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What Happens To Your Body During a Prolonged Fast?

by | Jan 28, 2021 | EATING/FASTING

You have seen the benefits of shorter periods of fasting, so you have decided to experiment with a longer fast. Before you embark on a longer fast, it is important to have clear expectations of what might happen so you can be prepared and set yourself up for success.

So, what can you expect if you stretch out your fasting time to 24 hours or longer?

What is a Prolonged Fast?

People have been engaging in prolonged fasts lasting 24 hours or longer for centuries. Fasting is part of many religious traditions. The length and the rules of the fast depend on the religion, but some can last for several days.

A prolonged fast is anything longer than 24 hours. Most define a long fast as anywhere from 4 to 7 days.

Some experts believe the benefit of prolonged fasting may be more aligned with the eating pattern humans had for many centuries. Unlike our modern world, our ancestors did not have food readily available 24-hours a day and likely did fast for extended periods between meals. Alternating periods of fasting with eating may be better aligned with our ancient metabolism, and slow-to evolve DNA.[1]

What is consumed (or not) during your prolonged fast can depend on the type of fast. Some may include just water or calorie-free beverages, while others allow for fresh juices or smoothies. For simplicity, this article will focus on what happens to your body when you consume calorie-free liquids for an extended period of time. 

What Happens to Your Body During a Prolonged Fast?

Here is a breakdown of what happens to your body during each phase of the fast:

The First 24 hours

Within about 8-12 hours of not eating, the majority of the glucose in your bloodstream will have been used up. Glucose, which comes from foods high in carbohydrates, circulates freely in the blood and is stored in the muscles and liver in the form of glycogen for fast access. But, there are limited storage sites for glucose in the body, and it is used up fairly quickly.

Since glucose is the body’s primary source of energy, once the glucose is gone, the body has two options as it quickly burns through its glycogen reserves. It can either make its own glucose via a process called gluconeogenesis or rely on fat stores.  

The body does not like to rely heavily on gluconeogenesis. It is extremely costly calorie-wise and uses up protein that the body would prefer to use elsewhere. If needed, your body will make just enough new glucose to feed the organs that need it.[2] During the early phase of fasting, the body may rely more heavily on converting amino acids and protein from your muscles into glucose as it ramps up the process of making energy from fat.   

Without carbohydrates, the other organs are able to use ketone bodies, which are made from fat, for energy. When there is inadequate glucose available, the body will begin to produce ketone bodies. Once the body relies primarily on ketones for energy, this is a metabolic state called ketosis.[3]

Within this first 24-hour period, hunger hormones, like ghrelin, may spike and make you feel quite “hangry.” Ghrelin levels cycle throughout the day – they have their own circadian rhythm. Most people find they are hungrier in the early to late afternoon. If you can get through this period of the first day or two, you might find that your sensation of hunger really starts to decrease the following day.[4]

24 Hours and Beyond

Between 18 and 24 hours without food, cortisol starts to rise due to increased stress from not eating. At this time, some people experience an increased feeling of alertness and mental clarity due to this hormonal shift. When cortisol is high, ghrelin is decreased. You may start to not feel quite as hungry during this time.[5]

You may also start to feel cold, especially in your extremities. Fasting can reduce heart rate and body temperature, activating the parasympathetic or “rest and digest” nervous system.[6] Feeling cold may also be a sign of low blood sugar, so if you start to feel dizzy or lightheaded as well, consider stopping your fast. It may also be a sign of mild or moderate dehydration, as your body isn’t absorbing water from food (which can make up as much as 40% of your normal daily water intake) and may not have the customary amount of salt. This can also lead to feeling dizzy or lightheaded. 

Many report a feeling of euphoria or extreme mental clarity during extended fasts. This may be related to ketosis and a mood-boosting chemical called brain-derived neurotrophic factor (BDNF). The presence of ketones increases BDNF in the brain, which has anti-inflammatory effects on the neurons. This increase in BDNF improves memory and mood.[7]

Most of the potential benefits of prolonged fasting are found in the 1-3 day range. Fasts longer than that may start to have significant metabolic consequences, increased risk for nutrient deficiencies, dehydration, irritability, lack of energy, and eventual starvation.[8]

Benefits of a Prolonged Fast

In addition to the metabolic changes during a prolonged fast, you can expect a few other things as you start to extend your fast. Here are a few:

Weight Loss

When you first start your fast, you may see a rapid amount of weight loss, which is likely coming mostly from water. Glucose storage holds on to a lot of water, which is released as the stores get emptied. 

After 12 hours, as you begin to switch into ketosis, something interesting happens. The body starts to burn energy primarily from body fat. This means that some of the weight loss you see beyond this point is fat. This increase in fat burning may be why muscle mass is generally preserved during fasting. Your body does not have to dig into protein (muscle) for energy.[9]

Fasting can also increase metabolism due to spiking levels of the hormone norepinephrine. This may result in even faster weight loss beyond just calorie restriction during a prolonged fast.[10] One thing to note, although the body does burn more calories initially, at some point, it will slow down metabolism in order to prevent starvation.

Reduced Risk of Chronic Disease

Fasting may improve your health beyond just helping you lose weight. There is some evidence that occasional fasts of 24-hours or greater may decrease your risk of chronic disease. The process of autophagy, or cleaning out waste and cellular debris that occurs during fasting, may help reduce the risk of illnesses, like cancer, heart disease, and diabetes, as suggested by animal studies, with some additional support in smaller human trials.[11]   

Fasting may also be beneficial in the treatment of certain illnesses. A 2016 study found that a fasting-like diet helped improve certain cancer treatments’ success rate in mice. Although this is emerging research around fasting and cancer treatment, it is still a possible benefit that should be explored.[12]

Better Memory

The process of autophagy that occurs during fasting may also be beneficial to long-term brain health. Due to the metabolic and hormonal shifts during fasting, people tend to experience more mental clarity and improved focus.

But, fasting may help long-term brain health as well. This cognitive benefit of fasting goes back to the process of autophagy or the clean-up of cellular debris. Brain cells are also cleaned up at this time. Autophagy may help prevent the plaques in the brain that are characteristic of degenerative cognitive illnesses. Fasting has been proposed as a potential treatment or possible prevention intervention for Alzheimer’s and other similar conditions.[13]

Improved Digestion

The short period of “rest” during a prolonged fast may help improve digestion and help the digestive system naturally “clean up” problematic cells and detoxify. The microbes in the gut may also appreciate periods of rest. Composition of the gut microbiome has been found to improve during periods of fasting.[14]

Improved Insulin Sensitivity

When the cells in the body lose sensitivity to insulin, known as “insulin resistance,” this leads to type 2 diabetes. Periods of fasting may help improve insulin sensitivity and lower the risk of developing diabetes.[15]

A 2005 study found that a 7-day fast helped improve insulin resistance, induced weight loss, and reduced inflammation in healthy men. Although this was a small study, it suggests that periods of prolonged fasting may help decrease the risk of diabetes.[16]

Possible Increased Longevity

Prolonged fasting and calorie restriction may help increase longevity. This is partially due to lower levels of inflammation and a lower risk of chronic disease, but it goes beyond that. 

Possible Dangers of Prolonged Fasting

Although there are many health benefits to fasting for longer stretches, some people still need to be cautious. People who should not fast for an extended period of time include:

  •  Children and teens
  • People with diabetes or hypoglycemia
  • Those with a history of eating disorders
  • Women who are pregnant or breastfeeding
  • Those who need to take medication with food
  • People with significant cardiovascular risks or cancer

If you want to know if fasting is right for you, always speak with your doctor before starting an extended fast.

Tips for a Successful Long Fast

If you are embarking on a longer fast, you can do a few things to help increase your chance of success.

The first step is to decide how long you want to fast. If you have never fasted before, it is best to start slowly. Practice with a 16/8 protocol, to begin with, then slowly extend your fasting period a few days a week until you are able to go for 24 hours. Once you can fast for 24-hours, you can then begin experimenting with longer periods, if desired.

The biggest hurdle you might face during a prolonged fast is staying hydrated and getting enough electrolytes. You may not realize it, but most of us get a lot of our daily hydration from food, so you will have to replace that water with other beverages.

Consider drinking calorie-free electrolyte-enhanced waters during this time; these can help you stay hydrated and maintain electrolyte balance. Coffee, tea, and no-calorie beverages can help as well. 

You may find yourself being hungry or having food cravings during a fast. This is normal. Your body wants you to eat! Be sure to drink plenty of liquids to help keep your stomach full. Also, try to keep yourself busy during fasting days; this can help keep your mind off food.

If you are going more than 24-hours without eating, try to get plenty of rest. It is best not to participate in intense exercise as this can significantly increase stress hormones, inflammation, and hunger. A light walk is fine, but don’t do a strenuous workout during this time.

When you are ready to break your fast, although you might be tempted to dive back into eating, it is best to start slow. Eating too much right-away can make you feel sick and cause digestive problems. We have a few ideas for ways to break your fast here.

You also want to replenish the nutrients you missed out on while you weren’t eating. Ideally, you want something nourishing that is also easy to digest. A light smoothie or a broth-based vegetable soup is a great choice. Wait an hour or two after this initial meal before diving into something heartier. 

No matter how much “science” there is to support the benefits of extended fasting, the most important thing is for you to listen to your body. If you start to feel overly exhausted, lightheaded, or dizzy, you can break your fast. There is no reason to push yourself to limits where you don’t feel your absolute best; remember fasting is supposed to help you feel better, not worse!

Read More:

[1] Evolution of human feeding behavior

[2] Gluconeogenesis and energy expenditure

[3] Liver and kidney metabolism during starvation

[4] Alternate day fasting in non-obese subjects

[5] Central effects of stress hormones in health and disease

[6] Intermittent fasting

[7] Flipping the metabolic switch: Understanding the metabolic and health benefits of fasting

[8] Potential benefits and harms of intermittent fasting

[9] Progressive alterations in lipid and glucose metabolism

[10] Resting energy expenditure in short-term starvation is increased as a result of norepinephrine

[11] Alternate day fasting and chronic disease prevention

[12] Fasting mimicking diet promotes T-cell mediated tumor cytotoxicity

[13] Short-term fasting induces profound neuronal autophagy

[14] Intermittent fasting and gut microbiota

[15] Effects of intermittent fasting on weight loss and metabolic disease risk markers

[16] Effect of intermittent fasting and refeeding on insulin action in healthy men

[17] Mitochondrial sirtuins and metabolic homeostasis

[18] Mitochondria and organismal longevity