Intermittent Fasting: Another Fad or Pathway to Healthier Living?

Unless you’ve been living under a rock for the past few years, you’ve likely heard of Intermittent Fasting. I’m super excited to share this article with you because of the fascinating research that has emerged on this topic. The eating trend has definitely picked up speed in the past couple of years and is continuing to grow. Admittedly, I’ve stubbornly rolled my eyes about it. After all, the concept was counterintuitive to many of the things I’ve learned as a nutrition coach and personal trainer. If you’ve been following me for a while, you probably have heard me discuss protein timing, and even mention that breakfast is the most important meal of the day….so IF or time-restricted feeding seemed to go against everything I had been taught. Also…how can you get your 120-150 grams of protein in only one or two meals?

However, as more and more people began to ask me about IF, I decided to dive deeper into the research, to understand the science behind this method of eating that is capturing growing interest both in the scientific community and among people in their quest for better health. Come to find out, there is a lot of quality research around intermittent fasting that supports it as not only a fat-loss tool but also a way to improve many other biological markers and overall health.

However, while intriguing, the research around the topic does not dismiss some of the other fitness and nutrition beliefs we have commonly held. In this article, we will dive into what the science actually tells us about intermittent fasting and who it may benefit, and who should steer clear.

In this Article:

The Science Behind Intermittent Fasting

Many of us have been conditioned to believe that consuming regular meals throughout the day is the key to maintaining a healthy metabolism. Intermittent fasting, on the other hand, turns this belief on its head. But before we dig deeper, let’s first understand what intermittent fasting is and what it means to be in a fasted versus a fed state.

What Happens in a “Fed” State

When we eat, our bodies start a series of processes to digest and absorb nutrients from the food. This period, often called the “fed” state, begins from the moment we start eating and continues as our bodies break down and metabolize the carbohydrates, proteins, and fats we’ve consumed.

During the fed state, our bodies’ primary source of energy comes from the glucose (sugar) derived from the carbohydrates in our food. As we digest and absorb these carbohydrates, our blood sugar levels rise, signaling the pancreas to release insulin. Insulin is often referred to as a “key,” unlocking our cells to allow the influx of glucose to be used for energy.

Furthermore, insulin helps with the storage of excess energy. The body prefers to use glucose as its immediate fuel source, but when we consume more glucose than the body needs for immediate energy, insulin helps convert that excess glucose into glycogen for storage in the liver and muscles.

What’s more, insulin also aids in the storage of excess energy in the form of fat. When glycogen stores in the liver and muscles are full, insulin signals the body to store any remaining excess energy in our fat cells.

For those with fat loss goals, it’s important to note that while insulin is high during the fed state, fat burning is inhibited. That’s because the body has an abundance of readily available energy from the food we’ve just eaten and the glycogen stores. Hence, the body doesn’t need to tap into its fat reserves for energy, meaning fat burning is put on the back burner.

You may also be surprised to learn that this fed state last anywhere from two to six hours or even longer after taking our last bite.

What Happens in a “Fasted” State

The “fasted” state kicks in several hours after the last meal, when your body has finished absorbing nutrients from the food. This state represents the opposite end of the metabolic spectrum from the fed state. While the fed state is characterized by digestion, absorption, and storage driven by high insulin levels, the fasted state is one of low insulin levels, energy release, and increased fat burning.

As the digestion and absorption process is completed, insulin levels begin to drop. This decrease in insulin triggers a series of hormonal events designed to release stored energy and maintain steady blood sugar levels until the next meal.

One of the significant players during the fasted state is glucagon, a hormone that acts in opposition to insulin. When insulin levels drop, glucagon levels rise, stimulating the liver to convert stored glycogen back into glucose to keep blood sugar levels stable.

However, glycogen stores only last for about 12-16 hours. If you don’t eat again before these stores are depleted, your body must turn to other energy sources, primarily your fat reserves. This shift is where the magic of fat burning happens during intermittent fasting.

During this state, another essential hormone comes into play: human growth hormone (HGH). HGH levels increase during the fasted state, promoting muscle growth and fat metabolism. In addition to helping maintain lean body mass, this hormone also aids in fat burning by signaling the breakdown of stored fat into fatty acids, which are then released into the bloodstream and transported to cells for energy.

The transition from a fed state to a fasted state is a natural part of our biological rhythm, one that our ancestors would have experienced regularly. They spent their days hunting and gathering food and fasted during the night (and sometimes longer when food was scarce). As a result, our bodies have evolved to function well, and perhaps optimally, with regular periods of eating and fasting. This balance between fed and fasted states – periods of energy storage and periods of energy use – might be a key element of human health.

The Benefits of IF — According to Clinical Research

Intermittent fasting (IF) isn’t just a weight loss fad; its potential benefits extend beyond the scales. This eating pattern may have profound implications for metabolism, brain health, cancer prevention, and even the microbiome. Here are some of the key benefits researchers have uncovered.

Weight Loss and Management

One of the most notable benefits of intermittent fasting is weight loss. Several studies show that IF helps reduce body weight and fat mass while preserving fat-free mass. According to a systematic review and meta-analysis, time-restricted feeding (TRF), a form of IF, showed greater weight loss compared to unrestricted-time regimens1. Another study confirms the effectiveness of TRF in weight reduction, showing a significant reduction in body weight and fat mass2. Finally, there is evidence that short-term fasting may improve resting caloric expenditure7, thus—promoting faster weight loss and more sustainable results. These studies provide solid evidence that intermittent fasting can be an effective tool for weight loss and management.

Improved Metabolic Health

Intermittent fasting can also improve metabolic health. Research indicates that TRF regimens significantly reduce fasting glucose concentrations1. This finding suggests that IF can be beneficial in managing blood sugar levels, potentially helping those with insulin resistance or diabetes. Moreover, TRE (Time-Restricted Eating) positively affected various cardiometabolic parameters, such as blood pressure, fasting glucose concentration, and cholesterol profiles2. This means that IF can potentially reduce the risk of heart disease and other metabolic disorders.

Autophagy and Cancer Therapy

One of the most interesting benefits of fasting to me, is the stimulation of autophagy, a process where cells recycle and renew themselves, which is crucial for maintaining cellular health4. Dysfunctional autophagy contributes to many diseases, including cancer.

Autophagy is a protective, self-cleaning mechanism where cells eliminate damaged components, including malfunctioning organelles and misfolded proteins, through a lysosomal degradation process. It’s vital for maintaining cellular health and responding to stress by recycling cellular components for renovation and homeostasis. Modulating autophagy could serve as a therapeutic strategy for a broad array of diseases, encompassing metabolic disorders, neurodegenerative conditions, cancers, and infectious diseases. Both fasting and calorie restriction are known to stimulate autophagy, aiding in cellular rejuvenation and potentially contributing to the anti-aging effects and longevity associated with these practices9.

Some evidence has already demonstrated positive effects on cancer tumors with short-term starvation or fasting8.

Enhanced Cardiometabolic Health

Accumulating evidence suggests that TRF may decrease energy intake spontaneously by 20-30% under ad libitum conditions, thus leading to modest, yet statistically significant weight loss of 1-4%. This decrease in energy intake can have broader implications. Not only does it promote weight loss, but it also alleviates the load on the digestive system during the fasting period, which might contribute to better overall gut health and efficiency.Moreover, TRF appears to significantly reduce systolic and diastolic blood pressure, independent of weight loss5.

This benefit may be attributed to the ‘resting period’ during the fast, allowing the body to maintain equilibrium without the added metabolic stress of constant digestion and nutrient assimilation. Reduced blood pressure is a critical aspect of cardiometabolic health, as it lowers the risk of heart disease and stroke.Furthermore, studies report improvements in fasting insulin levels and insulin resistance following TRF1.

Enhanced insulin sensitivity is pivotal in managing or preventing conditions like Type 2 diabetes, as it allows the body to utilize glucose more effectively, reducing blood sugar levels. Insulin resistance is also linked to heart disease; thus, its improvement can further bolster cardiometabolic health.Overall, the emerging research suggests that TRF and other forms of intermittent fasting may support cardiometabolic health by promoting weight loss, lowering blood pressure, and improving insulin sensitivity.

Management of Non-Alcoholic Fatty Liver Disease (NAFLD)

Intermittent fasting has emerged as a promising intervention in managing Non-Alcoholic Fatty Liver Disease (NAFLD), a condition characterized by an excessive accumulation of fat in liver cells, which is not caused by alcohol consumption. This disease is a significant concern, as it can lead to more severe liver conditions, such as cirrhosis, liver cancer, or liver failure6.

A systematic review and meta-analysis provided evidence that intermittent fasting is beneficial for weight management in patients with NAFLD6. As obesity is one of the leading risk factors for NAFLD, the weight loss associated with IF can contribute to a decrease in liver fat content. The weight loss may be due to IF’s ability to reduce energy intake and improve metabolic parameters, which together facilitate a healthier weight status.Moreover, IF has been found to contribute to an improvement in liver enzymes in NAFLD patients6. Abnormal liver enzyme levels are often indicative of liver damage or inflammation; thus, their reduction is a favorable outcome. The exact mechanism behind this improvement remains unclear. However, it might be related to the overall reduction in caloric intake, improved insulin sensitivity, or the effects of autophagy – a cellular ‘housekeeping’ process that IF enhances.

This promising evidence suggests that IF can be a powerful tool in managing NAFLD. However, it’s important to note that IF is an adjunctive lifestyle modification, not a substitute for comprehensive medical care. Further research is required to refine IF protocols best suited for patients with NAFLD and to understand the long-term impacts of such dietary patterns.

Mental Health Benefits

Intermittent fasting has been associated with various mental health benefits11. This is due to the fact that periods of fasting can have an impact on the body’s neurochemistry, which may, in turn, influence mood, cognition, and overall mental health. Notably, intermittent fasting has increased levels of brain-derived neurotrophic factor (BDNF), a protein that plays a critical role in neuroplasticity, learning, and memory. Reduced levels of BDNF are associated with various mental health disorders, including depression and anxiety. Additionally, by reducing inflammation, improving sleep, and promoting autophagy, intermittent fasting may help alleviate symptoms of mood disorders and improve overall mental well-being.

Impact on Gut Microbiome

Intermittent fasting can also profoundly affect the gut microbiome10 – the community of trillions of bacteria that live in our digestive tract. Recent studies have indicated that fasting can modulate the composition of the gut microbiota, leading to increased diversity – a trait generally associated with better overall health. This change in microbial composition can positively impact metabolism, immune function, and mood regulation, among other aspects of health. Moreover, intermittent fasting may promote the growth of certain beneficial bacterial species that can produce compounds like short-chain fatty acids, which have been linked to improved gut health and reduced risk of chronic diseases10.

  1. Effects of time-restricted feeding on body weight and metabolism. A systematic review and meta-analysis
  2. Beneficial Effects of Time-Restricted Eating on Metabolic Diseases: A Systemic Review and Meta-Analysis
  3. Autophagy and intermittent fasting: the connection for cancer therapy?
  4. Impact of intermittent fasting on health and disease processes
  5. Cardiometabolic Benefits of Intermittent Fasting
  6. Effect of Intermittent Fasting on Non-Alcoholic Fatty Liver Disease: Systematic Review and Meta-Analysis
  7. Resting energy expenditure in short-term starvation is increased as a result of an increase in serum norepinephrine
  8. Fasting cycles retard growth of tumors and sensitize a range of cancer cell types to chemotherapy
  9. The effect of fasting or calorie restriction on autophagy induction: A review of the literature
  10. Gut Microbiota and Time-Restricted Feeding/Eating: A Targeted Biomarker and Approach in Precision Nutrition
  11. Fasting in mood disorders: neurobiology and effectiveness. A review of the literature

Different Methods of Intermittent Fasting

There are several popular methods of intermittent fasting, each with a distinct schedule and potential effects on the body. The major methods include:

16/8 Method

The 16/8 method, also known as time-restricted feeding (TRF), involves fasting for 14-16 hours and confining all calorie consumption within an 8-10 hour window each day. You may, for instance, skip breakfast and consume your meals between 12 PM and 8 PM.

During the fasting window, the body enters the post-absorptive state, initially relying on stored glycogen for energy. As these reserves deplete, usually after about 12 hours, the body transitions into a fasting state where it begins to break down stored fat for energy and produces ketone bodies.

This method can lead to weight loss, improved blood glucose control, and potentially enhanced autophagy during the fasting window. The convenience of this method and its alignment with the body’s circadian rhythm may also promote better sleep and metabolic health.

The 16/8 method may be ideal for those new to intermittent fasting, as it is often easier to follow than some other fasting schedules. It may also be a good fit for people seeking weight loss or improved metabolic health.

5:2 Diet

The 5:2 diet, or the Fast Diet, involves eating normally for five days of the week and restricting caloric intake to 500-600 calories on two non-consecutive days.

On low-calorie days, the body is in a calorie deficit and will likely resort to using stored glycogen and fat for energy, leading to weight loss. However, as these fasting periods are relatively short, the body might not fully transition into a deep fasting state involving significant ketone production or autophagy.

This method can result in weight loss and improvements in metabolic markers such as blood pressure, cholesterol levels, and insulin sensitivity.

he 5:2 diet may be suitable for individuals who prefer to focus their weight loss efforts on just two days of the week and enjoy more freedom on other days. It’s also beneficial for those who find daily calorie counting or restriction difficult to sustain.


This approach involves one or two full 24-hour fasts per week. During these fasts, no solid food is consumed, though zero-calorie beverages like water, coffee, or tea are allowed.

A 24-hour fast allows the body to deplete glycogen stores fully and transition into a deeper fasting state. This state enhances fat oxidation, raises ketone production, and potentially triggers cellular repair processes like autophagy.

The Eat-Stop-Eat method can promote weight loss, improve metabolic health markers, and possibly enhance cellular repair and longevity due to increased autophagy.

This method might appeal to those who prefer a more intense, less frequent fasting approach and can tolerate going a full day without eating solid food.

Alternate-Day Fasting

As the name suggests, alternate-day fasting (ADF) involves fasting every other day. There are different versions of ADF, some involving no caloric intake on fasting days, and others allowing for about 500 calories.

Fasting every other day ensures frequent entry into the fasting state, during which glycogen stores are depleted and fat oxidation and ketone production become the body’s primary energy sources. This frequent fasting may stimulate adaptive cellular stress responses more often, which can promote autophagy and other beneficial processes.

ADF can lead to significant weight loss and improvements in various health markers, including insulin sensitivity, blood pressure, and cholesterol levels.

ADF may be beneficial for those seeking substantial weight loss or health improvements who can tolerate full-day fasts every other day.

Extended Fasting

Extended fasting involves fasting for periods exceeding 24 hours, often ranging from 48 hours to a week or more. During the fast, only zero-calorie beverages are allowed.

Extended fasting leads to profound metabolic shifts, including substantial glycogen depletion, increased fat oxidation, and elevated ketone production. It also allows for more prolonged periods of autophagy and other cellular repair processes.

Extended fasting can result in substantial weight loss, metabolic health improvements, and potentially enhanced cellular repair and longevity.

Extended fasting might be an option for experienced fasters looking for significant health or weight loss results.

Contraindications for Fasting

While the benefits of intermittent fasting make it a compelling eating schedule for most healthy individuals, there are some negative effects that you should be aware of.

Impact on Testosterone Levels and Sperm Count

Intermittent fasting, like any significant change in diet or lifestyle, can have effects on hormone regulation. For instance, some studies suggest that intermittent fasting can potentially lower testosterone levels. This may result from metabolic stress that fasting can place on the body, potentially leading to hormonal imbalances. Similarly, intermittent fasting might negatively affect sperm count although research in this area is still somewhat limited and the mechanisms aren’t fully understood.

Interference with Muscle Hypertrophy

For athletes and bodybuilders, or anyone interested in muscle hypertrophy, intermittent fasting might not be the optimal approach. While fasting, the body may not get enough protein frequently throughout the day to stimulate muscle protein synthesis, an important process for muscle growth. Furthermore, post-workout feeding windows may not align with fasting schedules, which could hinder recovery and muscle growth.

Risk of Disordered Eating

Intentional, controlled fasting is distinct from disordered eating, but it’s essential to note that for some people, the structure of intermittent fasting might unintentionally perpetuate or exacerbate disordered eating behaviors. Individuals with a history of eating disorders may find that the rules and restrictions of intermittent fasting could trigger unhealthy patterns of behavior. Therefore, intermittent fasting is not suitable for everyone and should be considered carefully, preferably with professional guidance, particularly for those with a history of eating disorders.

What Breaks a Fast?

Understanding what breaks a fast is essential in making intermittent fasting effective and beneficial. Fasting, in its most fundamental form, is a state of not eating, where the body shifts from using immediate dietary calories for energy to using stored calories. The determination of what technically ‘breaks’ a fast largely depends on why you’re fasting in the first place, as different goals might dictate different fasting protocols and standards. For instance, if your main goal is weight loss, your fasting rules may differ from someone who is fasting primarily for autophagy.

In the most traditional sense, consuming anything with caloric content technically breaks a fast. However, the influence of non-caloric or minimal-caloric consumables like water, black coffee, tea, and even some supplements on a fast can be subject to debate, but most likely do not break a fast.

How to Get Started with Fasting

Starting with intermittent fasting can seem intimidating, but it doesn’t have to be. It is a flexible dietary practice that can be tailored to your individual needs and lifestyle. Here are some guidelines to help you begin your fasting journey.

  1. Identify Your Goal: Before you begin fasting, it’s essential to identify why you want to fast. Are you trying to lose weight, improve your metabolic health, increase your lifespan, or improve your mental clarity? Your goal will determine the fasting method you should adopt.
  2. Choose the Right Fasting Method: Once you’ve identified your goal, the next step is to choose the right fasting method that suits your lifestyle and dietary preferences. It can range from time-restricted eating (like the 16/8 method) to alternate-day fasting. Beginners might find it easier to start with a less restrictive method like the 12/12 or 16/8 approach before moving on to longer fasting periods.
  3. Plan Your Eating Window: If you choose a time-restricted eating method, decide on your eating window. Some people prefer eating earlier in the day (e.g., from 7 AM to 3 PM), while others might prefer later windows (e.g., 12 PM to 8 PM). Choose a window that you can stick to consistently.
  4. Stay Hydrated: During your fasting hours, it’s crucial to stay hydrated. You can drink water, unsweetened tea, or black coffee.
  5. Eat Balanced Meals: During your eating window, focus on consuming nutrient-dense, balanced meals. Try to include a good mix of protein, healthy fats, and complex carbohydrates. Avoid the temptation to overeat or indulge in unhealthy food choices because you’re fasting.
  6. Listen to Your Body: Everyone’s body responds differently to fasting. It’s crucial to listen to your body and adjust your fasting regimen if needed. If you feel dizzy, excessively tired, or unwell, it might be best to end your fast and consult with a healthcare professional.
  7. Seek Medical Advice: If you have any underlying health conditions or are on medication, it’s crucial to seek advice from a healthcare provider before starting any fasting regimen. Certain individuals, like those with diabetes, pregnant or breastfeeding women, and those with a history of eating disorders, should not fast without medical supervision.
  8. Be Patient and Consistent: Like any other lifestyle change, it takes time to see the benefits of fasting. Be patient and consistent with your fasting regimen. Over time, your body will get used to the new eating pattern, and you might start to see the positive effects.

Closing Thoughts

Intermittent fasting represents a dynamic and flexible approach to health enhancement that goes beyond the traditional focus on what we eat to also consider when we eat. It has shown promise in various domains, including weight management, cardiometabolic health, mental well-being, and even longevity. Different methods of intermittent fasting can cater to individual preferences, lifestyle, and health goals. However, like any dietary intervention, it’s not a one-size-fits-all solution.

Fasting can positively influence numerous biological pathways, from enhancing cellular repair through autophagy to potentially reshaping gut microbiota composition. Still, more research is needed to fully understand its long-term effects and potential drawbacks. Intermittent fasting might not be suitable or beneficial for everyone, particularly for individuals with certain health conditions, those who are pregnant or breastfeeding, or individuals with a history of disordered eating. Thus, while exploring the benefits of intermittent fasting, it’s crucial to approach it with mindfulness and informed guidance.

Always remember that successful health interventions should fit within the context of a balanced, healthy lifestyle rather than act as a standalone ‘magic bullet.’ It’s not merely about eating and fasting, but about making conscious choices for long-term health and well-being. As with any significant dietary change, it’s essential to consult a healthcare provider before starting intermittent fasting. Understand your body’s unique needs and responses, listen to it, and adapt as necessary. Intermittent fasting can be a powerful tool for health and wellness, but it should serve your needs and lifestyle, not the other way around.

Disclaimer: I am a certified nutrition coach and personal trainer, not a registered dietician or physician. The information in this post are meant to be for educational purposes only and should not be considered as medical advice. Please consult a licensed professional for specific medical advice.

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