Amanda Pasko holds a Master’s in Nutritional Epidemiology from the Harvard T.H. Chan School of Public Health and a Master’s in Nutrition from the UNC Gillings School of Global Public Health. To her, it is rewarding to help others reach their goals.
As with many other dietary practices, intermittent fasting (IF) has its fair share of critics and skeptics.
A common concern among those who are considering IF is the fear that it may send the body into “starvation mode,” which could slow down metabolism, making weight loss more difficult or potentially causing weight gain in the long term.
This is a significant deterrent for people who are looking to improve their health or shed a few pounds.
While there’s some evidence that long-term weight loss can influence metabolic rate, the matter isn’t as straightforward as it’s made out to be.
In order to truly understand the impact of intermittent fasting on metabolism, it’s important to dissect the available research, analyze how the body responds to periods of fasting, and clear up any common misconceptions. Here’s what you need to know about the complex relationship between intermittent fasting and metabolic rate.
Fasting for extended periods can cause a drop in metabolism.
The Minnesota Starvation Experiment, which was conducted during World War II, demonstrated that prolonged calorie restriction can reduce resting metabolic rate (RMR) by as much as 40% (10).
However, this study involved extreme fasting for up to six months, which is far beyond the scope of intermittent fasting.
For example, a popular IF approach known as alternate-day fasting involves eating less than 500 calories on fast days – a stark contrast to the rigorously restricted diets in the Minnesota experiment.
In addition, our understanding of metabolism has advanced significantly since then, and many other factors can influence RMR beyond just calorie intake.
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Starvation mode, which is scientifically called adaptive thermogenesis, is a survival mechanism that our ancestors used to survive famine and prolonged periods of fasting.
In this state, the body adjusts its metabolic rate to conserve energy, burning fewer calories than normal. This natural response is triggered when the body senses an energy deficit and tries to prevent further weight loss (1).
However, this does not occur until a person reaches a dangerously low level of body fat. Research has suggested that starvation mode may only be activated in individuals with less than 10% body fat, which is considered extremely underweight.
While practicing IF, the body is not in a prolonged state of deprivation, which makes it highly unlikely that you will enter this extreme state.
Studies have shown that IF can increase metabolic rate while promoting weight loss. A study involving 11 obese adults found alternate-day fasting increased the metabolic rate of participants by 14% (7).
Hormones play a vital role in regulating metabolism and energy expenditure. Studies have found intermittent fasting can lead to hormonal changes, including an increase in human growth hormone (HGH) and norepinephrine, both of which can promote fat burning (5).
In addition, IF has been shown to increase insulin sensitivity in the body, which can improve metabolism and help regulate blood sugar levels.
Insulin is a hormone that is responsible for regulating blood sugar levels in the body.
When we eat, our bodies produce insulin to help transport glucose from the bloodstream into our cells where it can be used for energy or stored as glycogen.
However, with prolonged periods of eating, our bodies can become desensitized to insulin, which leads to high blood sugar levels and an increased risk of developing type 2 diabetes.
Research has shown intermittent fasting can improve insulin sensitivity, which allows our bodies to use insulin more effectively (3).
This helps regulate blood sugar levels and can lead to improved metabolic health in the long term(11).
Human growth hormone (HGH) is a vital hormone that promotes growth and development in children and helps regulate metabolism, body composition, and muscle growth in adults.
Studies have shown that fasting can significantly increase HGH levels, which can lead to increased fat-burning and improved metabolic health (5).
HGH also plays a role in preserving muscle mass during weight loss, which can help prevent a decrease in metabolism that often occurs with dieting (4).
Norepinephrine is a hormone and neurotransmitter that plays an important role in regulating metabolism and the body’s response to stress (8).
Studies have shown that intermittent fasting can increase norepinephrine levels, which leads to an increase in fat-burning and energy expenditure (6).
In addition, high levels of norepinephrine can decrease appetite and food intake, which makes it easier to stick to a healthy calorie deficit while practicing IF.
Read more: Green Tea Intermittent Fasting: Benefits and Side Effects
There’s not enough quality research to confirm whether or not metabolism returns to its original rate after extended fasting or intermittent fasting.
A study of participants on the reality TV show “The Biggest Loser” found that after significant weight loss, their metabolism remained lower than expected even six years later (9). This suggests that long-term weight loss may have an impact on metabolic rate.
More research is required to understand how intermittent fasting may specifically affect metabolism in the long term.
Are you interested in starting a fasting journey? Read our 30-day fasting challenge blog.
“Resetting” metabolism is a term that is used to refer to the idea of restoring metabolic rate after a period of fasting or weight loss. Unfortunately, no set duration of time guarantees metabolic reset as every person’s body and metabolism are unique.
What we know is that weight loss through intermittent fasting can increase metabolic rate and promote fat burning in the short term.
To maintain these effects long-term, it’s essential to adopt sustainable lifestyle habits, including regular physical activity and a balanced diet that is rich in whole foods.
Furthermore, avoiding drastic calorie restriction and extreme fasting may prevent a decrease in metabolic rate over time.
Fasting doesn’t make your body store fat, but it can cause temporary water retention.
When we restrict our food intake, our bodies use stored glycogen (carbohydrates) for energy first before tapping into fat stores.
As glycogen is broken down, it releases water, which can lead to a temporary increase in weight and bloating (14).
This phenomenon may explain why some people experience weight fluctuations when they start intermittent fasting.
Once the body adapts to using fat for energy, this temporary water retention often subsides, and weight loss may occur.
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The most effective intermittent fasting window for losing belly fat is broadly recognized as the 16:8 method. This involves fasting for 16 hours each day and restricting your eating to an 8-hour window.
The simplicity and flexibility of the 16:8 intermittent fasting belly fat strategy make it highly feasible for many, which is why it’s so popular. Remarkably, those who adhere to this regimen often report notable 16:8 fasting weight loss results in just 1 week.
Customizing intermittent fasting according to belly type and lifestyle can further enhance its effectiveness. For those who are wondering how they can lose belly fat overnight, it’s important to understand that while there are no outright quick fixes, consistent application of fasting methods combined with healthy eating and exercise can yield quick initial results, mostly due to the initial loss of water weight.
Prolonged fasting periods can intensify fat burning, including stubborn belly fat. However, a balanced approach that includes knowing how long to fast is essential for avoiding negative health impacts. Typically, longer fasts beyond 24 hours should be approached with caution and ideally under medical supervision.
Pairing intermittent fasting with specific routines, such as how to lose belly fat exercise plans, can magnify the fat loss effects. Exercises that target core strength and overall cardiovascular health complement fasting efforts by accelerating fat burn and improving muscle tone (13).
Find out more about this in our 15/9 Intermittent Fasting article.
Lipolysis is the process of breaking down fat stores for energy, and it typically starts after 12 hours of fasting (2). Therefore, during a 16-hour fast, your body will be burning fat for energy.
However, you should keep in mind that the total amount of fat burned is dependent on a variety of factors such as age, gender, body composition, and physical activity levels.
In addition to tapping into fat stores for energy, fasting also increases the body’s production of growth hormone, which promotes fat-burning and muscle growth.
People who practice intermittent fasting can lose an average of 4-7% of their body weight in a month. This includes both fat and muscle mass.
However, it’s important to note that the rate of weight loss may vary significantly between individuals. Factors such as starting weight, metabolic rate, and dietary habits can greatly impact the rate of fat loss.
A single 36-hour fast may not be enough to get rid of belly fat. However, consistently practicing intermittent fasting can lead to significant fat loss in both overall body weight and belly fat.
It should be noted that 36 hours falls under the category of prolonged fasting, and it should be approached with caution and under medical supervision.
Also, spot reduction (targeting fat loss in a specific area) is not possible. Fat loss occurs throughout the body and varies between individuals.
We go into more detail about this concept in our Fasting vs Starving article.
Yes, you may lose belly fat if you stop eating, but this isn’t a sustainable or healthy approach to weight loss.
Losing weight and body fat is about creating a calorie deficit, but this should be achieved through balanced nutrition and regular physical activity (12).
Simply stopping eating can lead to malnutrition, muscle loss, and other negative health effects.
Intermittent fasting offers a more sustainable and healthier approach to achieving fat loss while still nourishing the body with proper nutrition. It allows for periods of eating and fasting, which promotes fat-burning while preserving muscle mass and overall health.
Check out this 30-Day Fasting Challenge for a sustainable approach to weight loss.
While prolonged calorie restriction can lead to a decrease in metabolic rate, intermittent fasting involves shorter periods of fasting, and studies have shown that it can actually increase metabolic rate while promoting weight loss.
IF has also been linked to hormonal changes that can help improve insulin sensitivity and promote fat-burning and muscle preservation.
Generally, intermittent fasting can be a safe and effective tool for weight loss and improving metabolic health if it is done correctly.
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