What Is Intermittent Fasting? An Expert Explains

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As much as I love to feast, I also love to fast. Forgoing food (or any calorie-containing beverages) for periods of time had a positive impact on both my brain and body during my 75-pound weight loss journey, leading me to become a certified personal trainer (CPT).

Yet even though people have been practicing intermittent fasting (IF) for thousands of years, it’s often miscategorized as just another fitness trend. But there’s plenty of anecdotal and scientific evidence to support its efficacy. While IF isn’t some magic pill that’ll melt away body fat or transform your brain into a supercomputer, it can provide many health benefits when implemented correctly.

To help you avoid some of the mistakes I made in the beginning, I’ve put together this comprehensive guide on everything you need to know to reap the rewards of intermittent fasting.

Editor’s Note and Medical Disclaimer: This article is intended for educational and informational purposes only. It is not intended as a substitute for medical advice. Weight loss programs are not recommended for people with a history of disordered eating. For health advice, contact a licensed healthcare provider. If you need help with disordered eating, contact NEDA.

What Is Intermittent Fasting?

Intermittent fasting is a simple concept that people love to overcomplicate. All it entails is alternating between periods of eating and abstaining from food or any calorie-containing drinks. Some IF followers can be even stricter, sticking to only water during their fasting window.

Unlike traditional diets that focus on what to eat, intermittent fasting focuses on when to eat. Popular forms include time-restricted eating (TRE), where you eat within a specific window each day, and alternate-day fasting (ADF), where fasting occurs every other day. While this may seem like a foolproof way to reduce the total number of calories you consume for the week, it’s still important to follow a healthy diet rich in protein, healthy fats, fiber, and micronutrients.

In other words: Don’t use fasting as an excuse to gorge on snacks, sweets, and treats.

How Does Intermittent Fasting Work?

What happens when you don’t put any fuel in your proverbial tank? While just the idea of skipping breakfast might sound scary, your body will be able to enter a fasting state where it shifts from using glucose as its primary fuel source to burning stored fat for energy.

Once your body exhausts its glycogen stores (glucose stored in your muscles and liver), you’ll start to shift into ketosis. This metabolic change boosts the production of human growth hormone (HGH)¹, which can help with fat loss, muscle gain, and cellular repair.

RELATED: What Is The Keto Diet?

Ironically, IF goes hand in hand with what your body’s programmed to do: take care of itself. Fasting promotes autophagy²—a natural process where your cells clean out damaged components and regenerate new ones. Aligning your eating pattern with your body’s natural rhythms and processes can be a powerful tool to have in your arsenal.

Forms of Intermittent Fasting

Just like there are many types of workout splits, there are many forms of intermittent fasting. From short periods of calorie restriction to going one or two days of the week without eating, let’s explore the different methods of implementing IF.

Time-Restricted Feeding

Time-restricted feeding involves consuming all your daily calories within a specific time frame. One of the most common forms of IF is 16/8, where you have a 16-hour fasting period, followed by an eight-hour eating window.

A less aggressive approach is 14/10, giving you two extra hours to get your calories in. Overall, TRE is popular for its simplicity and flexibility.

Alternate-Day Fasting

Alternate-day fasting entails fasting every other day. Some versions allow you to consume a small amount of food (up to 500 calories) on fasting days. This method can be more challenging but is effective for significant calorie reduction.

According to a 2020 meta-analysis³ on the health effects of alternate-day fasting, this method is a viable diet strategy for weight loss. Pairing this protocol with a well-formulated eating plan and a structured strength training routine can help you preserve lean muscle as you shed body fat.

5:2 Diet

The 5:2 diet involves eating normally for five days a week and restricting calories to 500 to 600 on the remaining two days. On non-fasting days, you should hit your target amount of calories by eating lean proteins, vegetables, fruit, and other whole food sources. This intermittent fasting protocol is known for its flexibility and sustainability, as you can strike a decent balance between fasting and non-fasting days.

A 2022 pilot study⁴ on the effects of a 5:2 intermittent fasting plan on overweight or obese adults showed that the fasting protocol more effectively led to weight loss than short-term daily calorie restriction.

Eat-Stop-Eat

The eat-stop-eat fasting diet involves fasting for 24 hours once or twice a week. This method is straightforward but can be especially challenging for beginners. However, it offers flexibility for those who prefer less frequent fasting.

The Warrior Diet

Developed by Ori Hofmekler, The Warrior Diet pays tribute to our ancestral roots. This type of intermittent fasting protocol emphasizes undereating during the day and overeating at night.

The Warrior Diet includes a 20-hour fasting window where you can only consume small amounts of raw fruits and vegetables. Then, you should eat a large dinner rich in protein, healthy fats, and carbohydrates. This method mimics the eating patterns of ancient warriors and is suitable for those who prefer eating one substantial meal a day.

Extended Fasting Periods

Extended fasting periods, such as 24-hour fasts or longer, involve abstaining from food for more extended periods. These should be approached with caution and preferably with the help of a registered dietitian or doctor, as they can lead to rapid weight loss and significant metabolic changes.

A 2024 review⁵ on the efficacy and safety of prolonged water fasting found that fasting for five to 20 days produces “potent increases in circulating ketones” and mild to moderate weight loss of 2% to 10%. However, two-thirds of the weight loss is lean muscle—something you don’t want if you care about looks or longevity.

Overall, I think an occasional longer-duration fast can be helpful, but it’s not a regular practice I’d recommend.

Benefits of Intermittent Fasting

While IF can help with weight management and fat loss, that’s not the only reason to incorporate it into your health and wellness routine. Let’s explore the benefits of this practice.

May Help With Weight Loss

Intermittent fasting can be highly effective for weight loss and fat loss due to its ability to reduce overall calorie intake and enhance metabolic processes. When fasting, your body shifts from using glucose to tapping into stored body fat. This metabolic switch, known as ketosis, can significantly aid in reducing body weight and body fat.

May Help Improve Metabolic Health

Intermittent fasting can also help improve your metabolic health by enhancing your insulin sensitivity—your body’s ability to efficiently utilize glucose. People who consume ultra-processed foods loaded with sugar and fat typically become insulin-resistant, which can lead to metabolic syndrome⁶, obesity, and a host of other issues.

However, research shows⁷ that IF can help lower fasting insulin levels, decrease insulin resistance, and improve overall glucose metabolism. Fasting can also help regulate blood sugar levels, reduce your risk of type-2 diabetes, and prevent metabolic syndrome and other related conditions.

May Support Cardiovascular Health

Intermittent fasting has been shown to positively impact cardiovascular health⁸ by reducing various risk factors. Regular fasting can lower blood pressure, decrease levels of LDL cholesterol (often referred to as “bad” cholesterol), and reduce triglycerides, all of which are critical for heart health.

Additionally, the reduction in oxidative stress and inflammation associated with intermittent fasting supports overall cardiovascular health.

May Support Healthy Aging

Intermittent fasting may help promote your overall health and longevity by enhancing your body’s stress response and cellular repair processes. The practice of fasting triggers autophagy, a process where the body cleans out damaged cells and regenerates new ones, which is vital for disease prevention and longevity.

May Help Enhance Mental Clarity and Cognitive Function

Many individuals who practice intermittent fasting report improved mental clarity and cognitive function. Fasting has been shown to increase the production⁹ of brain-derived neurotrophic factor (BDNF), a protein that supports neuron growth and brain health. Higher levels of BDNF are associated with improved cognitive function, memory, and learning.

Drawbacks and Potential Side Effects of Intermittent Fasting

Intermittent fasting can cause several side effects—including headaches, dizziness, and fatigue—especially during the initial stages as your body adapts to the new eating pattern. These symptoms are often referred to as the “keto flu¹⁰” and typically subside within a few weeks.

In addition, there are potential drawbacks to implementing IF that you need to consider.

Dealing With Hunger and Cravings

The fasting period can lead to intense hunger and cravings, especially when you’re starting. This can make it challenging to stick to your fasting schedule and might lead to overeating during your eating window.

Can Cause Nutritional Deficiencies

Intermittent fasting can pose nutritional challenges if not properly managed. Ensuring a balanced intake of healthy foods during eating windows is crucial, especially if you want to preserve lean muscle mass. This makes supplementation particularly important, as a high-quality multivitamin can help make up for dietary deficiencies.

May Be Unsafe For Individuals With Eating Disorders

Intermittent fasting may not be suitable for individuals with a history of eating disorders such as anorexia nervosa, bulimia nervosa, or binge-eating disorder. The restrictive nature of intermittent fasting can trigger unhealthy eating behaviors¹¹ and exacerbate symptoms of these disorders.

If you’ve experienced disordered eating patterns, you should consult with a healthcare professional before you implement intermittent fasting.

Isn’t Suitable For All Populations

Certain populations should approach intermittent fasting with caution or avoid it altogether. For example, breastfeeding mothers have increased nutritional needs to support milk production and their own health. People with certain medical conditions, such as diabetes, hypoglycemia, or heart disease, should consult with a healthcare provider before starting intermittent fasting.

Furthermore, individuals on medications or undergoing treatment for chronic conditions should seek medical advice before adopting an intermittent fasting regimen to ensure it doesn’t interfere with their treatment plan.

Potential Negative Psychological Impact

While some people find fasting empowering and beneficial for mental clarity, others may experience increased stress, anxiety, or an unhealthy relationship with food. The psychological strain of adhering to a strict eating schedule can sometimes outweigh the physical benefits.

May Not Be Sustainable

Maintaining intermittent fasting over the long term can be challenging for many individuals. Between birthday parties, vacations, family outings, and lifestyle changes, many potential roadblocks can stand in your way of adhering to a strict fasting schedule.

Tips for Implementing Intermittent Fasting

I first tried intermittent fasting in 2011 when I was a collegiate rugby player looking to trim some unwanted pounds. Over the last decade-plus, I’ve tried different forms and discovered some important lessons along the way.

Here are my tips and tricks for implementing IF in your life.

Start Slowly

Begin with a shorter fasting period and gradually increase the duration as your body adapts. This approach can help mitigate side effects and make the transition smoother mentally and physically.

Stay Hydrated

Drink plenty of water and consider electrolyte supplements to stay hydrated and support bodily functions during fasting periods.

Choose Nutrient-Dense Foods

When it’s time to eat, make your calories count. Consume lean protein sources such as beef, chicken, fish, or eggs, and load up your plate with plenty of leafy greens and clean carbohydrates like potatoes and sweet potatoes. To satisfy your sweet tooth, stick to fruit, Greek yogurt, and dark chocolate—all of which provide more nutrients than sugary snacks and desserts.

Take Measurements

Track your weight, body fat percentage, and muscle mass to monitor your progress. Use tools like body composition scales, measuring tapes, and progress photos to get a comprehensive view of your results to assess your fasting protocol’s effectiveness.

Adjust Your Calorie Intake

As you progress, you may need to adjust your calorie intake to align with your goals. If you’re trying to gain muscle, increase the amount of protein and your overall caloric intake during your eating window. For fat loss, ensure you’re in a calorie deficit while maintaining adequate nutrition.

Modify Your Fasting Schedule

You may need to adjust your fasting schedule based on your lifestyle, goals, and how your body responds. Don’t hesitate to experiment with different fasting periods and eating windows to find what works best for you.

Intermittent Fasting: Final Thoughts

Let me be clear: Intermittent fasting isn’t a foolproof solution to sculpt your dream physique or fix your health problems. It is, however, a research-backed, time-tested practice that offers valuable upside for your physical and mental health. Experiment with different protocols, document how you feel and perform, and decide which form of fasting works best for you.

Intermittent Fasting: FAQs

What are the basic rules of intermittent fasting?

Intermittent fasting involves setting specific periods for eating and fasting. In strict cases, you may only consume water during your fasting window.

Is intermittent fasting effective for weight loss?

Intermittent fasting can be effective for weight loss as it helps reduce overall calorie intake and boosts metabolic processes, leading to fat loss and improved body composition.

What is an example of intermittent fasting?

An example of intermittent fasting is the 16/8 method, where an individual fasts for 16 hours each day and eats all their meals within an eight-hour window, typically from noon to 8 p.m.

How does intermittent fasting work for beginners?

Beginners can start with a manageable fasting period, such as the 12/12 method (12 hours fasting, 12 hours eating), gradually increasing the fasting duration as their body adapts.

These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any diseases.

References

Ho, K. Y., Veldhuis, J. D., Johnson, M. L., Furlanetto, R., Evans, W. S., Alberti, K. G., & Thorner, M. O. (1988). Fasting enhances growth hormone secretion and amplifies the complex rhythms of growth hormone secretion in man. Journal of Clinical Investigation, 81(4), 968-975. https://doi.org/10.1172/JCI113450
Parzych, K. R., & Klionsky, D. J. (2014). An Overview of Autophagy: Morphology, Mechanism, and Regulation. Antioxidants & Redox Signaling, 20(3), 460-473. https://doi.org/10.1089/ars.2013.5371
Cui, Y., Cai, T., Zhou, Z., Mu, Y., Lu, Y., Gao, Z., Wu, J., & Zhang, Y. (2020). Health Effects of Alternate-Day Fasting in Adults: A Systematic Review and Meta-Analysis. Frontiers in Nutrition, 7, 586036. https://doi.org/10.3389/fnut.2020.586036
Kang, J., Shi, X., Fu, J., Li, H., Ma, E., & Chen, W. (2022). Effects of an Intermittent Fasting 5:2 Plus Program on Body Weight in Chinese Adults with Overweight or Obesity: A Pilot Study. Nutrients, 14(22). https://doi.org/10.3390/nu14224734
Ezpeleta, M., Cienfuegos, S., Lin, S., Pavlou, V., Gabel, K., & Varady, K. A. (2024). Efficacy and safety of prolonged water fasting: a narrative review of human trials. Nutrition reviews, 82(5), 664–675. https://doi.org/10.1093/nutrit/nuad081
Fahed, G., Aoun, L., Zerdan, M. B., Allam, S., Zerdan, M. B., Bouferraa, Y., & Assi, H. I. (2022). Metabolic Syndrome: Updates on Pathophysiology and Management in 2021. International Journal of Molecular Sciences, 23(2). https://doi.org/10.3390/ijms23020786
Yuan, X., Wang, J., Yang, S., Gao, M., Cao, L., Li, X., Hong, D., Tian, S., & Sun, C. (2022). Effect of Intermittent Fasting Diet on Glucose and Lipid Metabolism and Insulin Resistance in Patients with Impaired Glucose and Lipid Metabolism: A Systematic Review and Meta-Analysis. International Journal of Endocrinology, 2022. https://doi.org/10.1155/2022/6999907
DONG, T. A., SANDESARA, P. B., DHINDSA, D. S., MEHTA, A., ARNESON, L. C., DOLLAR, A. L., TAUB, P. R., & SPERLING, L. S. (2020). Intermittent Fasting: A Heart Healthy Dietary Pattern? The American Journal of Medicine, 133(8), 901. https://doi.org/10.1016/j.amjmed.2020.03.030
Seidler, K., & Barrow, M. (2022). Intermittent fasting and cognitive performance – Targeting BDNF as potential strategy to optimise brain health. Frontiers in Neuroendocrinology, 65, 100971. https://doi.org/10.1016/j.yfrne.2021.100971
S. Bostock, E. C., Kirkby, K. C., Taylor, B. V., & Hawrelak, J. A. (2020). Consumer Reports of “Keto Flu” Associated With the Ketogenic Diet. Frontiers in Nutrition, 7. https://doi.org/10.3389/fnut.2020.00020
Blumberg, J., Hahn, S. L., & Bakke, J. (2023). Intermittent fasting: Consider the risks of disordered eating for your patient. Clinical Diabetes and Endocrinology, 9. https://doi.org/10.1186/s40842-023-00152-7

Stephen Sheehan, CPT, CNC

Stephen is an ISSA-certified personal trainer and nutritionist with an immense passion for health and fitness. A two-time University of Florida graduate and longtime rugby player, he brings more than a decade of strength training experience, plus a diverse writing and editing background. When he’s not creating content or hitting the gym, he enjoys traveling, cooking, and attending music festivals around the world.

What Is Intermittent Fasting? A Beginners Guide to Implementing IF