Does Fasting to Boost Your Metabolism Work?

Key takeaways:

• Intermittent fasting describes an eating pattern where you only eat during a specified window of time. 
• It promotes “metabolic switching,” which leads to your body burning fat stores for energy.
• Recent studies have found that the weight loss achieved with intermittent fasting is about equal to the weight loss achieved with daily calorie restriction.
• Some studies show that intermittent fasting does not have a negative impact on metabolism, and may even boost metabolism in the short term.
• Intermittent fasting is another form of restriction. All forms of restriction are difficult to maintain for long term sustainable weight management and health.

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Intermittent fasting is an alternative to continuous calorie restriction, with studies showing that the weight loss achieved through intermittent fasting is similar to weight loss achieved through traditional dieting methods.

But is intermittent fasting bad for your metabolism?

What is intermittent fasting?

Weight management in the long-term is very complicated. However, short-term weight loss is typically achieved by expending more calories than you consume. This is usually done by reducing energy intake (e.g., calorie restriction) and/or increasing energy expenditure (e.g., exercising more).

Intermittent fasting aims to make restriction easier by restricting the times during which you eat. There are several types of intermittent fasting, including:

• Alternate day fasting: fasting for 24 hours every other day.
• 5:2 fasting: eating about 25 percent of your energy needs for two uninterrupted days per week, followed by five days of normal eating.
• Time-restricted feeding: probably the most recognized form, it involves only eating during defined “food windows.” An example would be 16-8 fasting, where an eating window lasts for eight hours, followed by fasting for the next 16 hours.
• Religious fasting: describes fasting that is done for religious or spiritual purposes.

With any fasting diet for weight loss, you can eat whatever you want during the eating window. For some, fasting may make it an easier diet to stick to because you’re not restricting any specific foods. By restricting the time during which you eat, you may find it easier to eat fewer calories. However, for most people, all forms of restriction are difficult to maintain in the long term, and long term is of utmost importance for sustainable weight management and health. 

What happens to your metabolism during intermittent fasting?

To answer the question, “Is intermittent fasting bad for my metabolism,” you need to understand how intermittent fasting affects your metabolism, since your metabolism is the process that occurs in your body in order to change the food you eat into the energy that you need to live. The main reason it’s thought that intermittent fasting can be beneficial for weight and fat loss is due to the concept of “metabolic switching.”

To understand the concept of metabolic switching, it’s important to know about the “fed-fast” cycle. This cycle has four stages:

1. The fed state (which occurs immediately after eating)
2. The post-absorptive state/early fasting state (which occurs 6 to 12 hours after the last meal)
3. The fasting state (which occurs about 12 hours after the last meal)
4. The starvation state (which occurs 2 to 3 days into a fast, when fatty acids released from fat stores are the primary fuel. If the starvation state continues, the body will start to use protein from the muscles as fuel)

Glucose (sugar), which we get from foods that contain carbohydrates, is the main fuel source for our body for most of the day. Fats can be an alternative fuel source during fasting. Whether we use glucose or fats for fuel depends on which stage of the fed-fast cycle our body is in.

In the fed state (i.e., after eating a meal), the breakdown of our body’s fat stores is turned off and glucose is the main fuel source. If we eat more calories than our body needs, they are turned into fats and stored in adipose tissue.

In the post-absorptive (early fasting) state, we get energy from the carbohydrates stored in our liver and muscles (also known as glycogen). During this stage, the process that breaks down fat from our body’s fat stores is turned back on, which leads to an increase in fats in the blood.

When we enter the fasting state (about 12 hours after our last meal), there is a metabolic shift, and our body switches to using fats for fuel because there isn’t enough glucose. The breakdown of our fat stores increases, and fats are used to produce ketone bodies. Ketone bodies are molecules produced by the liver from fats when glucose is not readily available. The three main ketone bodies are acetoacetate, 3-beta-hydroxybutyrate, and acetone. They can be used as an alternate fuel source for our brain in the absence of glucose. 

In other words, immediately after eating a meal, we use glucose for energy. When we enter the fasting state (about 12 hours after our last meal), we’re able to use our fat stores for energy. This is why it’s thought that intermittent fasting can be an effective way to lose weight – it turns on the process that allows us to burn our body’s fat stores for energy. 

Does intermittent fasting help with weight loss?

A review article published in 2022 found that the three main forms of intermittent fasting (alternate-day fasting, the 5:2 diet, and time-restricted eating) all produced weight loss of 3 to 8% from starting weight over short durations (8 to 12 weeks). The article also found that the weight loss achieved with intermittent fasting was about equal to the weight loss achieved with other diets that involve daily calorie restriction. However, more research on the long term benefit for weight loss and weight maintenance with intermittent fasting, is needed. 

What does this mean for you? Whether or not a fasting diet for weight loss and weight maintenance will be effective, likely relates to how sustainable a particular way of eating is for you. If you find intermittent fasting more sustainable than other dietary plans, you may be more likely to stick to it and therefore have better long term weight management than on other “traditional” diets. 

Does intermittent fasting slow metabolism?

Resting metabolic rate (metabolism) is the amount of energy that your body needs to perform basic functions (like breathing and maintaining a stable body temperature) at rest. Studies show that starvation (prolonged fasting) leads to a reduction in resting metabolic rate. But is intermittent fasting bad for metabolism? Probably not. A small study of non-obese men and women found that three weeks of alternate-day fasting did not significantly reduce resting metabolic rate. The study also found that alternate-day fasting led to increased fat-burning. 

Researchers looked at the effects of Ramadan fasting on resting metabolic rate and total daily energy expenditure. It found that fasting had no effect on either marker of metabolism.

Overall, it appears that intermittent fasting likely does not have a negative impact on metabolism, and may even boost metabolism in some cases in the short term. Remember that your metabolic rate is affected by many things, especially your activity level, and not just the timing of your food intake.

How long does it take for intermittent fasting to work?

Research shows that you need to fast for at least 12 hours to initiate the switch from glucose-burning to fat-burning. One of the simplest ways to achieve this is through 16-8 fasting. With this type of fasting, you’ll fast for 16 hours and eat during the remaining eight hours. For example, you may choose to have an eating window from 12:00 pm to 8:00 pm (i.e., skip breakfast and eat a normal lunch and dinner).

How do I reset my metabolism to lose weight?

Intermittent fasting may provide a way to reset your metabolism to lose weight. This is because when you fast for at least 12 hours, your body switches from using glucose as its main fuel source, to fat (which we can get from our body’s fat stores) as its main fuel source. By mobilizing the body’s fat stores, intermittent fasting can lead to weight and fat loss.

Intermittent fasting may be an effective way for some people to lose weight in the short term, especially if you find fasting to be easier to stick to than a traditional, continuous calorie reduction diet. By fasting for at least 12 hours, your body will likely switch from using glucose to fat as its main fuel source.

Many people worry that intermittent fasting is bad for their metabolism. It is unclear the effect that it has on your metabolism, but current studies show that it probably does not lower your resting metabolic rate significantly. 

Intermittent fasting is not suitable for everyone. If you’re interested in trying intermittent fasting for weight management, speak with a licensed healthcare practitioner before starting, to ensure it is safe and appropriate for you.

It's important to remember that, for most people, all forms of restriction (including intermittent fasting) are difficult to maintain in the long term, and the long term is of utmost importance for sustainable weight management and health. 

Felix’s holistic weight loss program not only includes weight loss treatment plans but also includes sustainable weight management courses that help build habits for long term weight loss. Learn more about how Felix can help you.