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What Happens to Your Body When You Fast?

Fasting can have many powerful effects on your body.

From burning fat and weight management to digestion support and lowering insulin, fasting can offer a number of benefits. But what’s actually happening in your body while you’re fasting? Also, what happens to your cells and hormones when you fast?

Let’s dive in and take a look at what your body is actually going through during a fast that leads to these awesome health benefits.

What Happens to Your Body When You Fast?

The First 4 Hours

The first four hours after eating is known as the anabolic growth phase.

Your body is using up the energy (glucose) you just ate to power your current activity and for cellular and tissue growth. Your body is flushed with the hormone insulin to remove this blood sugar and convert it into glycogen. This spike in the hormone insulin (which is produced in the pancreas) can result in a sugar crash, so people tend to start feeling hungry after four hours of a meal.

The higher the glycemic index of your meal, the greater your crash will be. Also, if your cells don’t use all the available glucose, it ultimately gets stored as fat.

4 to 16 Hours

The second phase kicks off after about four hours and can last until about 16 hours after your last meal. This is where blood sugar starts to fall, and insulin is no longer produced.

Around the 12 hour mark, your digestive system goes to sleep, and your body begins its healing process. At around 14 -16 hours, your body has converted to using stored fat, chiefly your triglycerides, as energy. The process of releasing fat and burning it up for energy releases chemicals known as ketone bodies for energy.

The rate at which your body reaches this stage depends on what you ate for the last couple of meals before you fast. For example, if you ate many carbohydrate-heavy foods, it will take a bit longer than if you ate mostly protein and healthy fats.

16 to 24 Hours

Once you pass the 16-hour mark, your body starts to ramp up the fat burning. Glucose in the cells and glycogen in the liver and muscles begin to dwindle rapidly, causing your body to burn stored fat to keep up with your body’s need for energy.

This is also when the human growth hormone (HGH) starts to increase dramatically. This is significant because, as we age, our bodies may produce lower HGH levels, and that’s associated with an increase in fat tissue and loss of muscles.

Also, in this stage, one of the most powerful features of fasting kicks off: autophagy.

Autophagy is a detox process your body undergoes to clean out damaged cells and regenerate new ones. Damaged cells can contribute to aging, cancer, and chronic disease.

You might also like: The Benefits of Intuitive Eating

24 to 72 Hours

It’s important to work with a professional if you plan to do a prolonged fast.

Once you pass 24 hours of fasting, ketones are released into the bloodstream, and your body will enter into a state called ketosis, where your body is reliant on burning your fat stores for energy. Burning ketones has major health benefits for cognitive function, including mental clarity.

Your brain gets a boost around the 24-hour mark from an increase in the production of brain-derived nootropic factor (BDNF).

BDNF supports the growth of brain neurons. Not only is it correlated with improvements in long-term memory and learning, but it’s also thought to be key in reducing the risk of developing Alzheimer’s disease later in life.

This becomes a stage of self-repair. Any damaged cells are broken down and consumed for energy; new and more effective cells are created. You may even start to see the impact on your skin.

72+ Hours

Once you’ve been fasting for three days or more, your body has entered into a deep state of ketosis where it is running on stored fat. Your immune cells also start regenerating at a very rapid pace.

All the previous benefits: fat loss, mental clarity, autography, and BDNF continue to increase.

Research suggests that during a fast, your cells get a little stressed out (in a good way), making you more resilient. It’s similar to the stress put on your heart and muscles during exercise—an immediate shock to the system that allows your body to get stronger over time.

All of this said, people with a history of eating disorders should avoid prolonged fasts, and it’s always a good idea to seek out medical supervision when fasting.

Will You Try Fasting?

Fasting is absolutely a powerful tool for improving your body’s longevity, fat loss, increasing resilience, and improving your overall health. Before you adopt the fasting lifestyle, consider these caveats.

Fasting and intermittent fasting may not be right for you if:

  • You’re suffering from chronic or adrenal fatigue.
  • Your hormones are out of whack.
  • You are trying to get pregnant.
  • You have an eating disorder.
  • You’re under a lot of stress at home or work.

Once again, fasting is not something to take on lightly and can damage your health if not done correctly. If you plan to do any kind of extended fast, please consult with a licensed medical practitioner!

Related article: Here’s Why Weight-Loss Diets Don’t Work

Kat Vernelli

Kat Vernelli

Kat is a virtual, Certified Holistic Nutritional Consultant. She has a serious passion for holistic living and loves educating people on nourishing foods and well-being. She believes in eating fresh, whole foods, cooking with real ingredients, keeping chemicals out of daily care products, and doing what makes you happy as... Read More

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