Intermittent fasting is a popular “diet” that restricts when you eat, not what you eat.
This method of eating lets you eat what you what within an eating window, but you must fast for the rest of the time. The length of time an individual may fast can vary drastically, depending on the method followed.
Some fasting schedules will restrict eating to a certain time period each day, while others will have you fast for an entire day, or sometimes every other day.
But with all the different variations of this, the question is, how many hours do you really need to fast with intermittent fasting?
In short, fasts between 16-24 hours will give the most health benefits. A shorter eating window can also make it easier to reduce your calorie intake without having to count calories or carbs.
Popular ways of doing intermittent fasting
First, let’s explore some of the varying ways in which people are implementing intermittent fasting into their lives…
Fast for 12 hours, eat during a 12-hour window.
Fast for 14 hours, eat during a 10-hour window.
Fast for 16 hours, eat during an 8-hour window.
Fast for 18 hours, eat during a 6-hour window.
Fast for 20 hours, eating during a 4-hour window.
One Meal A Day (OMAD)
Eat just one meal per day, so the eating window would be somewhere between 30 minutes and 1 hour.
Eat Stop Eat
Fast for a full 24 hours, once or twice per week. It’s important to note this method still involves eating every day. For example, you could eat breakfast, then fast for 24 hours until breakfast the next day.
Every Other Day
There are different variations of this method, but typically it involves eating normally followed by strict calorie reduction every other day (eating about 500 calories on a fasting day, or 25% of your normal intake).
But how many hours do you really need to fast per day while following an intermittent fasting plan?
This was the question on my mind when I first started researching intermittent fasting.
I mean, after what point does it become effective? Is a 12 hour fast beneficial? 14 hours? 16? Or do you really need to fast for a full 24 hours to see results?
According to the research, the fat-burning benefits of fasting start to begin around the 12-hour mark and escalate between 16-24 hours. It’s different for each individual.
And Brad Pilon outlines the benefits of a full 24 hour fast, along with the research, within his popular book Eat Stop Eat.
In other words, the longer you fast, the more beneficial it can be (this is true up until the 24-hour mark).
To get the full benefits from fasting, it seems anywhere between 16-24 hours is ideal. Keep in mind these benefits aren’t necessarily just weight loss – fasting has been proven to improve glucose control, lower cholesterol, and even increase your lifespan.
12 hours may not be an ideal fasting period, but it’s a great start.
Another (big part) of this equation is the longer you fast, typically the easier it is to naturally eat less food. If you have free reign to eat 12-hours per day and aren’t tracking your food intake, you’re probably going to eat much more than someone who is restricted to eating only 1 hour per day.
The stricter the fasting schedule, the more likely you’ll lose weight without purposely trying to restrict food intake.
Fasting is not magic – calories in = calories out
The thing is, it’s important to realize fasting is not magic.
You can still gain weight (or maintain) with fasting if you aren’t eating in a caloric deficit.
Because while fasting can make weight loss easier, you still have to eat less. Fasting doesn’t break the fundamental science behind weight loss – that is, if you eat fewer calories than your body needs, you’ll lose weight.
Fasting just makes it much easier, because if you can go 16, 18 or 24 hours without eating, you’re going to have much less time to be eating.
And many people might find that their body reacts very positively to an extended break in calories. If losing weight has always been a struggle, intermittent fasting could be worth a try.
How much should you eat? What is too much?
There isn’t a set answer to this. Everybody’s body burns a different amount of calories depending on a ton of different factors. My best advice is if you have no idea how much you should be eating, use a free calorie calculator to get a baseline idea.
My favorite is this one – https://www.calculator.net/calorie-calculator.html
You can adjust it based on your lifestyle. The Basal Metabolic Rate (also known as BMR) will tell you how many calories you burn just existing – that is, if you literally did not move your body the entire day.
My current BMR is 1,932.
That’s just the BMR. It increases based on activity level. Even sedentary people burn more calories than their BMR. My sedentary caloric level is around 2,300.
This won’t be 100% accurate, but it’s a start. You can adjust it as you learn more about what works for you.
Now to be clear, fasting doesn’t require strict calorie counting, but if you’re coming from a place where you have no idea what “eating normally” means, it’s probably a good idea to learn what your body actually needs to function.
Now you have a baseline to go from. You know that if you (theoretically) eat less than your maintenance calories, you will lose. If you eat more, you will gain.
No matter what fasting program you follow, you still have to eat less to lose weight.
It’s just that for many people, only being able to eat in restricted time frames can seriously help limit their eating, which leads to fewer calories and in turn, weight loss without really having to specifically track food.
The Bottom Line – Longer fasting periods are better for weight loss
Ultimately, if you truly want to follow an intermittent fasting diet without having to restrict food or count calories, it’s better to aim for longer fasting periods.
For one, the fat-burning benefits of fasting don’t truly ramp up until around the 16-hour mark. And secondly, it’s also much easier for most people to naturally reduce calories if they have a stricter eating window.
The trick is to find what works for you. During my time spent 16:8 fasting, I found it was enough to naturally reduce calories without trying too hard, and it was a short enough fast that I never felt extremely ravenous.
But if you’re looking for something you don’t even need to follow every day, a 24 hour fast once or twice per week could be more than enough for many people to naturally reduce calories enough to actually lose weight. If you can make it through fasting occasional 24 hour periods, it may be easier to stick to if you can eat more normally the rest of the time.
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