer fasts would yield more pronounced results. I should remind you, if you’re considering doing this, to examine your intentions since any extended period of voluntarily skipping meals can be a sign of disordered eating.

I’ve tried to reconcile all the anecdotal fasting with conversations I’ve had with with doctors and dieticians to figure out what might happen to my body if I commit to this increasingly popular fad-within-a-fad and eat nothing for 72-hours straight. Oddly, I haven’t tried this personally, but based on the below, I just might.

You’ll be ravenous. Then, not so much.

For many of us, skipping breakfast is NBD, particularly when you’re sufficiently distracted and quaffing black coffee all morning. Skip lunch, however, and by mid-afternoon your brain is screaming at you to refuel. It’s not literally screaming, of course. It just makes you behave like an irritable and petulant toddler until someone else recognizes the tell-tale signs of hanger and shoves a donut in your face.

A recent study looked into why hanger is a thing and concluded that a disruption in homeostasis of the brain can provoke complicated emotional response involving an interplay of biology, personality, and environmental cues. This, perfect shitstorm along with flagging energy levels, and a talkative abdomen can and do make getting through the first part of a 72-hour fast extremely challenging.


But if you can ride it out, things tend to greatly improve at day two or three. “The gradual decrease in hunger is well documented in physiological studies showing gradual decrease in ghrelin over multiple days of fasting,” says Jason Fung, Toronto-based nephrologist and co-author of The Complete Guide to Fasting. Ghrelin, he explains, is a hormone that makes you feel hungry. It’s secreted in greater amounts when your stomach is in a non-stretched state. Fung goes on to explain that an abatement of hunger happens more often than not during an extended fast.

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I should probably mention here that 72 hours is a much shorter duration than it would take a healthy person to starve to death. In an editorial published in the British Medical Journal, a review of the pertinent literature on the subject found that humans can survive without any food for 30-40 days provided they are adequately hydrated.

As Alan D. Lieberson told Scientific American, how long a person survives without food really depends on “factors such as body weight, genetic variation, other health considerations and, most importantly, the presence or absence of dehydration.” Dying of thirst, however, can happen within mere hours. In another Scientific American article, professor of biology at George Washington University Randall K. Packer, said that an adult in comfortable surroundings could potentially last a week sans-liquid.


Your breath may smell

When Fung talks about your body using your fat for fuel, he’s talking about ketosis. To get into ketosis, you don’t give your body any of its preferred grab‘n go fuel—glucose—and force it to look for alternatives. When there’s nothing coming into your piehole, the body will start shaking down fat cells for energy. That’s why all those ripped bros are so into fasting and the ketogenic state it puts them in. They’ll tell you that fasting and ketosis is what’s gotten their body fat percentage down into the single digits and studies have shown that they may be right about that. What they don’t talk too much about is that those abs may have come at a high price.

A byproduct of that conversion of paunch into available energy are ketone bodies. “One way the body releases ketone bodies is through exhalation therefore making the breath sweet and fruity,” says New York-based dietician Amy Shapiro, putting somewhat of a positive spin on the odor. Research has shown that breath acetone is reliable indication that you have gone into fat burning mode. You release ketone bodies through your breath—and the smell is often unpleasant enough that the people you hangrily scared away by threatening to flay the next person you find pilfering your yogurt will stay away in fear of having their faces melted by your hellacious mouth farts.

You’ll lose weight

Keep in mind, Shapiro doesn’t consider the 72-hour fast as a way to achieve meaningful weight loss. “You will likely lose more water weight than actually fat as your body uses its glycogen stores for fuel before dipping into actual fat,” she says. “As you release glycogen, you lose water and that is usually the reason for the rapid weight loss. Losing fat takes more time.” Fasting proponent Fung, however, disagrees and maintains that you could lose 1.5 pounds of fat over a 72-hour period. For that reason, he recommends that people with a Body Mass Index (BMI) of less than 20 could put themselves at risk of malnutrition. “Most people have much more fat than that,” he says.

Your body starts running on emergency power

Traditionally, not eating for three days would be seen by most as a not smart move. In fact, in times and places of food scarcity, it would likely be viewed at the last word in stupidity. Provided you can get something to eat on Thursday however, padlocking your pantry on Monday may actually improve brain function—according to rodent studies, at least.

Researchers at Yale started injecting ghrelin into mice and found that their performance in learning and memory tests was increased by 30 percent. Another study at Swansea University in Wales added the hormone to mouse brain cells grown in a dish. The infusion it switched on a gene known to trigger neurogenesis, a process in which brain cells divide and multiply.



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As mentioned, ghrelin production tapers off after a few days of not eating. In the interim, the stomach is secreting plenty of it. Shapiro says that this could be an adaptation from a time when food was often scarce and getting at it had as much to do with your cognitive ability as it did with how well you could throw a spear. “During times of starvation, the body preserves two organs and then shrinks the rest,” she explains—the preserved organs are the brain and, in men, the testicles. “Biologically, this is likely linked to the necessity of mental clarity to get out of starvation times or to survive long periods without food and to continue to grow the species.”

You might get an opportunity to practice mindfulness

Fasting is said to be a mental, physical, and spiritual reset,” says Virginia Beach-based dietitian Jim White. He explains that people who have fasted for three days often report that it causes them to face their bottled-up emotions so that they are more mentally stable after fasting is completed. “Additionally, those who fast learn to appreciate the little things that they may take for granted in everyday life, such as having a cold glass of water to drink or a bed to sleep in at night. By focusing on spiritual and mental connections during fasting instead of food and life inconveniences, mental clarity can be achieved.”

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This Is What Happens To Your Body During a 72 Hour Fast? ›

Aside from weight loss, additional 72 hour fast benefits include reducing oxidative stress and chronic inflammation, decreasing LDL (“bad”) cholesterol and triglycerides levels, promoting autophagy (cell cleanup) and cellular health, boosting the secretion of human growth hormone (HGH), improving blood sugar regulation ...

What happens to your body on a 3 day fast? ›

What To Expect When Fasting For 3 Days? Fasting can result in vitamin and mineral deficiencies, muscle breakdown, and diarrhea. Other Side effects of fasting include dizziness, headaches, low blood sugar, muscle aches, weakness, and fatigue.

What are the stages of a 72 hour fast? ›

The four stages of fasting.

Stage 1: Fed state (0-3 hours) Stage 2: Early fasting state (3-18 hours) Stage 3: Fasting state (18-48 hours) Stage 4: Long-term fasting state (48+ hours)

How much will I lose if I fast for 72 hours? ›

You'll lose weight

“As you release glycogen, you lose water and that is usually the reason for the rapid weight loss. Losing fat takes more time.” Fasting proponent Fung, however, disagrees and maintains that you could lose 1.5 pounds of fat over a 72-hour period.

What is the hardest day of a 3-day fast? ›

From what we've heard, Day 2 is usually the hardest on a 3-day fasting journey. This seems to ring true even during a longer 7-day or 14-day fast. So, we went to bed unsure of what was coming the next day. Below is our sleep data, showing a typical night's sleep for us.

What happens to your brain when you fast for 3 days? ›

Fasting can increase BDNF levels, which may lead to improved cognitive function, learning and memory. Reduced inflammation: Intermittent fasting can help decrease inflammation in the body, including the brain.

Will a 72 hour fast burn fat? ›

Here are some potential benefits of a 72-hour fast that forces your body into ketosis: Weight loss: When you're in ketosis, your body becomes more efficient at burning fat for energy, which can lead to weight loss.

Does fasting for 72 hours reset your immune system? ›

Now the question is, how long should you fast? Valter Longo, one of the world's leading authorities on fasting, recommends 48-72 hour fasting to enhance the immune system. With this longer fast, the body eats up the old immune cells and regenerates new immune cells.

What not to eat after a 72 hour water fast? ›

Foods and drinks that can be even more shocking to your system after a fast include those like a greasy cheeseburger, slice of cake, or soda. Even high-fiber raw produce, nuts, and seeds may be difficult to digest.

Is a 72 hour fast worth it? ›

However, one of the biggest advantages of fasting for 72 hours is the production of stem cells, immune cells, and reduction in oxidative stress. New cells are produced since autophagy helps clean out all the weak cells. It also improves brain function and prevents conditions such as Alzheimer's.

What should I eat during a 72 hour fast? ›

Additionally, people must consume adequate water to stay hydrated during the fast. This is considered water fasting. 72 hours of fasting can cause severe hunger pangs. People can consume zero-calorie drinks like black coffee and herbal teas to get through this.

What should I eat after 3 day water fast? ›

You can break your 3 day fast with bone broths or fresh fruit and vegetable juice made with no preservatives. If you want something to chew on, stick with simple raw fruits and vegetables for the first day after your fast then start introducing soft cooked foods and vegetables.

How much weight will I lose after a 3-day fast? ›

One 3-day fast is not going to have a significant impact on your weight. You may notice some changes on the scale, but don't mistake these changes for long-term weight loss. Any weight that you lose during a fast is mostly water weight (6). Once you start eating again, you quickly regain this weight.

Can I workout while fasting for 3 days? ›

Whether you fast for 24 hours or 24 days, you can still exercise. Your muscles may take up to 2 weeks to become fat adapted, though. During the first 2 weeks of fasting, you may need to take it a little easy, but you should quickly recover after that.

Can I take vitamins while fasting? ›

Intermittent fasting is a health and fitness trend, widely used to manage weight and generally promote health. “Taking Care/of vitamins, for example, won't break a fast. But if you're taking vitamins with additives, sugar, and carbs, then those vitamins will indeed break your fast.”

How much weight can you lose in 3 days of water fasting? ›


Water fasting helps you lose weight quickly. According to studies, you can lose 0.9 kg per day by drinking water in 24 to 72 hours. However, this loss of body mass is only from water, carbs and even muscle mass, not fat.

What happens if I don t eat for 3 days and only drink water? ›

Although water fasting may have some health benefits, it comes with many risks and dangers. For example, water fasting could make you prone to muscle loss, dehydration, blood pressure changes, and a variety of other health conditions.

What happens after not eating for 3 days? ›

After two or three days without food, your body starts to break down fatty tissue. Your muscles use the fatty acids created during this process as their main source of fuel. Fatty acids are also used to form ketones in the liver. Ketones are another substance the body can use for energy.

Will I lose muscle if I fast for 3 days? ›

Get enough protein daily, and you shouldn't lose muscle mass intermittent fasting. Further research has backed this up: subjects who underwent 70 days of alternate daily fasting (eating one day and fasting the next) lost NO muscle mass.

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