Understanding the Stages of Fasting: What to Expect at Each Phase

If you have ever used apps to track your fasting windows, you have likely come across fasting zones. Once you start your timer, the app provides insights into what is supposed to happen in your body, whether digestion, fat burning, or ketosis.

When I tried fasting myself, I was somewhat perplexed by the stages of fasting displayed in these apps. What does it even mean to be in an ‘early fasting zone?’ What are the benefits of fasting longer and reaching ketosis? With these questions, I decided to do some research of my own.

I realized one important thing: you can't know your exact stage by looking at the clock. Our bodies can indeed go through a few stages of fasting, although the apps I have tried either exaggerate the details or present a vague understanding of fasting.

Based on current scientific knowledge, this article will explore fasting further, focusing on the potential stages. I aimed to gather as much of the important details as possible, accompanied with expert insights, into one place.

What is prolonged fasting?

Generally speaking, a prolonged fast is a fast that lasts longer than an intermittent fast. This is where it gets tricky as definitions may vary depending on the literature.

For instance, some academic literature describes any fast that lasts shorter than 36 hours as intermittent fasting. However, the most popular fasting practices are considered those where people fast for around 16 hours.

A prolonged fast is typically considered to last at least five days, but that also may vary depending on the source you are reading. Many researchers would consider any fast that lasts 48 hours or more to be prolonged.

During a prolonged fast, the body undergoes a metabolic switch, gradually transitioning from using carbohydrates (glucose) to relying mostly on fats and ketone bodies for energy because of low insulin levels and depleted glycogen stores.

Potential health risks and considerations of prolonged fasting

Although fasting is often associated with fitness and even the bodybuilding community, prolonged fasting tends to lead to greater loss of lean mass compared to body fat. As one study indicates, weight loss achieved usually consists of around two-thirds lean muscle mass and one-third fat mass.

When it comes to weight loss attained by other means, e.g., caloric restriction and increased activity levels, the proportion of fat versus lean mass loss tends to be more favorable, meaning more lean mass is preserved. Time-restricted eating with fasting windows of 12–20 hours every day may also pose less risk of losing lean mass compared to prolonged fasting.

Although to some degree all weight loss also causes loss in non-fat mass, it is important to understand that muscle atrophy is not a desirable side effect. Loss of muscle can lower your resting metabolic rate, making it hard to maintain the results. Moreover, lower muscle mass may be associated with increased risk of insulin resistance — the exact opposite of what many people aim to achieve through fasting.

Besides that, extended fasts are advised to be carried out under medical supervision, ideally in a clinical setting. This is because it can pose a risk of nutrient deficiencies.

In general, malnourished people should avoid fasting due to an increased risk of refeeding syndrome. The risk of developing this potentially fatal condition is also increased in those who have a history of alcohol misuse.

Other groups that are not suited for fasting include pregnant and breastfeeding individuals, people with eating disorders, children, and older adults. This applies to intermittent fasting as well.

Fasting and female hormones

For this section, we have asked Nicole Bendayan, a certified nutritionist focused on female health, to give her opinion on fasting. When asked whether fasting can affect female hormones, she replied affirmatively.

Fasting may affect female hormones, and the way it’s typically promoted doesn’t work for some women. For women with a natural cycle, fasting can add extra stress to the body, which can throw hormones off balance. Potentially leading to things like irregular cycles, worsened PMS symptoms, low progesterone, and even missing periods (hypothalamic amenorrhea).

Nicole Bendayan

Are the potential benefits of prolonged fasting scientifically proven?

Some of the benefits associated with fasting are supported by scientific evidence, though ongoing research continues to explore its long-term effects and mechanisms.

Fasting for weight management

Fasting, including short-term or intermittent fasting, can be part of a weight management approach. Studies point out that fasting may lead to body weight reduction, however, it is unclear how much of it comes from general calorie restriction. Its long-term effectiveness compared to traditional methods is still being studied.

Fasting for overall health

Research studies indicate that fasting may support some physiological indicators related to health, including blood pressure levels, insulin sensitivity, body composition, and blood lipid profiles. Fasting is generally associated with benefits for managing metabolic syndrome (a cluster of risk factors for cardiovascular disease and type 2 diabetes).

However, some of these health benefits may happen due to a decrease in weight, or ketosis. For instance, potentially improved insulin resistance is often the goal for those who choose to fast, yet, some data indicates that this benefit may be achieved because of calorie restriction, not fasting itself.

This is not to say fasting is not beneficial — some people find it a great and easy strategy to follow in their weight management journey. However, some of the fasting-related benefits seem to be associated with fat loss or ketosis rather than fasting as a main mechanism.

Stages of fasting

Stage 0: pre-fasting state

Physiological processes: this stage happens after you eat. As your body absorbs nutrients from food, your blood sugar levels increase and insulin is secreted.

Insulin is needed to use sugar as energy as it facilitates the movement of glucose into cells. In other words, once insulin is secreted, it will start lowering blood sugar levels by promoting the glucose uptake in your cells.

Extra glucose that is not utilized for energy will be transferred to the liver or muscles as glycogen for later use.

Subjective experiences: during this state, you may feel satiated due to the interplay of hormones like ghrelin and leptin that are involved in controlling hunger signals. Some people may also feel drowsy after a meal.

Stage 1: early fasting state

Physiological processes: after eating, glucose and insulin levels begin to decline, and the body starts relying more on stored energy. The liver breaks down glycogen to maintain blood sugar levels.

This process can sustain energy needs for approximately up to 24 hours, depending on factors like activity level, metabolic rate, and overall dietary choices.

This stage lasts until you reach ketosis. If your preferred fasting strategy is intermittent, whether it is alternate day fasting or time restricted eating, you mainly stay in the early fasting state.

Fasting may potentially trigger the release of human growth hormone, which, in turn, may inhibit protein breakdown, thus, preventing muscle breakage.

Subjective experiences: now, you may start feeling less full. At some point, you will start getting hunger signals again as the levels of ghrelin hormone involved in appetite control gradually rise.

During this period, it may be hard to exercise for some people. If you are adapted to fasting or a low-carb diet, you may not see any negative effects on your workout performance.

Stage 2: fasting state (ketosis)

Physiological processes: after around 24 hours of fasting, your glycogen stores are mostly depleted, with not much to be converted back to glucose. This means your body will need to find other energy sources. This is when ketosis happens — instead of relying on glucose, your body uses ketone bodies.

Ketone bodies come from fatty acids, while glucose comes from carbohydrates.

This is why ketosis can happen in two different scenarios: either by fasting or eating a low-carb (ketogenic) diet.

Although 24 hours of fasting would most likely put most people into ketosis, ketosis can start to happen earlier or later as it highly depends on the individual metabolism, the last meal consumed, and overall diet. For instance, some people may reach ketosis after fasting for 16 hours, meaning that during intermittent fasting they would switch between the fed state, the early state, and ketosis. In general, it is considered that the switch between early fasting and ketosis happens after 12–36 hours.

At-home tests can show ketone levels in your urine and can help you determine whether you are in ketosis.

Subjective experiences: in this stage, you might start feeling less hungry as ketosis is associated with appetite suppression.

Some people may experience headaches, especially those who are new to fasting or transitioning into ketosis for the first time. This is often referred to as the ‘keto flu’ and can be accompanied by fatigue, brain fog, dizziness, and irritability.

Stage 3: prolonged fasting state

Physiological processes: during the prolonged fasting state, typically occurring after 36–72 hours of fasting, the body has fully transitioned to using ketones as its primary energy source.

Lipolysis, the breakdown of fat cells into free fatty acids and glycerol, accelerates. However, protein breakdown is also occurring, meaning you would experience muscle wastage during an extended fasting period.

Subjective experiences: some individuals may experience dehydration or electrolyte imbalances, which can contribute to dizziness, cramps, or heart palpitations. Staying hydrated and ensuring adequate electrolyte intake may help mitigate these side effects.

You may also have the aforementioned symptoms of the ‘keto flu’.

Managing fasting-related concerns

Fasting, especially prolonged, may lead to suboptimal electrolyte levels. For instance, for the first four to five days, a fasting person is expected to lose around 40–45 mEq of potassium per day (1,560–1,755 mg), while sodium loss is the highest in the first 24 hours, standing at 50–250 mEq (1,150–5,750 mg) per day. Conversely, some side effects such as headaches may be relieved by introducing electrolytes while fasting.

Electrolytes themselves will not break your fast, but make sure you are choosing a product that does not have other ingredients that can cause a spike in blood glucose levels. For instance, electrolyte drinks are often made to suit the needs of long-distance runners and, thus, contain glucose since it can help retain endurance while working out. However, glucose would break your fasting period.

Breaking a fast

Another thing that you should be keeping in mind if you choose to try fasting is a refeeding syndrome. This condition may happen when food is introduced too fast, and is related to extreme shifts occurring in one's body. Changes in electrolytes can play a role in refeeding syndrome, but other deficiencies may be involved as well, including vitamins and minerals.

For this reason, people involved in extended fasting (more than five days) are advised to do that in clinical settings. The medical team supervising the fast is equipped to help end it or manage complications that may arise.

Moreover, the refeeding syndrome is linked to malnutrition in general and some people are at a higher risk, including those with low BMIs or experiencing anorexia nervosa. For people with these conditions, it is advised to avoid fasting altogether.

Final thoughts

Fasting is a complex process that can lead to various changes in the body, including ketosis, and contribute to potential health benefits such as improved insulin sensitivity and lipid profiles, as well as support to weight loss.

However, it also comes with certain risks, such as loss of muscle tissue, potential nutrient deficiencies, and electrolyte imbalances, especially if not managed properly.

There are a few stages of fasting as your body goes through different physiological states, from the pre-fasted state to the early fasting state, ketosis, and finally prolonged (sustained) fasting. Each stage involves distinct metabolic processes and subjective experiences, such as changes in hunger, energy levels, and potential side effects like headaches or fatigue.

While some people may prefer fasting, similar metabolic effects such as weight loss or insulin sensitivity may be supported by calorie restriction or low-carb diets.

Long term fasting, especially if it lasts for more than 5 days, should be supervised by a healthcare professional.