MythBusters: Intermittent Fasting Slows Down Your Metabolism

intermittent fasting and metabolism

Have you considered intermittent fasting for weight loss, but read or heard from a friend that it would slow your metabolism? That seems counterintuitive, especially if your goal is to lose unwanted body fat. Thankfully, you don’t have to believe everything that you read or hear.

No evidence shows intermittent fasting slows your metabolism. On the contrary, periods of time-restricted eating can improve your metabolism and offer other health benefits, like a reduction in your blood pressure and improvement in your sleeping habits. 

If you’re considering a new healthy lifestyle but are concerned about the adverse effects intermittent fasting could have on your metabolism, Simple has all the facts you need to make an informed decision. 

What’s the Big Deal About Metabolism?

Chances are you’ve seen the word “metabolism” in all of your research about weight loss. But what is metabolism? Metabolism is a chemical reaction that occurs in your cells to convert your food into energy. The metabolism process typically occurs when you’re asleep or at rest.

Once food enters your digestive system, enzymes break proteins into amino acids, transfer fat into fatty acids, and convert carbohydrates into glucose for fuel. You may think of this as the calorie-burning or energy creation process. When you eat more calories than you need to fuel your body, you store those excess calories as body fat. 

Scientists use your basal metabolic rate (BMR) to measure how quickly your body burns calories for energy while at rest. If you have a low BMR, you’ll consume fewer calories at rest, and possibly gain more body fat than a similar-sized individual with a high BMR who eats the same amount. But how does intermittent fasting affect your BMR? 

How Intermittent Fasting Affects Your Body at the Metabolic Level

Many specific metabolic changes happen when you fast for an extended period. For one, your body finds alternative ways to create sources of energy. Typically, your body breaks down carbohydrates from your food into glucose, which powers your body. When you refrain from eating, your body must make glucose from noncarbohydrate resources instead. This process is called gluconeogenesis.

When your body enters gluconeogenesis, it uses glycogen reserves in your liver and water stored within your tissue to create glucose for energy. Once your glycogen stores are depleted, your body turns to other noncarbohydrates for fuel, such as amino acids. The more your body searches for alternative sources of energy, the more levels of specific metabolites in your blood increase. 

In particular, scientists have identified a significant increase in products of the citric acid cycle, which helps your cells release stored energy. As citric acid metabolites increase, your cells are thrown into overdrive, creating and storing excess energy to power you through your fast. In other words, your body doesn’t slow down from lack of food; it goes into overdrive. 

Does Intermittent Fasting Increase Fat-Burning Hormones? 

intermittent fasting metabolism

Hormones are your body’s natural messengers. They coordinate most of your bodily functions, including weight regulation, appetite, and muscle growth. Hormones also influence the amount of fat you store or burn and heavily influence metabolism. There’s a link between intermittent fasting and improvements in the production of your metabolism-boosting hormones, including insulin, human growth hormone (HGH), and norepinephrine. 

Insulin is a hormone at the forefront of fat metabolism. It dramatically affects your BMR by telling your body when to store fat and when to stop breaking it down. High insulin levels can not only make it more challenging to lose weight but also increase your risk of heart disease and type 2 diabetes. Studies show intermittent fasting can reduce insulin levels by 20 to 31 percent, making it easier to control weight loss and increase your metabolism. 

Human growth hormone (HGH) is a critical hormone for retaining muscle mass and promoting fat loss. Scientists found a 24-hour fast can increase HGH production by 2,000 percent in men and 1,300 percent in women. An increase in HGH can stimulate protein and carbohydrate metabolism, helping to improve both your BMR and overall weight loss efforts. 

Lastly, there’s also a link between intermittent fasting and an increase in norepinephrine, a stress hormone that helps your body’s fat cells to release fatty acids. As more norepinephrine circulates in your bloodstream, a more significant amount of fat becomes available for your body to burn. This release of fat helps you lose weight more effectively and can improve metabolic processes. 

Does Intermittent Fasting Slow Your Metabolism?

It’s a common misconception that intermittent fasting will slow your metabolism when, in reality, it has the potential to do the exact opposite. Researchers have found that fasting for short periods, such as a 16:8 protocol, can increase your metabolism

A study by the University of Vienna found that a three-day fast can increase your metabolism by 14 percent. Researchers believe the cause for a spike in metabolism is due to an increase in norepinephrine, which helps make more fat available to burn. While a three-day fast may seem extreme, you can accomplish the same fluctuation in norepinephrine with a milder fast. Research suggests that even 12-hour fasts increase metabolic processes and support weight loss. 

Traditional Diets vs. the Intermittent Fasting Effect on Metabolism

intermittent fasting effect on metabolism

You may be wondering if traditional diets, like calorie counting, can have the same effect on your metabolism as intermittent fasting. Unfortunately, that’s not physically possible. The hormonal and metabolic changes that occur during intermittent fasting only happen because your body is without food entirely. Though calorie-restricted diets lessen the amount you eat, you still are constantly intaking food. Therefore, your body will utilize the carbohydrates or sugars you’ve digested for energy, without using any of your body’s existing fuel sources. 

In essence, it’s the extended periods of restricted eating that allow your body to experience positive changes in metabolism. Even by cutting carbs and eating less per meal, you can’t mimic the effects of intermittent fasting by counting calories. 

Why Calorie Counting Can Cause Starvation and a Plateau Effect

The basis of intermittent fasting is if you eat correctly and nutritionally during your feasting window, you can fill your body with the proper nutrients to withstand a fast healthily. In comparison, calorie restriction minimizes the amount you eat over the entire day, potentially causing you to run low on nutrients despite eating all day. Over an extended period, the severe calorie restriction can cause your body to enter “starvation mode.”

Starvation mode is your natural defense against starvation—which can result when you restrict calories beyond what your body needs. When you enter starvation mode, you may feel sluggish, cranky, and tired. What’s worse, is it can cause your BMR to drop. And research has found starvation caused by calorie restriction can decrease metabolism for good. 

Once your metabolism has slowed, it can lead to a plateau effect, where you no longer seem to lose weight. Following a plateau, a calorie-restricted diet is also more likely to cause you to regain the weight you’ve lost. 

Myth Busted: Intermittent Fasting and Metabolism 

Does intermittent fasting slow down your metabolism? All research points to no. If you’ve been waiting to try intermittent fasting, now is the time to reap all the benefits it has to offer—no loss in metabolism needed. The free Simple app has all the tools you need to start efficiently, including meal, hydration, and fasting trackers; personalized health insights; and tons of useful articles.

Author's bio


Grace Trumpfeller

Grace is an avid health and wellness advocate with nearly a decade of experience in the diet and nutrition industry. With a vast background in journalism and creative writing, her core focus is to empower individuals to reclaim their health and vitality by providing expert advice rooted in ...