Fact Checked By Dr B Lee
Intermittent fasting and its impact on metabolism have been topics of great interest in the health and wellness community. Many people believe that intermittent fasting can boost metabolism, leading to weight loss and other health benefits. However, there is still some debate and conflicting information on how intermittent fasting truly affects metabolism. In this article, we will delve into the research and explore the relationship between intermittent fasting and metabolism.
Understanding Intermittent Fasting
Intermittent fasting, also known as IF, is an eating pattern that involves alternating periods of fasting and regular eating. This approach to eating has gained popularity due to its potential benefits for weight loss, disease prevention, and even longevity. During the fasting periods, individuals restrict their calorie intake, allowing their bodies to tap into stored fat for energy.
Types Of Intermittent Fasting
One popular method is the 16/8 or time-restricted eating, where individuals fast for 16 hours and consume their meals within an 8-hour window. This method allows for flexibility in adjusting the fasting and feasting periods to suit individual preferences and lifestyle.
Another approach is the one-meal-a-day (OMAD) method, which entails fasting for 23 hours and consuming all meals within a one-hour window.
The 5:2 method involves eating without restrictions for five days and severely restricting calorie intake (typically 25% of total calorie intake) on the remaining two days of the week.
The 16/8 method is generally considered relatively healthy, whereas OMAD and 5:2 have significant drawbacks.
For example, OMAD severely limits muscle protein synthesis as does 5:2 and generally they lead to side effects of intermittent fasting, like difficulty sleeping. 5:2 and OMAD also has the disadvantage of lowering NEAT  (the amount of movement that people do subconsciously through the day) and can actually end up lowering your caloric burn in many cases.
The Relationship Between Intermittent Fasting and Metabolism
A key concern for individuals considering intermittent fasting is its potential impact on metabolism. After all, a healthy metabolism is crucial for maintaining an optimal weight and overall well-being.
So, does intermittent fasting have a positive or negative effect on metabolism? Let's explore the scientific evidence and expert opinions to find out.
The Benefits of Intermittent Fasting for Metabolism
Numerous studies have indicated that intermittent fasting may have a positive impact on metabolism. One significant benefit is the potential for increased fat burning. When we fast, our body's hormone levels and metabolic processes undergo changes that promote the breakdown of stored fat for energy. This shift in fuel utilization can enhance weight loss and improve body composition. 
Furthermore, intermittent fasting has been shown to influence the production of certain hormones involved in metabolism. Hormones such as growth hormone and norepinephrine increase during fasting periods, leading to enhanced fat mobilization and utilization.  This hormonal response can contribute to a more efficient metabolism and facilitate weight loss. However, as we'll come to later there's a bit more to the the effects of intermittent fasting.
Additionally, intermittent fasting has shown promise in improving insulin sensitivity, a crucial factor in metabolic health.
Research suggests that intermittent fasting can enhance insulin sensitivity and reduce insulin resistance, leading to better blood sugar control and decreased risk of type 2 diabetes. 
These metabolic benefits may contribute to a healthier metabolism and overall improved metabolic health.
The Role of Caloric Restriction
It's important to note that intermittent fasting often involves a reduction in overall calorie intake, especially in methods like the 5:2 approach or OMAD.
Caloric restriction has long been recognized as a strategy for weight loss and metabolic improvement. By consuming fewer calories, individuals create an energy deficit that can lead to weight loss and improved metabolic markers. 
However, it is essential to distinguish between the effects of intermittent fasting itself and the effects of caloric restriction. Some studies have compared intermittent fasting to continuous calorie restriction and found similar weight loss outcomes, suggesting that the metabolic benefits of intermittent fasting may be attributed to the caloric deficit rather than the fasting pattern itself.
Therefore, the benefits observed in intermittent fasting studies may not be exclusive to the fasting periods but rather a result of overall calorie reduction.
The Psychology Of Intermittent Fasting
Ultimately a lot of what makes intermittent fasting work, is that some people are simply better at sticking to diets with time restrictions than food restrictions. It’s more finite. If you’ve missed your window, then so be it. 
And in cases of diets like OMAD, there is only so much time to consume calories.
It can lead to some negative psychological issues as a result of low blood sugar and poor sleep, most notably irritability, as well as have a negative impact on cognition overall. 
The Effect of Intermittent Fasting for Weight Loss
One of the primary reasons people turn to intermittent fasting is for weight loss.
The idea behind this approach is that by restricting calorie intake during fasting periods, the body will burn stored fat for energy, resulting in weight loss.
When the body's metabolism is in a fasted state, it can efficiently utilize stored fat as an energy source.
You do however, subconsciously move less, which does counter balance this somewhat. As NEAT makes up between 5-15%  of calorie burn per day, you could inadvertently lose 10% of your daily burn.
Once again, intermittent fasting comes down to whether the restrictions help you stay in a caloric deficit by eating less. Although there is evidence that it could raise metabolism, as much as there is evidence that it could lower metabolism.
Other Potential Health Benefits Of Intermittent Fasting
Intermittent fasting has been associated with various health benefits beyond weight loss. Research suggests that it may help lower blood pressure, improve metabolic health,  and even reduce the risk of cardiovascular disease. A Harvard study found that intermittent fasting could potentially extend lifespan and improve overall health by activating certain cellular processes.
The Impact on Metabolism Over Time - Or When Intermittent Fasting Can Lower Your Metabolism
The relationship between intermittent fasting and metabolism is a complex one. While some studies suggest that intermittent fasting may increase metabolism, others indicate that it has little to no effect. Let's explore the different factors that may influence how intermittent fasting affects metabolism.
Short-term intermittent fasting has been shown to have a positive impact on metabolism. Some studies have found that fasting for brief periods can actually increase metabolism, rather than slowing it down. For example, a study involving 11 healthy men revealed that a three-day fast increased their metabolism by 14%. 
This increase may be attributed to the rise in the fat-burning hormone norepinephrine, which is stimulated during fasting. 
Hormones play a crucial role in metabolism and weight regulation. Insulin, in particular, is a hormone that affects fat metabolism. High levels of insulin can make it challenging to lose weight and have been linked to various health conditions. Intermittent fasting has been shown to effectively lower insulin levels, which can contribute to improved metabolism and weight loss.
Another hormone affected by intermittent fasting is human growth hormone (HGH). Fasting has been found to increase blood levels of HGH,  which promotes fat loss and muscle preservation. However, the effects of fasting on HGH levels may vary between men and women,  and more research is needed to fully understand its impact.
Norepinephrine,  a stress hormone, is also involved in the fat-burning process. Fasting has been shown to increase norepinephrine levels, leading to the release of fatty acids for energy. This can potentially enhance fat burning and contribute to weight loss.
While short-term intermittent fasting may have positive effects on metabolism, the long-term impact is less clear. Severe calorie restriction over an extended period can cause the body to enter a state known as adaptive thermogenesis, where it conserves energy to prevent starvation.
This can lead to a decrease in metabolic rate and hinder weight loss efforts. 
A study involving participants from "The Biggest Loser" reality show demonstrated the long-term effects of weight loss and metabolic rate. Even after regaining most of the weight, the participants' metabolic rates remained significantly lower than expected for their body size. This highlights the challenges and potential metabolic adaptations that can occur with sustained calorie restriction.
The Impact of Fasting Duration and Gender
The duration of fasting periods and individual factors, such as gender, may also influence the effects of intermittent fasting on metabolism. Studies have shown that longer fasting periods may lead to more significant metabolic changes, including increased fat burning and improved insulin sensitivity. However, it's important to find a fasting duration that is sustainable and suitable for your individual needs and lifestyle.
Moreover, some research suggests that the metabolic response to intermittent fasting may vary between males and females. A study published in the Journal of Endocrinology  found that intermittent fasting led to more prominent metabolic benefits in male rats compared to female rats. More research is needed to determine the extent of these gender-specific differences in humans.
Preserving Lean Body Mass
Preserving lean body mass, such as muscle, is crucial for maintaining a healthy metabolism. Losing weight often results in a combination of fat and muscle loss. However, intermittent fasting may help preserve muscle mass better than continuous calorie restriction. 
Human growth hormone, which increases during fasting, plays a role in preserving muscle mass. It promotes fat burning while preserving lean tissue.
Some studies suggest that intermittent fasting is more effective at retaining muscle during weight loss when combined with weight training specifically  than traditional low-calorie diets.
However, the research on this topic is mixed, as there is also some evidence in the other direction and further studies are needed to draw definitive conclusions.
Practical Considerations and Tips for Intermittent Fasting
If you are considering incorporating intermittent fasting into your lifestyle, there are a few practical considerations to keep in mind. Firstly, it is essential to consult with a healthcare professional or registered dietitian before starting any new dietary regimen, especially if you have any underlying health conditions or concerns.
Here are some tips to help you navigate intermittent fasting successfully:
Start gradually: Begin with shorter fasting periods and gradually increase the duration as your body adapts.Stay hydrated: Drink plenty of water during fasting periods to stay hydrated and help manage hunger.Choose nutrient-dense meals: When breaking your fast, prioritize whole, nutrient-dense foods to ensure you meet your body's nutritional needs.Listen to your body: Pay attention to your hunger and fullness cues. If you feel unwell or experience any adverse effects, adjust your fasting approach accordingly or consider alternate strategies.Maintain a balanced diet: While intermittent fasting can be beneficial, it is still important to consume a balanced diet with adequate macro- and micronutrients to support overall health.
Does Intermittent Fasting Work? Conclusion
In conclusion, intermittent fasting has gained popularity as a potential tool for weight management and improved metabolic health if used foor the short term.
The evidence suggests that intermittent fasting can slightly enhance fat burning, improve insulin sensitivity, and potentially lead to weight loss.
However, it's important to note that the metabolic benefits may be largely attributed to the overall caloric restriction rather than the fasting pattern itself. And that there is evidence of some risk to metabolic behaviour when used for the long term.
What is important is whether intermittent fasting works for you over other diet options and it’s something that you can stick to till you reach you goals.
Ultimately weight loss requires a caloric deficit and there are other ways to achieve this, but for those who like the structure of intermittent fasting it’s a perfectly valid approach.
Individual factors, such as fasting duration and gender, may also play a role in the metabolic response to intermittent fasting. Finding a fasting approach that suits your lifestyle and preferences is crucial for long-term adherence and success.
About Fact Checker Dr B Lee
Dr B Lee Dr Lee is a member of Center TRT's research team, he has more than 200 scientific publications, and has been a member of the global obesity center. With more than 20 years experience in health, focusing on wholistic treatment, lifestyle and dietary changes to improve health outcomes on a regional and nationwide level. View Profile
1 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4516560/
2 - https://pubmed.ncbi.nlm.nih.gov/26374764/
3 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC329619/
4 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7856758/
5 - https://www.nih.gov/news-events/nih-research-matters/calorie-restriction-immune-function-health-span
6 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8754590/
7 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8470960/
8 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6058072/
9 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9512108/
10 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8932957/
11 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8839325/
12 - https://pubmed.ncbi.nlm.nih.gov/26653760/
13 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9182756/
14 - https://pubmed.ncbi.nlm.nih.gov/7723665/
15 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9036397/
16 - https://pubmed.ncbi.nlm.nih.gov/35371260/
17 - https://pubmed.ncbi.nlm.nih.gov/34179054/
18 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7468742/