My wife and I have been intermittent fasting for over ten years, and it’s been life-changing for us. It’s by far the easiest way for us to control our weight and keep our body fat levels low. There is so much science behind why fasting works so well. One of the questions I get most frequently is when does fat burning start when fasting.
I like to break it down into three stages when explaining when fat burning starts during a fast. Phase one is the first four hours right after eating a meal. Phase two is from 4 to 16 hours when you start dipping into energy reserves, and Phase three is when all that excellent stuff starts to happen from 16 to 24 hours.
You can always speed up the fat burning process by eating a low-carb diet, but this is how it works.
Right after you’ve eaten a full meal with protein, fat, and carbs, your pancreas releases insulin, the storage hormone to store away the excess calories. Keep in mind, carbohydrates spike insulin the most. Excess carbohydrates get stored in the muscles and the liver in the form of glycogen. One of the controlling factors is that your muscles can only hold about 1500 to 2000 calories of glycogen, and your liver can only hold up to 400 calories of glycogen.
A significant factor in regulating when your body will start burning fat while fasting is how much glycogen (carbs) you have stored in your muscles and liver. If your muscles and liver are full to the brim, you probably won’t start burning fat until well after four hours of your last meal. But if you’re going into a fast with low glycogen levels in your muscles and liver, you can potentially start burning fat right away.
This brings us into Phase two, 4 to 16 hours. Phase two is when fat burning can begin. After you process the meal over the last four hours, you start to dip into the energy stores. Inverse hormones to insulin-like glucagon start kicking in, releasing stored energy carbs and fats. Once you begin depleting your glycogen stores in your liver, you can begin burning ketones. You may have heard of the keto diet. When your liver gets low on carbohydrates (glycogen), it will begin turning fat into ketone bodies. This is when you start burning body fat.
Once again, Phase two can be different for all people depending upon how full your muscles and liver were with glycogen when you started the fast. If you were eating a low-carb diet, you could be in ketosis within 12, 13, 14 hrs. In a short while, other people eating a high carb diet may not deplete their glycogen stores until Phase three.
In Phase three, just about everyone is getting very low in glycogen. Sixteen to 24 hours of a fast is when all the good stuff starts happening. Glycogen levels are being depleted. Ketosis is kicking in along with the counter-regulatory hormones like human growth hormone, cortisol, and epinephrine. These hormones can increase metabolism and help with releasing energy. This is when fat-burning kicks in big time. Even high carb eaters will have depleaded just about all of the glycogen fasting for 24 hours.
This whole process can be speeded up by eating a low-carb diet. If you want to burn more fat while fasting, try to eat only 50 to 100g of carbs per day. I recommend a whole natural food diet that is low in carbohydrates, and is mostly high in fiberous vegetables. Take in an adequate amount of protein, about .9g per pound of lean body weight, and eat plenty of healthy fats.
I hope this article helps and check out the video. It breaks everything down in detail.
Best – Mike Cola