What Happens When You Stop Eating (Science-Based)
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What happens to your body if you stop eating for a day, a week, or months?
Find out what will happen step by step.
Do healing benefits come from fasting for an extended period of time?
Discover what the scientific evidence has to say about intermittent fasting, water fasting, and extended duration fasting.
It’s well known that when you eat your body begins a series of biological processes to digest and use the nutrients in food.
These processes include repairing muscle, creating new cells, and storing the extra fuel as fat or glycogen for later use.
However, it’s not common knowledge that when you stop eating your body enters a completely different series of biological processes.
These are just as necessary and beneficial as the biological processes that occur after eating.
Of course, if you don’t eat for long enough, the negative effects will start to outweigh the positive ones.
But there are definitely myths and misconceptions about what happens exactly to your body when you stop eating.
So, today I want to go over that process step by step based on the scientific evidence.
Is skipping meals unhealthy?
First, let’s address the conventional advice that it’s unhealthy to go for long periods without food.
I don’t mean days or weeks. I’m talking about simply skipping meals.
We’re told that we should eat small but frequent meals.
The truth, however, is that there’s nothing “natural” about eating five or six meals, spread out evenly throughout the day.
This is because, over the course of our evolution, we didn’t always have ready access to food. (1)
So, you could argue that a “fasting and feasting” eating cycle is more “natural” than eating breakfast, lunch, and dinner every day.
Because our ancestors had to cope with cycles of food shortage and food abundance, we’ve developed metabolic pathways that help us cope with prolonged fasting periods.
In fact, fasting for long periods can actually benefit your health, rather than hurt it.
But it definitely doesn’t feel that way. That’s because one of the first things that you’ll experience when you stop eating is unpleasant hunger.
Hunger and fat utilization
Now, the intensity of this hunger highly depends on how accustomed your body is to burning fat for fuel.
You see, most people regularly eat a high-carb diet. So, their bodies rely primarily on glucose for energy as opposed to fatty acids or ketone bodies.
Your body can store lots of fatty acids in the form of body fat which can be used for energy later on, when food is scarce.
Your body does so by converting the fatty acids into ketone bodies in the liver.
However, with glucose, it’s an entirely different story.
Your body can only store a limited amount of glucose in your muscles and liver for future use.
This amount gets depleted after fasting for only about 24 hours.
Yet, depending on how many carbs you eat regularly, it can take as little as 8 hours or up to 48 hours.
After that, your body will have to rely on a different energy source in the form of fat.
If your body isn’t effective at burning fat for fuel, this will likely lead to severe cravings, especially for high-carb foods.
Now, if you’ve trained your body to use fat for fuel (by, let’s say, following a low-carb diet, a keto diet, or through long-duration fasting), then you won’t experience the same severity of hunger/cravings.
In fact, many people that fast frequently don’t experience hunger at all.
This isn’t only because their body is used to burning fat for fuel, but it’s also because hunger levels increase at the times of the day that you normally eat.
Ghrelin – the hunger hormone
This is due to the hunger hormone ghrelin, which has the effect of stimulating your appetite when released.
During the first 24 hours that someone stops eating, studies show that ghrelin will spike automatically for most people around the times of the day that they typically have breakfast, lunch, and dinner. That spike will make you feel hungry. (2)
However, if you’re someone that’s already become accustomed to fasting every day, you won’t experience these sharp spikes in ghrelin during your 16-hour fasting window.
In other words, you won’t feel as hungry as someone who’s used to more meals per day. (2)
But even if you’re used to breakfast, lunch, and dinner, data shows that the spike in ghrelin is short-lived.
If you just ride out the hungry feeling that you get, within about two hours from the initial spike in ghrelin, it should drop back to normal levels, along with your appetite.
The biochemistry behind fasting
Regardless, after about the first 24 hours your body will burn your remaining glycogen stores and switch almost entirely to burning fat for fuel which will slowly lead to weight loss.
Your brain relies primarily on glucose to function. In its absence, your body will separate the triglycerides from your fat stores into glycerol and free fatty acids.
The free fatty acids can be used immediately for energy. Instead, the glycerol can be sent to the liver for conversion into glucose through a process known as gluconeogenesis.
This process can temporarily supply the brain with the glucose it needs, without breaking muscle down just yet.
Contrary to popular belief, your body will actually lower its protein breakdown rates substantially after just 24 hours of fasting to try to preserve muscle during these initial stages.
During this phase, you will start to feel cold for two reasons – lower body temperature from not digesting food, and lower thyroid function.
An extreme example… not to follow
As you fast for more and more days, the pounds of fat lost will start to add up substantially.
For example, there’s evidence from a study on a 27-year-old man that didn’t eat for longer than a full year.
He did this under the supervision of doctors, that had him living on multivitamins with yeast for 382 days.
He only drank water, coffee, tea, and diet soda the entire time. (3)
During that time his body weight went from 456 pounds to 180 pounds. (4) So, by not eating he was able to lose about 3/4 of a pound per day.
Even though this is a crazy achievement, for the average person, doing something like this is extremely risky.
And going even way less than 382 days without food can lead to death.
The man in this study was severely overweight, which is probably why he was able to survive by using all that excess fat for energy.
But it must also be acknowledged that this evidence comes from a single study on a single individual whose metabolic features and adaptability may well be unique.
In other words – don’t try this at home assuming that your fat stores alone will be able to pick up the slack of the entirety of your whole body’s metabolic demands.
Gluconeogenesis and muscle mass
Indeed, for most people after just a few days of not eating, you won’t only be losing body fat, but you’ll also lose a lot of muscle mass, especially if you’re already relatively lean.
That’s because if you don’t have a lot of fat, your body is more likely to convert protein into glucose through that same process as before, known as “gluconeogenesis.”
Muscle loss shouldn’t be a big issue until about 72 hours after you stop eating.
That’s because, during those first few days, your body will be able to get the energy and glucose it needs from body fat and glycogen.
Your body will also ramp up your human growth hormone levels as an anti-starvation response (5).
The growth hormone will assist with the release of fat from fat cells and it’ll help stimulate fat oxidation in general.
This also helps spare amino acids and preserve muscle mass during the first couple of days that you’re not eating.
Of course, muscle loss can start happening even sooner than 3 days. This will depend on your own body’s efficiency to burn fat for energy, or if you’re already really lean.
The longer you go past 3 days without food the more your muscle loss will accelerate.
For example, when a slightly overweight man only had water for 44 days, by the end of his fast he reduced his weight by 25%.
This would’ve been an amazing outcome if at least 2/3 of that weight wasn’t muscle loss. (6)
Fasting – days versus weeks
Now, going for some time without food not only impacts your body weight and physical appearance, but food deprivation will also set off a unique series of metabolic events.
For example, after the first couple of days, your body will activate a sort of cellular clean-up process known as autophagy.
Autophagy is like a recycling process where your body gets rid of the dead and dysfunctional cells and turns that waste into materials that can be used for growth and repair.
This helps reduce inflammation throughout the body which in turn helps fight many diseases. (7)
These benefits will also extend to your skin cells, potentially slowing down wrinkles, age spots, and acne.
Your immune system will also improve thanks to autophagy. Your body will repair and produce brand new white blood cells which will be stronger at fighting diseases.
Unfortunately, these positive effects on your immune system will come crashing down when you go longer than just a few days without food.
This is because not eating for very prolonged periods will slowly start to lead to malnutrition.
Your body won’t have access to the vitamins and minerals that it needs to maintain a strong immune system.
This is why research shows that starvation increases the risk of infections. (8)
For example, the same man from before that lost mostly muscle by fasting for 44 days straight, developed several nutrient deficiencies, including a deficiency in thiamine, riboflavin, and vitamin K.
So, a few days without food and your immune system is likely to get a boost.
But go for a few weeks without food and you’ll be much more susceptible to diseases and infections.
Fasting consequences – long- and short-term
As further time passes, your body will continue breaking down the last of its muscle, fat, glucose, and even bones, until it’s forced to start slowing or shutting down organs to save energy.
At this point, many of the opposite effects of autophagy will kick in.
For example, you can lose hair, develop brittle nails, have frequent infections, and have trouble healing wounds.
Your brain will also go through a roller coaster ride during this process.
When you fast for just a few days there will be a rise in neurogenesis rates in the brain. This is the growth and development of new brain cells and nerve tissues.
Higher rates of neurogenesis have been linked to increased brain performance, and improved memory, mood, and focus. (9)
While fasting you’ll begin producing more protein in your brain and one of these important proteins is known as BDNF which has been referred to as “miracle grow for your brain.”
This protein assists with the production of new brain cells and even helps protect your current brain cells while improving memory and learning as well.
Unfortunately, just like the other things I mentioned, the longer you fast the more these positive effects start turning into negative effects.
If you don’t eat for long enough past the point where your body has already broken down significant amounts of fat and muscle, it’ll become progressively harder for your brain to function properly.
Remember your brain runs on glucose. Without eating, converting protein and fat into glucose will only get you so far.
If you go long enough without food, eventually, your brain will even metabolize its own grey matter, literally shrinking the size of the brain to stay alive.
Your body will do the same with your heart and other organs as well.
Fasting – for how long is it healthy?
Ultimately, if you still don’t eat over time, you will die most likely from organ failure.
Exactly how long this will take is unknown, because it’s unethical to study starving populations.
Although some sources estimate that you won’t last longer than 2 months without food, it can take more or less time depending on how efficiently your body can utilize fat stores.
So, overall, not eating for a short period ranging anywhere from 16 to 72 hours can be very beneficial for fat loss, productivity, immunity, and your brain.
But when you don’t eat for longer than 3 days, you’ll start to experience negative effects, like muscle loss.
As you go for weeks or months without food, that’s when everything – organs, muscles, bones, brain, immunity – will go rapidly downhill.
That about wraps it up. I really hope you guys were able to learn something new from this article.
I’ve been helping my clients set up intermittent fasting plans for over a decade.
I have found a lot of success with intermittent fasting, not only for my clients but also for myself.
So, if you’re looking for some help on how to set up your intermittent fasting diet plan effectively to burn fat, my team and I can definitely help.
We’ll give you a customized intermittent fasting diet plan based on a schedule and meals that are comfortable for you.
You’ll also get a recipe book that’ll go hand in hand with your meal plan, workouts (3 days a week), and an accountability coach to answer any questions you have as you transform your body.
To find out more you can click the link below.
- Hunter-gatherer anthropoids, including those living today, often eat intermittently depending upon food availability.
- Ghrelin (the hunger hormone) automatically spikes during the times of the day that you’re used to eating. If you ignore the feeling of hunger, within about 2 hours, it should subside or at least dull the hunger.
- A 27-year-old male once fasted for more than a year. Under supervision, he lived on multivitamins with yeast for 382 days. He only drank water, coffee, tea, and diet soda.
- The result? He dropped his bodyweight from 456 pounds to 180 pounds in the process.
- Your body will also ramp up your human growth hormone levels as an anti-starvation response.
- In one study, a slightly overweight but otherwise healthy man drank only water for 44 days. “At the end of the fast, body mass had decreased by 25.5%, of which a quarter to a third was due to loss of fat and the remainder to fat-free mass, predominantly muscle.”
- RIF attenuates inflammatory status of the body by suppressing proinflammatory cytokine expression
- Starvation increases the risk of infections, particularly malaria.
- “Fasting improves cognition, stalls age-related cognitive decline, usually slows neurodegeneration, reduces brain damage.”
My passion for fitness began when I was 14 years old. I naturally fell in love with training and haven’t stopped since. At 18 years I acquired my first personal training certification from ACE after which I opened my first of 3 transformation studios in 2011. I love to share my knowledge through personal training, my online courses, and youtube channel now with over 3,000,000 subscribers! I can happily say that we've helped over 15,000 people get in great shape over the years. I'm always here for my customers so if you need help don't hesitate to send your questions to email@example.com