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After a certain point, it's not healthy.
By Aly Walansky — Written on Mar 04, 2019
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. But how long can you survive without food?
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.
"The team at LifeOmic went on a 120-hour fast with the Prolon 'fasting-mimicking' diet that restricts calories and elevates ketone levels. The Prolon fasting-mimicking diet has been used in dozens of human research studies and clinical trials, including some recruiting and active trials.
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 the 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.
When you're fasting, how long can you survive without eating anything? It depends on a few factors, it seems.
1. It's a matter of 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. Evolution made humans store energy in fat deposits to be used for those times without much food,” says board certified cardiologist and weight management specialist, Dr. Luiza Petre.
2. Our body has reserve.
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. But you should be aware of the stages and signs.
It's not just the 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 point 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 while you are fasting. 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 is more dangerous than 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, I would say that the limit of complete fasting safety is around 72 hours.”
5. You can begin to enter starvation mode.
“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 advises.
6. It's a downward spiral from there.
Yes, one can still live for 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 lifestyles writer. Her work appears in dozens of digital and print publications regularly. Visit her on Twitter or email her at alywalansky@gmail.com.