Intermittent fasting offers many health benefits as an eating pattern on its own but can also be a powerful tool in fat loss.
Be sure to check out this prior post on How to do Intermittent Fasting.
I’ve also written about the myriad amazing health benefits of intermittent fasting as well as building muscle while intermittent fasting.
- Does fasting burn body fat?
- Why am I gaining weight on Intermittent Fasting?
- Does your body burn fat or muscle first when fasting?
- How many hours of fasting before body burns fat?
- Does the time of day when I fast help me burn more fat?
- Can fasting reduce belly fat?
- Will I lose weight if I fast for only one day?
- How much weight can you lose in a month with intermittent fasting?
- How soon will I see results from intermittent fasting?
Does fasting burn body fat?
The only thing that actually burns body fat is eating in a caloric deficit which means eating fewer calories than you burn every day.
Fasting describes the timing of when you eat, not how much you eat, so on its own fasting does not burn body fat but it can be a very helpful tool in making it easier to eat less and lose weight.
Why am I gaining weight on Intermittent Fasting?
Weight gain is not related to Intermittent Fasting nor to the timing of eating but weight gain is related to how much you eat.
It doesn’t matter when you eat the calories, if you are eating more calories than you burn over time, you will gain weight.
Does your body burn fat or muscle first when fasting?
This depends on how active you are at the time.
Your body will always burn glycogen (your carbohydrate stores in your liver and muscles) first no matter how active you are. (This is not burning the muscle protein tissue itself but the glycogen happens to be stored in muscles.)
Once you’ve used up your glycogen stores, your body will turn to both burning fat and muscle protein for energy. Your body can only burn fat at a very slow rate. If this slow rate is enough to provide energy for your activity at the time (like minimal household activities or resting) then your body will stick to only burning fat. If you are being active (exercising or doing more strenuous household activities) then this slow rate will not be enough energy for those activities so your body will turn to burning muscle protein to use as energy in addition to the small amount it is using from fat.
For this reason, exercising in the morning prior to eating will tend to burn a significant amount of muscle tissue since your glycogen stores have been used up overnight. If you like exercising in the morning and you would like to preserve your muscle, it is a good idea to eat at least some light carbohydrates (like fruit) before exercising to have enough glycogen to prevent your body from going into burning muscle tissue.
How many hours of fasting before body burns fat?
As above, your body will burn through its glycogen stores first. The time it takes to burn through your glycogen stores is the amount of time until your body burns fat.
On average, it takes about 12 hours to burn through your glycogen stores and start burning fat.
If you are normally very active, your body adapts to this activity by building up larger amounts of glycogen stores after eating. So maintaining that activity will still put you in the 12 hour range (but less activity will make your glycogen last longer).
If you are less active, your body will maintain a smaller amount of glycogen after eating. Maintaining your level of activity will still put you in the 12 hour range (but sudden increases in activity will burn through glycogen faster – though your body will adapt and store more glycogen after a couple days).
Of note, if you are in a caloric deficit (eating less than you burn over time), then your body will not be able to build up as much glycogen as it would otherwise want to and therefore your body will go into fat burning mode sooner. This has not been studied but is likely around 9-11 hours if you are in a reasonable caloric deficit. (Severe caloric deficits will be much less time but have serious health effects and are NOT recommended).
This is key in how Intermittent Fasting can be used to help with weight loss.
Intermittent Fasting gives you more time each day without glycogen stores for energy and therefore gives you more fat burning time. This can be easily undone if you eat enough to replenish your fat stores when you start eating again. This is why you still need to eat in a caloric deficit over time. But intermittent fasting makes it easier to do so.
Does the time of day when I fast help me burn more fat?
Interestingly, the time of day when you eat can affect whether your food is used as energy or stored as fat.
Eating later in the day and evening, your body will tend to store more of the calories as fat. Eating earlier in the day, your body will tend to store more of the calories as temporary energy storage (called glycogen) and less as fat.
This trend is minimal (as in only a very small amount more or less is stored as fat vs glycogen) but can be meaningful as small effects really do add up to significant amounts over time.
To minimize fat gain, it can be helpful to regularly eat early in the day rather than later in the day.
Can fasting reduce belly fat?
Intermittent Fasting itself won’t help you lose belly fat unless you are eating in a calorie deficit (eating fewer calories than you burn every day).
Intermittent Fasting dieting (in caloric deficit) compared to traditional dieting shows about equal reductions in belly fat. What Intermittent Fasting dieting has over traditional dieting is that it has been shown to be much easier to stick to over the long term.
Will I lose weight if I fast for only one day?
Again, the only factor in weight loss is calories in vs calories out over time.
If fasting for one day per week means that ultimately you eat fewer calories than you burn in the week, then yes, you will lose weight that way.
However, if you fast for one day and then eat more in the following days to make up for the lost calories or eat more to satiate increased hunger during that time due to the fasting day, then you will not lose weight.
Looking at the numbers, you probably won’t lose much noticeable amount of weight by fasting for only one day.
One pound of fat is 2500 kcals. If you burn 2500 kcals more than you eat then you will lose 1lb of fat.
Most people burn about 2000 kcals per day. So if you don’t eat anything for 24 hours, you will burn about 2000 kcals which wouldn’t even be one pound of weight.
It’s much more effective and much safer to eat in a smaller calorie deficit regularly over time to lose weight (ex. eat 300-400 kcal less than you burn each day).
How much weight can you lose in a month with intermittent fasting?
The answer to this lies in how many fewer calories you eat in the month vs how many calories you burn in the month, not in whether you use intermittent fasting or not.
Again, one pound of fat is 2500 kcals. So if you eat 2500 kcals less than you burn over the month, you’ll lose one pound. If you eat 10,000 kcals less than you burn over the month, you’ll lose four pounds.
Intermittent Fasting makes it easier to stick to a calorie deficit diet and eating earlier in the day helps you store less fat. So Intermittent Fasting is a great tool for weight loss but the weight loss itself comes from eating less than you burn.
Losing weight slowly over time is the best way to lose only fat and spare losing muscle along the way.
Losing weight slowly over time is also the best way to prevent rebound weight gain later.
I’d recommend aiming to lose maximum 1lb per week or 4lb per month (which you can do by eating 300-400 kcal less than you burn each day).
How soon will I see results from intermittent fasting?
This depends on what result you are looking for. There are so many incredible benefits of Intermittent Fasting!
You’ll likely see the improved mental performance and better sleep within a couple days of intermittent fasting.
Some of the other benefits such as decreased blood pressure and lower blood sugar levels you’ll likely start to see after a month or two.
How soon you’ll see weight loss from intermittent fasting will depend on how many fewer calories you eat each day compared to how many you burn. Feeling the results of weight loss will also depend on how much weight you are starting with.
Either way, losing about 1lb per week (eating 300-400 kcal less than you burn each day) is the easiest, safest, and most effective way to lose weight in the long-term.
Losing about 1lb per week, you’ll likely start seeing and feeling the weight loss within about a month.
Intermittent fasting makes eating in a calorie deficit like this easier to stick to in the long term and eating earlier in the day can help avoid putting on more fat during the day.
Ultimately, the only way to lose fat is to eat fewer calories than you burn. However, Intermittent Fasting is a powerful tool in dieting as it is much easier to stick to your calorie deficit this way and eating earlier in the day can help avoid putting on more fat during the day.
Now that you’re convinced to try it out, read this introductory post on How to do Intermittent Fasting.
For even more, I’ve also written about the myriad amazing health benefits of intermittent fasting as well as building muscle while intermittent fasting.
Antoni, R.; Robertson, T.; Robertson, M.; Johnston, J. (2018). A pilot feasibility study exploring the effects of a moderate time-restricted feeding intervention on energy intake, adiposity and metabolic physiology in free-living human subjects. J. Nutr. Sci. https://doi.org/10.1017/jns.2018.13
Gabel, K., Hoddy, K. K., Haggerty, N., Song, J., Kroeger, C. M., Trepanowski, J. F., Panda, S., & Varady, K. A. (2018). Effects of 8-hour time restricted feeding on body weight and metabolic disease risk factors in obese adults: A pilot study. Nutrition and healthy aging, 4(4), 345–353. https://doi.org/10.3233/NHA-170036
Garaulet, M., Gómez-Abellán, P., Alburquerque-Béjar, J. J., Lee, Y. C., Ordovás, J. M., & Scheer, F. A. (2013). Timing of food intake predicts weight loss effectiveness. International journal of obesity (2005), 37(4), 604–611. https://doi.org/10.1038/ijo.2012.229
Jakubowicz, D., Barnea, M., Wainstein, J., & Froy, O. (2013). High caloric intake at breakfast vs. dinner differentially influences weight loss of overweight and obese women. Obesity (Silver Spring, Md.), 21(12), 2504–2512. https://doi.org/10.1002/oby.20460
Moro, T., Tinsley, G., Bianco, A., Marcolin, G., Pacelli, Q. F., Battaglia, G., Palma, A., Gentil, P., Neri, M., & Paoli, A. (2016). Effects of eight weeks of time-restricted feeding (16/8) on basal metabolism, maximal strength, body composition, inflammation, and cardiovascular risk factors in resistance-trained males. Journal of translational medicine, 14(1), 290. https://doi.org/10.1186/s12967-016-1044-0
Rynders, C. A., Thomas, E. A., Zaman, A., Pan, Z., Catenacci, V. A., & Melanson, E. L. (2019). Effectiveness of Intermittent Fasting and Time-Restricted Feeding Compared to Continuous Energy Restriction for Weight Loss. Nutrients, 11(10), 2442. https://doi.org/10.3390/nu11102442
Sutton, E. F., Beyl, R., Early, K. S., Cefalu, W. T., Ravussin, E., & Peterson, C. M. (2018). Early Time-Restricted Feeding Improves Insulin Sensitivity, Blood Pressure, and Oxidative Stress Even without Weight Loss in Men with Prediabetes. Cell metabolism, 27(6), 1212–1221.e3. https://doi.org/10.1016/j.cmet.2018.04.010