What fat burns first when fasting?

When fasting, your body goes through several stages of burning fat and energy sources. The first type of fat to be used is glycogen, which is the stored form of glucose in the liver and muscles. Glycogen is depleted within the first 24 hours of fasting. After glycogen is used up, your body starts burning fat stored in fat cells. The pattern of fat burning during extended fasting depends on several factors.

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Stages of Fasting

There are three main stages of fasting and fat burning:

12-24 hours: Glycogen stores are depleted first. The body burns glucose from glycogen for energy.
24-48 hours: Glycogen is depleted and the body starts burning fat. Triglycerides from fat cells are broken down into glycerol and fatty acids.
48+ hours: The body transitions to burning stored body fat. Ketones are produced from the breakdown of fatty acids.

So in summary, glycogen is the first energy source to be used up, followed by fat from triglycerides and fatty acids, and finally stored body fat in the form of ketones.

Where Does Body Fat Burn First?

The pattern of where body fat burns first depends on several factors:

Genetics: Genetics play a role in where your body stores fat and burns fat first.

Sex: Women tend to store more fat in the hips and thighs compared to men.
Fat cell size: Fat cells with more volume tend to release fat faster than smaller fat cells.
Insulin sensitivity: Areas with more insulin sensitivity release fat more readily.

Alpha/beta receptors: Alpha receptors inhibit fat breakdown while beta receptors enable fat release.

Taking these factors into account, the general pattern of fat burning goes:

Fat closest to the organs: Visceral fat around the organs burns first.

Upper body fat: Fat stored in the chest, shoulders, neck, face, and arms.
Belly fat: Subcutaneous belly fat is used next.
Lower body fat: Hips, thighs and buttocks are the last to slim down.

This pattern may vary between men and women due to differences in body composition and fat storage patterns.

The Role of Insulin

Insulin plays an important role in fat storage and burning:

When insulin is high, it signals your body to store fat and inhibits fat burning.

During fasting, insulin levels drop dramatically, enabling your body to burn stored fat.
Areas with more insulin sensitivity like the belly, love handles, and upper body burn fat first.
The lower body including hips and thighs may have less insulin sensitivity and take longer to slim down.

In summary, lowering insulin through fasting enables your body to access and burn its fat stores. Areas with high insulin sensitivity like the belly tend to burn fat faster during fasting.

Exercise and Fat Burning

Exercise along with fasting can help boost fat burning:

Exercise depletes glycogen forcing your body to turn to fat for fuel.

High intensity interval training (HIIT) is especially effective for burning fat.
Weight training helps build muscle and raises your metabolic rate.
Aerobic exercise utilizes fat for energy expenditure.

When combined with fasting, exercise can accelerate the use of fat stores for energy and help you lose fat faster. Targeted exercise can also help tone problem areas.

Tips to Maximize Fat Burning

Here are some tips to help maximize fat burning during fasting:

Lower insulin – Minimizing spikes in insulin allows greater access to fat stores.

Reduce calories – Eat at a caloric deficit to force your body to pull energy from fat.
Increase protein – Protein helps retain muscle mass which raises metabolism.
Lift weights – Building muscle boosts your resting metabolic rate.

Add HIIT – High intensity interval training helps burn fat stores.
Sleep well – Poor sleep increases insulin and disrupts metabolic hormones.
Manage stress – High cortisol slows fat burning and makes burning fat difficult.

Conclusion

In summary, glycogen is used for the first 24 hours of fasting. After glycogen is depleted, the body starts burning fat, starting with visceral fat around organs, then upper body, belly, and lower body fat. Insulin levels play an important role in regulating the ability to burn fat. Combining fasting with strategic exercise, HIIT, strength training, stress reduction, and a proper diet can help maximize fat burning, especially from stubborn areas.