Last Updated on November 9, 2023 by Fasting Planet
Losing weight on a fast is a two-pronged approach. First, you restrict calories to the point where you burn more calories than you eat. Your body also begins torching fat, which can lower the number you see on the scale. As you fast, how long will it take for fat burning to occur?
Your body may begin burning fat 12 to 14 hours after you begin fasting. This number can vary depending on how much glucose you have stored, as that must be burned before your body can switch to burning fat as your main energy source.
If you’re about to fast for the first time and you want to maximize your fat-burning potential, this is the article for you. In it, we’ll delve deeper into how your body burns fat on a fast, why it takes upwards of 12 hours for it to happen, and whether you can speed up the process.
Let’s get started!
How Does Your Body Burn Fat on a Fast?
Before we talk about the timeframe for fat burning, let’s start with an explanation of the basics, such as how your body burns fat when fasting. This something that’s come up on this blog a lot, but if you’re new here, you’ll want to check out this recap.
When you’re not in a fasted state, you probably eat between two to three meals a day and snacks in between. Each time you put food in your system, especially carbohydrates and sugar, you increase your body’s supply of glucose.
Glucose is a type of simple sugar that acts as our primary source of energy. Our body takes that glucose and makes it into glycogen through a process known as glycogenesis. It’s easier for our liver and muscles to store glycogen than glucose, but it’s still a sugar that gives us energy.
On an average, non-fasting day, you eat enough to keep your glycogen supply up. Eventually, you might have more glycogen than you need, so your body holds onto it for later.
Let’s say you started fasting. Now all the glucose your body gets from food is not being supplied to it. That’s okay though, because it still has all that leftover glycogen. It will begin using that for an energy supply instead.
Those glycogen stores don’t last forever. When your body depletes even the leftover glycogen, what happens next?
Your body needs an energy source to prevent it from shutting down, so your system switches gears. It stops burning sugar and begins burning fat and some muscle protein. Once you reach this state in your fast, you have the greatest potential for weight loss.
How Long Does It Take Your Body to Start Burning Fat During a Fast?
Okay, so how long will you have to wait for your body to switch from burning glycogen to fat instead? As we mentioned in the intro, the average timeframe is 12 to 14 hours.
This data comes from a 2017 article in Live Science. The article cites a study where participants fasted at different times. The first group ate over a span of 12 hours, from 8 o’clock in the morning until 8 p.m. The second group ate for only six hours, beginning again at 8 a.m. but wrapping up at 2 p.m.
The group with the smaller eating window had much more favorable results. Despite that both groups ate the same number of calories, the group that fasted longer reported their hunger levels felt “more stable.” They also burned fat at a faster rate of six percent.
The researchers concluded that you must fast for at least 12 hours to trigger fat burning, and that that’s why the group with the smaller eating window experienced the results they did.
Now, a word of caution. That 12-to-14-hour span is a good guideline, but it’s by no means the same for everyone. If you’ve never fasted before and you go 12 hours without food, you may still have more glycogen than someone who fasts every week. Thus, it may take you longer than 12 hours and even more than 14 hours to start burning fat.
It’s not necessarily easy to tell when you’re burning fat, either. Sure, eventually your clothes might fit looser, and you should have more energy, but right away? None of those things happen. It’s not like there’s a light switch that goes off in your body and you feel different when burning fat. It happens silently behind the scenes.
Do know that even if it takes you a little longer than the average person to begin burning fat, it will happen if you keep up with your fast.
Is There Any Way to Speed up Fat Burning When Fasting?
If you’re new to fasting, the idea of going 12 to 14 hours without food can seem daunting. That’s doubly true if you’ll be awake for your whole fasting window. Is there any way you can make fat burning happen even faster?
Siim Land, a trusted resource for intermittent fasters, suggests a few methods you might want to incorporate.
Make Sure You Eat Adequately Between Fasting Periods
To lose weight, your body has to burn more calories than you eat in a day, as we mentioned earlier. That might make you think that staying in a long-term fasted state may be best for your health. You can lose the most weight and burn more fat while you’re at it.
That’s actually not true. Your body doesn’t understand why you went from eating regularly every single day to now depriving it of food. Yes, it’s an intentional choice on your part, but your body views it as an emergency, as it thinks you’re starving.
Your metabolism can begin to slow down the longer you restrict calories so your body can hold onto any calories you do ingest. You could accidentally turn your weight loss results around and make it harder for yourself to lose more weight with ultra-prolonged fasts.
Siim Land suggests that you spend a few days per month eating a significant number of calories. This will even out your metabolism so it doesn’t drop.
Get Plenty of Protein
As you prepare your diet for your next fast, incorporate plenty of protein. Your thermic effect of food goes up with more protein, which can encourage better food nutrient absorption and digestion. Also, you’ll retain your lean muscle and you may even feel fuller for longer once your fast begins.
Measure your lean body mass and then try to eat 0.8 to 1.2 grams of protein per pound of your mass.
Exercise on the Fast
You may want to lay off some exercises when fasting, especially those that involve heavy lifting. Resistance training is an acceptable form of fitness for fasters. Siim Land mentions this training increases fat burning by boosting total daily energy expenditure.
You can also go on a few walks each day to speed up fat burning. Try to plan for a walk that’s between 30 to 60 minutes each. This will help your fasting period go by a lot more quickly, too.
Consume Tea and Coffee
You don’t just have to drink water on an intermittent fast. Green tea and black coffee are acceptable beverages. Both can get you into a state of ketosis, another fat-burning process that happens during a fast. Your fat oxidation also increases, helping along your fat burning capacity.
How Much Fat Can You Burn on a Fast?
You’re all about results. If you’re going to fast, then how much fat can you expect to trim off your body by the time you’re finished?
Like with how long it takes you to start burning fat, the answer to this question is not a perfect science. In a 2014 report published in Translational Research, when participants fasted for three to 24 weeks, their body weight decreased by three to eight percent.
Now, the fasting period there is huge, as there’s a big difference between fasting for about a month (three weeks) to five and a half months (24 weeks). Luckily, the researchers shared data that’s a little more specific and thus useful.
For each week the participants intermittent fasted, they lost roughly 0.55 pounds. If they went on an alternate-day fast, the weight loss accelerated to 1.65 pounds weekly. If you’re not familiar, an alternate-day fast is one in which you spend 24 consecutive hours fasting, then the next 24 hours eating normally.
Overall, both forms of intermittent fasting led to a waist circumference decrease of four to seven percent.
Conclusion
When you stop providing your body glucose for energy through food, it starts using fat for that energy instead. This can take about 12 to 14 hours to happen, although for some fasters, it may be longer. You can accelerate fat burning by drinking black coffee and green tea, walking, doing resistance training, and loading up on protein ahead of your fast.
Now that you know how much fat loss to expect on an intermittent fast, you can begin planning for success. Best of luck!