The Different Stages of Fasting: What to Expect


Fasting can have many powerful effects on your body. More than just helping you lose weight, research shows that if you fast, even on a sub-regular basis, you can unlock a vast number of physical and mental health benefits. Intermittent fasting can promote mental clarity and mood [1], improve immune function and cellular repair [2], and increase muscle growth [3]. These are only some of the benefits associated with fasting. More of the benefits will be covered in tile 3.

Fasting comes in many forms, such as prolonged fasting or different options of intermittent fasting. Each method comes with its own set of benefits, depending on the nature and the length of your fast. To understand how the benefits of each fasting method get unlocked, we’ll need to dive into the various stages our bodies go through, hour by hour when we are fasting.

Stage 0 or Pre-fasting State (0-8 Hours): Anabolic Growth Phase

Though we’re not actually fasting in this stage, it is still worth understanding what is happening in your body in the pre-fasting state. 

The first four hours after you eat are known as the anabolic growth phase. In this window, your body uses the food you just ate as fuel to power your current activity and for cellular tissue growth. This is what happens in your body every day after consuming a meal. The exact duration of this window depends on what and how much you ate in your last meal.

The meal you consume before fasting can also affect your fast as well. Veteran fasters report that they experience higher levels of hunger, and even withdrawal symptoms after eating a lot of sugary foods before entering into a fast.

Stage 0 Continued: Food Digestion and Insulin Peak

The blood sugar level in your blood tends to increase after eating a meal, particularly a meal high in carbohydrates. This state of elevated blood sugar can last up to four hours. As a response to this, your pancreas produces a hormone called insulin, which allows your body to remove the glucose that was released into your bloodstream after your meal, and convert it into glycogen.

The dreaded ‘sugar crash’ happens when your blood sugar rises rapidly, which causes a spike of insulin, which in turn leads to a sudden fall in blood glucose levels.

The higher the glycemic index of your meal the greater your sugar crash will be. The glycemic index is a value given to carbohydrate-containing foods that represent their ability to increase blood sugar (glucose in the blood). This will make you feel hungrier if your last meal had a higher sugar content compared to a nutritionally well-balanced meal.

4 to 8 hours after your last meal, your blood sugar levels start to drop and will steadily return to its normal value, bringing us to the first stage of fasting.

Stage 1 (8-12 Hours): Falling Blood Sugar Levels

Your body begins to enter the fasted state about 8 hours after consuming your last meal. This stage is characterized by a lowering of your blood sugar levels. For many people, this period can feel a bit uncomfortable. This is why the general practice is to start a fast before bedtime as it allows you to sleep through at least a few hours of this discomfort.

After around eight hours, your blood glucose begins to dip and you may experience hunger, fatigue, food cravings, and trouble concentrating. These symptoms usually subside very quickly. 

Between 8 and 12 hours after your last meal, your body will start to tap into your glycogen stores (sugar stores), and your blood glucose levels will stabilize [4]. 

When your glycogen stores are depleted, your body needs to turn to an alternative source of fuel – enter ketosis, which is a metabolic state that allows your body to burn fat for fuel instead of sugars or glycogen.

In ketosis, your body produces ketones from fatty acids in the liver. Ketones can fuel the vast majority of the cells in your body. However, for the few other types of cells that need glucose, your body will synthesize it from protein a natural process called gluconeogenesis.

Stage 1 continued: Stable Blood Sugar Levels

At around the 12-hour mark after a meal, the first real benefit of fasting begins. At this stage in your fast, you will enter into the early stages of ketosis, where your body stops relying on carbohydrates for fuels and switches to burning your body fat stores instead [5]. At this point you’re already beginning to lose weight from burning fat.

During this period, your body is fueled by both metabolic pathways – Ketosis and Glycolysis. Glycolysis is the pathway the body uses to convert glucose from food to energy it can use. 

The time it takes to reach this stage varies a lot depending on the last meal you ate before your fast. If you ate a lot of carbohydrates (sugars or starches) it will take longer than if you ate mainly fats and protein.

As a side note, short-term fasting (8-12 hours of fasting) may also lower blood pressure and increase insulin sensitivity, making it especially useful for people with type 2 diabetes [6].

Stage 2 (12-18 Hours): Partial to Full Ketosis

Depending on your last meal, you should be in full ketosis after about 16 to 18 hours of fasting [4]. If you had a high-carb meal, this stage might take a little longer to achieve. In Stage 2 of fasting, you start to switch into fat-burning mode. Intermittent fasting for 16-18 hours a day allows you to burn through your body fat which makes it easy to stay in a calorie deficit and lose weight.

Entering a state of full ketosis means that your body primarily utilizes fats for energy. Your liver starts to convert some of this fat into ketone bodies or ketones.

You can think of ketones as an alternative fuel source produced from fatty acids by the liver. Ketones are only released after glycogen stores are depleted.

As a result, insulin levels begin to fall, which according to several studies, could be very beneficial for a list of patients that struggle with chronic diseases.

High insulin levels are a significant risk factor for chronic diseases. Low insulin levels are therefore favourable for preventing medical conditions such as obesity, metabolic syndrome, type 2 diabetes, cardiovascular disease, cancer, Alzheimer’s disease, and Parkinson’s disease [7]-[10].

Stage 2 Continued: Benefits of Ketosis and BDNF

One of the known effects of ketones is that they suppress appetite [11]. In addition to leading to lower levels of ghrelin (the hunger hormone) and stable insulin levels, being in a state of ketosis can lessen food cravings. This means that fasting can actually get easier the longer you fast.

Ketosis has another often overlooked benefit that kicks in at this stage of fasting: increased mental clarity. In a state of ketosis, many people report feel higher alertness and increased mental acuity.

One explanation for this is thought to be a boost in brain-derived neurotrophic factor (BDNF) – a protein produced in your brain to protect existing brain cells. According to medical professionals, BDNF also supports the growth of brain neurons and enhances long-term memory, learning [12], and boosts mood [13].

Intermittent fasting practitioners recommend fasting for at least 16 hours to reap these benefits. Note, 16 hours is just a guide – everyone is different and it could take you more or less time to see these benefits.

Stage 3 (24-54 hours): Autophagy, growth hormone, reduced insulin levels

After about 24 hours into your fast, autophagy starts to occur, and continues throughout the fast.

At around the 36 hour mark, the body goes into a long-term fasting state. As you undertake multiple days of fasting, your growth hormone levels begin to rise significantly, which stimulates faster than normal muscle repair and helps preserve lean muscle mass. Studies show that human growth hormone could also speed up the healing process for wounds and other injuries [14].

After fasting for 54 hours, your insulin levels will be at their lowest. This is especially beneficial for persons struggling with diabetes or people who have been diagnosed with pre-diabetes.

Prolonged high levels of insulin can cause insulin resistance, which could lead to diabetes. In contrast, increased insulin sensitivity could lead to a healthier body fat percentage, reduce inflammation and further activate autophagy. Along with autophagy, fat loss and mental clarity continue to increase during this stage.

Stage 4 (72+ hours): Stem Cells and Immune Function

At stage 4 you will have been fasting for 3 days. Research shows that in this fasting stage, you may unlock a further array of health benefits, such as stem cell generation and immune cell development. This is caused by a reduction in insulin-like growth factor (IGF-1), a hormone involved in growth and development, will decrease [15]. 

However, a 3-day fast is a serious undertaking, fasts of this duration require adequate preparation. Staying hydrated is key as is consuming supplemental electrolytes like sodium, magnesium, and potassium to prevent possible complications during a fast of this length, such as light-headedness, dizziness, and excessive fatigue. As always, you should consult your healthcare professional before embarking on a fast of this length.

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