Photo: axelbueckert via Canva
After a certain point, it's not healthy.
By Aly Walansky — Last updated on May 29, 2023
There are a lot of reasons people may not eat for a certain amount of time. They may be ill and not able to digest food. They may be on a fast or some sort of cleanse. Or, they may be starving and have no food available.
Regardless of circumstances, not eating for periods of time is essentially fasting. And while the human body can survive for about three days without water, what about going without food?
How long can you survive without food?
You can go without food for approximately 1-2 months. This answer is linked to a 2009 German study that determined that people can survive without food and drink for about 8-21 days, but just going without food could extend that to around 1-2 months.
According to Don Brown, MD, founder and CEO of LifeOmic, "Water fasts (when you are not consuming any calories) up to 24 to 36 hours in duration are generally safe and well tolerated based on clinical studies."
There are options for fasting safely beyond the recommended 36-hour maximum, though.
"From a weight loss and maintenance perspective, 24-hour water fasting on a regular basis can be difficult to stick to and adopt as a long-term health practice, which is why Prolon and other fasting protocols used in human studies and clinical trials call for the consumption of some calories. Thus, when practicing a multi-day fast, you can plan to take in some limited amount of calories, especially if this is your first fast beyond 18 to 24 hours," he advises.
But your body goes through some interesting changes when fasting.
After the first day without food
Your body uses glucose as its main energy source, so when you don't eat, your body begins to use your glucose reserve, which will be gone after one day.
This means your body will release a hormone called glucagon, which will tell your liver to make glucose. This source will only be used to "feed" your brain.
After the first 2-3 days without food
After going 2 or 3 days without food, your body will begin to break down your fatty tissue as another source of energy for the body.
Your fatty acids can become broken down into ketones in your liver. Ketones are then released into your bloodstream to help feed your brain. What your body is trying to do is keep your brain aware and alert.
After one week without food
After about a week without food, your fatty acids will be used up. This means your body is going to look for another source of energy that will be protein. After a week of not eating, your body will start breaking down muscle to get that protein.
After the first two weeks without food
Once you hit day 14 of not eating food, your muscles will have been depleted enough for you to start losing function in your heart, liver, and kidneys. If you are not careful, this is what will ultimately lead to your death.
After the first month
If you have survived one month without food — which, if you are hydrating properly you could do, but it is strongly unadvised.
At the one-month mark, your body will have shed most if not all of your fat reserves causing you to have lost a lot of weight. You could also suffer from horrible exhaustion, hallucinations, organ failure, and mood swings.
Be careful once you start eating again, though. Once you start to bring nutrients back into your life, your body could go into shock and overproduce insulin.
What happens when you go without food?
Once you stop providing nutrients for your body, it is only a matter of time before you succumb to death. Our bodies weren't made to fast for longer than 36 hours (if that). When you do start to starve, your metabolism slows in an effort to keep as many nutrients in your body as possible.
Our body will also stop being able to regulate its temperature. You will lose the functioning of your kidney, liver, and heart. Your immune system will be weakened, making you vulnerable to bacteria and viruses.
In many women, if they lose enough weight and nutrients, they could also lose their menstrual cycle.
Other health issues include:
- Bone loss
- Muscle weakness
- Feeling cold
- Thinning or loss of hair
- Dry skin
- Constipation
- Shortness of breath
How long can you survive without eating when you're fasting?
It depends on a few factors, it seems.
1. Evolution
“In theory, our body is made to survive for long periods without food, as food was scarce and obtained sporadically during the Stone Age," says board-certified cardiologist and weight management specialist, Dr. Luiza Petre. "Evolution made humans store energy in fat deposits to be used for those times without much food.”
2. Your body's reserves
Our liver alone has sugar storage to hold us for two to three days, called glycogen. “This is one of the reasons why entering ketosis, or fat burning mode, takes three days,” reveals Dr. Petre.
3. How well you track the stages of fasting
It's not just hunger that can be difficult when embarking on a multi-day fast; monitoring your time between eating is critical for understanding and tracking your various stages of fasting.
For example, Dr. Brown says that by 12 hours, you’ve entered the metabolic state called ketosis. In this state, your body starts to break down and burn fat. By 18 hours, you’ve switched to fat-burning mode and are generating significant ketones. Within 24 hours, your cells are increasingly recycling old components and breaking down misfolded proteins linked to Alzheimer’s and other diseases. This is a process called autophagy.
By 48 hours, without calories or with very few calories, carbs or protein, your growth hormone level (which helps build and repair tissue in the brain and other organs) is up to five times as high as when you started your fast. By 58 hours, your insulin has dropped to its lowest level since you started fasting and your body is becoming increasingly insulin-sensitive (which helps with inflammation and can help protect you from chronic diseases).
"It’s best for your metabolic health to stick to a fasting regimen you can easily maintain over time, with the caveat that you should stop fasting and see a physician if you experience light-headedness or significant discomfort, or if you are at risk of becoming underweight. You should also ensure that you are getting the recommended daily amounts of electrolytes. Intermittent fasting is not the only way to improve your metabolic health; it’s important to find the meal composition and timing plan that works best for you long term," Dr. Brown warns.
4. Dehydration and starvation
But when it comes to the safety of lengthy fasting, there are other aspects to consider, namely fluid intake and electrolytes.
“Dehydration and imbalance of electrolytes like magnesium, potassium and sodium should be kept in check on a daily basis,” suggests Dr. Petre. “If one takes in enough fluid and replenishes electrolytes by mouth, the limit of complete fasting safety is around 72 hours.”
“Beyond that, the metabolism starts to enter starvation mode, slowing down energy consumption for non-vital functions. Immunity, reproduction ability and hormone production will lower,” Dr. Petre continues.
Yes, one can still live for a longer time without any food, but that is dangerous and can spiral into something worse.
According to Dr. Petre, “Without any fluids, dehydration can become severe after only two days, and the kidneys shut down. Electrolyte deficiencies can also kick in after 2 days, leading to cramps and significant cardiac rhythm disturbances that can cause sudden death.
In all intermittent fasting protocols, fluid intake and replenishing electrolytes are the key to staying healthy and safe.
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Aly Walansky is a NY-based lifestyle writer who focuses on health, wellness, and relationships. Her work appears in dozens of digital and print publications regularly.