Stages Of Fasting – When Does The Body Start Burning Fat During Fasting

 by Dr. Steve Hruby
Reviewed by Dr. Steve Hruby

I am a chiropractor, father, husband, coach, entrepreneur, and motivational speaker. Feeling good about yourself is a choice: My mission is to transform lives through optimized health and overall well-being.

  Fact Checked
 by Rhealyn Tropia, RMT
Reviewed by Rhealyn Tropia, RMT

I’ve been called a Data Ninja, The Fact Checker, and a Human Search Engine. I’m an indie content curator who does research daily, and quizzes myself on the important issues of the day.

when does the body start burning fat during fasting

Did you know almost half of the US population is trying different weight loss regimens yearly? People are always looking for the next big thing to help them slice those extra pounds, from fad diets to juice cleanses.

One weight loss method gaining popularity is fasting. Fasting is an ancient practice used for both religious and health purposes. There are many fasting ways, but the most common is abstaining from meals for their desired duration.

Now, when does the body start burning fat during fasting?

The body starts to break down fat for energy after about 12 hours of fasting. However, maximum burning begins 16-24 hours into the fast. So, if looking to burn fat, you must fast for at least a day.

Fasting for shorter periods, like 12 hours, can still help you lose weight. However, you’ll mostly be losing water weight and not stored body fat.

Continue reading to discover how to fast to lose weight.

Weight Loss and Fat Burning

Does intermittent fasting burn fat? Intermittent fasting is an excellent weight loss technique that lowers your fat reserves through caloric restriction.

However, not all fasting is created equal. To see results, you need to ensure you’re doing it correctly.

Here’s how intermittent fasting works.

When you fast long enough, your body uses stored energy, which means it’ll start burning fat for fuel. This ketosis process can help you lose weight.

How to Fast Intermittently

Everyone is different, and there’s no “perfect” way to fast. Intermittent fasting is a flexible practice that you can tailor to your needs and liking. Experiment and find what works best for you.

woman in gym clothes holding bottled water

Once you’ve determined that fasting is safe for you, you can use one of these methods to achieve your goals.

#1. 8/16

This type of intermittent fasting involves eating within eight hours and restricting calories during the 16-hour fasting period. This approach is popular for those chasing weight loss and aiming to improve their health.

#2. 5:2

The 5:2 intermittent fasting is basically eating five days and restricting calories for two days. You can use this approach with complete or partial fasting. Men typically consume 600 calories on fasting days, while women consume 500 calories.

#3. 24 and 2

This technique entails no meals for 24 hours, twice weekly, followed by normal eating. You can do this on a breakfast-to-breakfast, lunch-to-lunch, or supper-to-supper basis.

#4. Alternating

The alternating intermittent fasting approach involves fasting one day and then eating normally the next. You can fast alternately as often as desired.

#5. Long-Term Fasting

Those accustomed to fasting can do so for several days at a time. This approach is typically for spiritual purposes.

How Long Should You Fast to Burn Fat?

What are the stages of intermittent fasting? When should you expect the fat reduction to commence?

The duration varies based on your fasting method and physique. However, fat-burning often begins after 12 or more fasting hours, based on meal type and habits.

Here’s what happens in each fasting stage.

foot stepping on a weighing scale

Fed Stage (Phase 1)

The fed stage is the period after a meal when your body is digesting it. During this stage, your gut is busy breaking down the food you’ve consumed and absorbing nutrients. 

Insulin levels are high during the fed state. Insulin helps supply glucose to cells with extra glucose stored as glycogen. Leptin levels rise while ghrelin levels reduce, minimizing appetite and hunger.

This stage typically lasts three to four hours after a meal, depending on the meal type and size.

Early Fasting Stage (Phase 2)

During this time, your body has finished digesting and is now in a “post-absorptive” state.

Your body starts to burn glycogen for energy. Energy is also available through amino acid breakdown. The body also starts burning fat as an alternate energy provider. However, those on a high-carb diet need to enter the fasting stage to burn fat.

This stage can last as much as 18 hours after feeding. You may experience hunger during this time, but it’s generally not as severe as it would be if you were in phase three.

Once glycogen stores are over, the body enters the fasting state.

Fasting Stage (Phase 3)

The fasting stage is when your body has emptied its stored energy and is now running on stored fat and protein. As a result, ketone production commences, and your body enters ketosis after some time into this stage.

Those on a low-carb diet tend to enter ketosis faster. However, for fasts under a day, you may not achieve ketosis. 

This stage lasts up to approximately 48 hours.

Extended Fasting Period (Phase 4)

The extended fasting or starvation period is any time beyond the fasting stage. It often commences 48 hours after fasting. Insulin levels continue plummeting while levels of ketone bodies like beta-hydroxybutyrate rise. 

The brain gets its energy requirements from the glucose produced in the kidney and ketone bodies. Branched-chain-amino-acid-breakdown lowers to conserve muscle mass.

The Risks of Prolonged Fasting

Fasting for long periods can be extremely dangerous. The risks of prolonged fasting include:

empty and sad plate with fork and knife on a pink background
  1. Dehydration and anemia: When you do a complete fast, your body isn’t taking in any fluids. That can lead to dehydration and anemia, which can be dangerous.
  1. Malnutrition: Fasting can also lead to malnutrition because you’re not meeting your nutrient requirements when you fast.
  1. Electrolyte imbalance: When you fast, your body can become imbalanced in electrolytes. This disorientation can lead to problems like irregular heartbeat, seizures, and even death.
  1. Kidney problems: Fasting can also cause kidney problems because your body isn’t getting the fluids it needs to function correctly.
  1. Psychological problems: Fasting can also cause anxiety, depression, anger,  tension, and irritability, among other psychological issues.

Excessive fasting may also cause tiredness, muscle pains, and weakness.

What Can I Eat While Fasting Intermittently?

Understanding what snacks to take, how to break your fast, and how to go about the post-fasting duration is crucial. So, what can you drink while fasting? What foods are suitable during fasting?

Focus on nutrient-dense, whole foods. Get plenty of veggies, fruits, whole grains, and healthy fats. These foods help you feel satisfied and provide the fuel your body needs to function optimally. 

You may want to limit or avoid processed foods, sugary drinks, and alcohol. Such foods can disrupt your fasting schedule and make it harder to stick to your plan. 

It’s a good idea to consume adequate water. You can also take unsweetened tea, black coffee, green beverages, apple cedar vinegar, and herbal teas. You must stay hydrated to avoid fasting side effects like headaches or dizziness. 

Eating a small snack is okay if you’re feeling hungry during your fast. Just be sure to choose something high in nutrients and low in calories. A piece of fruit or a handful of nuts is a good option. 

FAQs: What Everyone Wants to Know on the Stages of Fasting

man holding a salad bowl and measuring waist

What Fat Burns First When Fasting?

Hard fat burns first when fasting. This fat is typically around organs like livers and kidneys and supplies energy when the body is fasting.

How Much Fat Does the Body Burn while Fasting?

The body burns about 0.55 to 1.65 pounds of fat a week while fasting. However, this only occurs if you fast correctly and consume the right calories. If you take excess calories after fasting, your body stores them as fat, canceling the initial fat-burning effects.

How Do You Burn the Most Fat While Fasting?

You burn the most fat while fasting by eating more often than a single massive meal. Several small meals allow your body to metabolize the food better, which helps burn more fat.

Toxin buildup in your body contributes to weight gain. Alternating fasting and other natural weight loss techniques like a 3-day gut cleanse can clear your body. That way, you’ll be closer to your goals.

How Do I Know When My Body is in Fat Burning Mode?

You know your body is in fat-burning mode when your appetite decreases and you feel better overall. You should find it easier to fit into your tight clothes, plus have better sleep and moods. Your bowel movements are often more effortless, chronic pain lessens, and blood pressure may improve.


The body starts burning fat during fasting when it depletes glycogen stores. Glycogen is a molecule stored in muscles and the liver, and it’s broken down into glucose to supply energy. When the glycogen stores are over, the body must start breaking down fat for energy.

Losing fat during fasting is useless if you’ll binge-eat it back after the duration. Familiarize yourself with what foods are best for a weight-loss-aimed fast. 

While at it, keep your conditions in mind, as some foods excellent for fasting may not be suitable for you. We’ve rounded up the worst foods for PCOS. Check them out, if battling the condition, to ensure you are in the safe zone.

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