Dr. Ahmed Most people think that fasting is simply about “starving” or going hungry, but it is actually a powerful biological process.
Normally, your body uses glucose for energy, but when you stop eating for a certain amount of time, something different happens. Instead of waiting for food (source of glucose), your body turns to the stored fat in your body.
During fasting, the ultimate goal is to use both fuels efficiently. Nowadays, people eat so much and so frequently that our body never has the chance to burn fat instead of glucose.
However, by understanding how you can enable your body to use fat rather than glucose, you can lose weight in a healthy way.
What Happens Hour-By-Hour?
When you stop eating, your body does not just sit idle. Here is exactly what happens inside you as the hours pass.
0-4 hours: the Fed State
Immediately after a meal, your blood sugar rises. To handle this, your body releases insulin, a hormone that acts like a storage manager.
During these first few hours, your body is busy using the energy you just ate and storing any extra in your liver and muscles.
In this stage, fat burning is completely turned off because the body has plenty of easy-to-use sugar available.
4-12 Hours: the Post-absorptive State
Now that your last meal has been processed, your blood sugar levels begin to drop back to normal. Since there is no new food coming in, your body turns to its first backup: Glycogen.
This is basically stored in your liver. During this window, you are not burning much body fat yet as you are mostly draining the sugar reserves you saved up from your last meal.
12-18 Hours: the Fat-burning Zone
This is where the magic happens for weight loss. Once your glycogen runs low, your body has no choice but to find a new fuel source.
It begins a process called lipolysis. Your insulin levels drop significantly, which sends a signal to your fat cells to open up and release stored energy. Your body is now officially burning fat for fuel to keep your brain and heart running.
18+ Hours: Ketosis and Autophagy
If you continue fasting beyond 18 hours, your body enters ketosis, where it creates ketones, a fuel for your brain made entirely from fat.
Moreover, a process called autophagy kicks in. This is where your body identifies old, damaged cells and recycles them into new, healthy parts. This stage is highly beneficial for anti-aging and deep internal healing.
The Science of Fat Cells: How Fat Actually Leaves the Body
Many people wonder where the fat actually goes when they lose weight. Here is the three-step process of how your body uses its fat stores.
● Step 1: Lipolysis
Inside your body, fat is stored in cells as triglycerides. When you fast, and your insulin levels drop, your body sends out a chemical message to your fat cells.
This message tells the cells to open up and release those oil droplets into your bloodstream. This “release” phase is called Lipolysis.
● Step 2: Fat Transport
Once the fat is released, it travels through your blood in the form of “fatty acids.” These fatty acids attach to proteins and travel to the places that need energy the most, such as your liver and your muscles.
● Step 3: Fat Burning (oxidation)
When the fat reaches a muscle cell, it enters the mitochondria. Here, the fat is combined with oxygen and burned to create ATP, which is the actual energy your body uses to move and breathe. This final burning process is called oxidation.
The Myth Buster: Where Does the Fat Go?
A common myth is that fat “turns into muscle” or “turns into heat.” In reality, fat is made of atoms like carbon, hydrogen, and oxygen. When you “burn” fat, it is converted into Carbon Dioxide (CO2) and Water (H2O).
The surprising truth:
You actually breathe out most of your lost fat! About 84% of the fat you lose leaves your body as CO2 through your lungs, while the remaining 16% leaves as water through your sweat, tears, or urine.
Fasting during Ramadan vs. intermittent fasting
While both methods involve periods of not eating, they affect the body in slightly different ways. Understanding these differences can help you manage your energy better.
● Dry Fasting (Ramadan):
During Ramadan, you practice dry fasting, which means no food or water from dawn to sunset. Interestingly, some studies suggest that dry fasting can accelerate fat metabolism [ref].
Because the body does not have water coming in, it may break down fat cells even faster to produce metabolic water, a small amount of water your body creates internally from burning fat.
● Water Fasting (Intermittent Fasting):
In standard intermittent fasting, you drink plenty of water, tea, or black coffee. Hydration is crucial here because it helps the kidneys flush out the metabolic waste products created when fat is burned. It also helps keep your energy up and reduces hunger pangs.
● The Importance of the “re-feed”:
Whether it is Iftar or your IF breaking-fast meal, what you eat matters. If you break your fast with high-sugar foods, your insulin will spike instantly, stopping the fat-burning process and signaling your body to store fat again. To see results, focus on whole foods and slow-digesting carbs.
Why Fasting Targets “Stubborn” Visceral Fat
Have you ever noticed that belly fat is the hardest to lose? This is because stubborn fat is highly sensitive to insulin. When insulin is high, your body locks your fat cells, making it impossible for them to release energy.
3 Tips to Maximize Fat Loss While Fasting
To get the most out of your fasting window and ensure your body is burning fat efficiently, follow these three simple rules:
1. Stay Hydrated (when allowed):
The chemical process of burning fat (lipolysis) actually requires water molecules. If you are dehydrated, your fat-burning furnace slows down. Drink plenty of water during your non-fasting hours to keep the process moving.
2. Prioritize Protein:
When you do eat, make sure you get enough protein. This tells your body, We have enough building blocks for muscle, so keep burning the fat instead. This prevents muscle loss and keeps your metabolism high.
3. Light movement (the fasted walk):
Taking a brisk walk 30 minutes before you break your fast is great. At this point, your sugar stores are empty, so your body is forced to get 100% of the energy for that walk from your fat stores.
This is often called fasted cardio, and it is incredibly effective for shifting stubborn weight.
Summary:
| Feature | Sugar burning mode | Fat-burning mode |
| Primary fuel source | Glucose (from carbs/sugar) | Fatty Acids & Ketones (from body fat) |
| Main hormone | Insulin (Storage hormone) | Glucagon (Release hormone) |
| State of fat cells | Locked (Storing energy) | Open (Releasing energy) |
| Impact on liver | Stores fat (De Novo Lipogenesis) | Clears fat (Oxidation) |
| Mental energy | Ups and downs (Sugar crashes) | Steady and clear (Ketone fuel) |
| Cellular health | Growth and building | Autophagy (Cellular cleanup) |
Conclusion:
Fasting is not about deprivation as it is about giving your body the time it needs to clean itself and use the energy it has already stored.
You do not need to be perfect to see results. Even a simple 14 or 16-hour fast can begin the process of lowering insulin and protecting your liver.
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