With intermittent fasting becoming more and more popular, it’s a good idea to understand how it works and how much weight loss is reasonable to expect. In this article, we’ll answer the question, “how much weight do you lose during a fast?”
John’s Hopkins medical doctors have provided key insights into how the body behaves during a fast. In turns out that our bodies will not only continue to function without daily food intake, but in some cases, will even thrive.
And this makes sense from an evolutionary perspective. Our ancient ancestors were not guaranteed daily meals. And, in fact, would need to perform grueling daily tasks to survive with or without eating.
Modern humans have inherited this adaptability, and in fact, learning to use the body’s natural fat-burning capabilities is one of the best ways to safely and effectively manage weight and energy levels.
Key Point: Through processes like ketosis, the human body has adapted very well to periods of fasting. Doing so provides a wealth of well-documented benefits, including weight loss.
Daily Metabolic Rate
To understand your personal weight loss potential, we first need to discuss daily metabolic weight and how it translates to the fat burning process. Here’s how it works:
- Your body burns a certain amount of calories throughout a given day.
- The amount of calories burned depends on the daily expenditure of energy on physical activities.
- Genetics also play a crucial role in daily metabolic rate.
- The more calories expended by your metabolism per day, the greater the weight loss potential achieved by fasting.
- The crucial distinction to make is whether the body is burning calories by burning fat or muscle.
Weight loss is a complicated subject, but keeping it simple: leveraging your metabolism to lose weight is one of the most straightforward and reliable methods.
Takeaway: Daily metabolic rate is the key measure to determine the rate at which you body will burn fat and facilitate weight loss.
Intermittent Fasting
Intermittent fasting is a type of regulated fasting, where you set periods of time to refrain from eating. By abstaining from food, your body begins to exhaust its store of sugar, and then turns to burning fat once the sugar is depleted.
There’s several schools of thought when it comes to setting an intermittent fasting schedule:
- Daily Fasting: Limit eating to an 8 hour period during the day, and fast for the rest. This is called 16/8 fasting, because you fast for 16 hours and fast for 8.
- 5:2 Approach: This approach involves eating regularly for 5 days a week, and then limiting yourself to one healthy meal per day for the remaining two days.
- Extended Fasting: Fasting up to 24 hours is supported by science, but anymore than that and you may encounter some detrimental side effects. And there’s not much literature to suggest extended fasting is anymore beneficial than more conservative intermittent fasting.
To develop a healthy and sustainable schedule for fasting, it’s important to reference what’s been proven to work. Straying from recommended schedules can result in the metabolism developing maladaptive responses.
The Caveat: Fasting too much or too often can result in your body entering ‘starvation mode’ where it begins to store more fat rather than burn it. Additionally, in a starvation state the body might even prioritize burning muscle over fat.
Obviously this is the exact opposite of what you would want if you’re aiming to lose weight while keeping lean muscle.
Determining Your Metabolic Rate
There’s many different factors that impact your daily metabolic rate. So there’s no easy way to quickly determine what yours is without some more in-depth calculations.
Thankfully, there’s some accessible tools to make this calculation without paying for a metabolic panel. Here’s a well-respected metabolic calculator to use.
The important thing to note is that your metabolic rate is measures calorie expenditure that does NOT include:
- Sleeping
- Eating
- Physical activity
- Exercise
So while exercise is not included in measuring your daily metabolic rate, consistent exercise has been linked to more efficient metabolisms. In other words, achieving your fitness goals can improve your daily metabolism, and thus improves the rate at which your body burns fat.
Key Point: Your daily metabolism rate is determined by a baseline of normal activity that does not include physical exertions or sleep. There are several methods for increasing your daily metabolic rate.
Factors of Daily Metabolic Rate
While it’s difficult to calculate your exact daily metabolic rate without assistance, we can say what they key factors of this equation are:
- Age
- Height
- Weight
- Ratio of Muscle to Body Fat
So, in general our metabolisms slow down as we age. Additionally, the more muscle we have and the larger we are (by height and weight) the higher our daily metabolic rate will be.
From this we can conclude that using our natural metabolisms to burn fat (by way of fasting) will be most effective when we’re young and physically active, which is when our daily metabolism rate shows the highest efficiency of burning calories.
These days, instead of just reporting your weight, smart scales offer loads of data that can help you determine how your metabolism might be functioning from day to day.
Key Point: These key factors are used to calculate your daily metabolism rate. These factors also play a crucial role in increasing your daily metabolism rate.
Age and Metabolism
Research indicates that the rate at which our bodies break down food into calories slows down by about 10% each decade after reaching 20 years of age. Here’s what that looks like:
- 20 years old: Your daily metabolism rate is at its ideal efficiency
- 30 years old: 10% slower
- 40 years old: 20% slower
- 50 years old: 30% slower
- 60 years old: 40% slower
- 70 years old: 50% slower
This means that by the time you’re 70 years old, your metabolism is roughly half as efficient at burning calories as when you were 20.
Conversely, this means that fasting is a noticeably more efficient way of burning fat while you’re young.
Key Takeaway: While your metabolism slows throughout the aging process, it remains one of the safest and most reliable ways to burn fat and lose weight (by way of fasting).
How Much Weight can You Lose by Fasting?
The primary process by which you lose weight during a fast is by creating a calorie deficit. To create a calorie deficit you need to eat less calories than you are burning per day.
To burn a pound of fat, you’ll need to burn anywhere from 3,436 to 3,752 calories.
So even if you do not fast fully, by eating less calories than are burned by your daily metabolism rate your body will begin to burn fat.
- The average basal metabolic rate (BMR) for women in the United States is 1,493 calories burned per day.
- The average basal metabolic rate (BMR) for men in the United States is 1,662 calories burned per day.
Here’s some examples of what your calorie deficit could look like while fasting:
- Daily Metabolic Rate of 1,200 calories: 1,200 calorie deficit during full-fast days in the 5:2 approach, or 700 calorie deficit with one meal consisting of 500 calories.
- Burns 700 to 1,200 calories per fasting day.
- Takes 3-5 days of fasting to burn 1 pound of fat (or 2 to 3 weeks of 5:2 intermittent fasting).
- Daily Metabolic Rate of 1,500 calories: 1,500 calorie deficit during full-fast days in the 5:2 approach; or 1,000 calorie deficit with one meal consisting of 500 calories.
- Burns 1,000 to 1,500 calories per fasting day.
- Takes 3-4 days of fasting to burn 1 pound of fat (or 2 weeks of 5:2 intermittent fasting).
- Daily metabolic rate of 2,000 calories: 2,000 calorie deficit during full-fast days in the 5:2 approach, or 1,500 calorie deficit with one meal consisting of 500 calories.
- Burns 1,500 to 2,000 calories per fasting day.
- Takes about 2 days of fasting to burn 1 pound of fat.
Conclusion: Due to variations in our basal metabolic rate (BMR), we burn fat through fasting at different rates. In general, younger people with a higher BMR will be more successful at burning fat through fasting.
What if You have a Low BMR?
Because a high BMR is associated with more successful weight loss during fasts, you might wonder what to do if yours is low. There’s two main points to make here:
- Either you will want to incorporate other methods of weight loss in addition to intermittent fasting;
- Or, you will want to consider methods to increase your BMR.
And, in fact, incorporating both practices in your fitness regimen represents the optimal path to safe and effective weight loss. Here’s some popular methods to increase your BMR in order to make your fasts more effective for weight loss:
- Eat protein-rich meals.
- Drink more green tea.
- Drink more cold water; less sugary drinks.
- Do high intensity workouts and lift heavy things.
- Stand up more, particularly if you sit a lot during the day.
- Get better quality sleep.
Summary: While the metabolism–measured as BMR–is the primary system by which the body burns fat during a period of fasting, there’s some great ways to increase the rate at which your metabolism burns fat.
