Weight loss is often the primary goal for individuals exploring fasting, and it’s indeed one of the noticeable outcomes. However, the mechanism of weight loss during fasting is frequently misunderstood. Let’s clarify this process before delving into the intricate details of what happens in your body when you fast.
Much of the initial weight reduction during a short fast is water loss.
This is fundamentally linked to carbohydrate consumption and water retention in the body. When you consume carbohydrates – found in bread, pasta, baked goods, rice, fruits, and even vegetables – your body stores this energy as glycogen.
Glycogen is stored along with water. Early in a fast, as your body breaks down glycogen for energy, the associated water is released into your bloodstream.
This leads to increased urination and a rapid decrease on the scale – sometimes as much as 1 to 2 pounds daily. However, this initial drop is primarily water weight, not fat loss.
Actual fat loss during fasting is typically around half a pound per day for most individuals. If you’re experiencing more significant weight loss, it’s likely water.
To fully understand the fat-burning process and the broader health benefits of fasting, let’s explore a detailed, hour-by-hour breakdown of what occurs in your body during a fast. This timeline will illustrate when significant fat loss begins and when the numerous longevity benefits start to become prominent.
Understanding the Fasting Timeline Hour by Hour
0 – 4 Hours: The Anabolic Phase (Growth Phase)
In the first 0 to 4 hours post-meal, your body is in Phase 1, known as the anabolic phase. This is essentially a growth phase. Your body utilizes the energy from your recent meal for immediate needs and for cellular and tissue repair and growth. During this phase, your pancreas releases insulin. Insulin’s role is to facilitate glucose uptake from your bloodstream into cells for energy or storage. Excess glucose is stored as glycogen in muscles and the liver.
4 – 16 Hours: The Catabolic Phase (Breakdown Phase)
From 4 to 16 hours, you transition into Phase 2: the catabolic phase.
This phase marks a shift to a breakdown state, where your body starts utilizing stored nutrients. Initially, your glycogen stores are broken down into glucose and used as the primary energy source.
As glycogen stores deplete, typically towards the end of this 16-hour window, your body begins to switch its fuel source to stored fat and ketone bodies.
Therefore, if you practice intermittent fasting with a common 8-hour eating window followed by a 16-hour fast, you primarily cycle within these initial two phases throughout your fast.
The time it takes to reach the fat-burning stage at the end of Phase 2 is influenced by your dietary composition. A diet higher in carbohydrates and starches will prolong the time needed to deplete glycogen stores and enter the fat-burning phase.
This is why it’s often recommended to reduce your intake of carbohydrates and starches in the days leading up to starting a fasting protocol. You don’t need to eliminate them entirely, but reducing them can expedite your transition into fat-burning.
A significant event during Phase 2 is the decrease in the growth regulator called mTOR (mammalian target of rapamycin). This reduction in mTOR activity is crucial because it initiates autophagy.
Autophagy is a key benefit of fasting. It’s a cellular “clean-up” process where old, damaged, and dysfunctional cellular components are removed and recycled. This process is incredibly beneficial as it eliminates cellular debris that can contribute to aging, cancer development, and chronic diseases.
16 – 24 Hours: Primary Fat-Burning Stage
Between 16 and 24 hours of fasting, you enter Phase 3: the primary fat-burning stage.
At this point, your body is actively burning fat stores to meet its energy demands because glucose from glycogen is largely depleted.
Another crucial element that becomes activated during this phase is an energy sensor called AMPK (AMP-activated protein kinase).
AMPK acts as a catalyst for autophagy, significantly enhancing the cellular clean-up process throughout your body.
24 – 72 Hours: Ketosis Stage
Stage 4 commences between 24 and 72 hours of fasting: this is the ketosis stage.
This phase is characterized by your body’s complete shift to burning fat for energy. Ketosis is the metabolic process where your body breaks down fat cells to release energy, producing ketone bodies as a byproduct. This stage amplifies all the benefits experienced in the previous phases.
Ketone bodies serve as an alternative fuel source for the brain when glucose is scarce.
It’s crucial to differentiate between nutritional ketosis, achieved through fasting or low-carb diets, and diabetic ketoacidosis, a serious medical condition. They are distinctly different states. Nutritional ketosis is a natural physiological response to fuel scarcity, whereas diabetic ketoacidosis is a dangerous complication of diabetes.
This built-in mechanism allows our brains to remain energized and functional during periods of food scarcity, enabling us to make sound decisions – a truly remarkable evolutionary adaptation.
Many individuals, including myself, report experiencing enhanced energy levels and improved mental clarity during this phase of fasting or when following a ketogenic diet. These benefits are largely attributed to ketone bodies.
Another remarkable event occurring around the 24-hour mark is the brain’s production of a chemical called brain-derived neurotrophic factor (BDNF).
BDNF plays a vital role in supporting the growth and survival of brain neurons. It is essential for long-term memory, coordination, and learning.
Researchers believe that BDNF is a significant factor in why fasting is so effective in reducing the risk of neurodegenerative diseases like Alzheimer’s as we age.
72+ Hours: Deep Ketosis and Enhanced Benefits
During fasts extending beyond 72 hours, you enter a state of deep ketosis. This intensifies all the benefits experienced in the earlier stages.
The advantages of weight loss, improved metabolic health, and longevity are significantly amplified with prolonged fasting.
In multi-day fasts, insulin and glucose levels remain consistently low, and the body stays in a sustained state of ketosis.
Due to nutrient restriction, your liver reduces the production of IGF-1 (insulin-like growth factor 1), a hormone involved in growth and development.
Short-term reductions in IGF-1 are linked to decreased oxidative stress and are believed to have anti-cancer and anti-aging effects.
Fasting for 72 hours or longer has also been shown to enhance the body’s resilience to toxin exposure and stress hormones.
In summary: Fasting, particularly beyond the initial water weight loss phase, offers a range of significant health benefits, from fat burning and cellular repair to enhanced brain function and potential longevity advantages. Understanding the timeline of fasting can help you appreciate the different stages and optimize your fasting practices for maximum benefit.
