Everything You’ve Always Wanted To Know About Fasting - But Didn’t Know Who To Ask

Key takeaways: 

• Fasting is rooted in both tradition and science, offering benefits that go far beyond weight loss.
  
  

• When done strategically, fasting can support fat burning, cellular rejuvenation, metabolic health, brain function, and healthy aging.
  
  

• There are many fasting methods, from intermittent approaches to more intensive strategies like prolonged fasting —so personalization is key.
  
  

• The Fasting Mimicking Diet (FMD) offers a scientifically backed way to gain many of the benefits of prolonged fasting without giving up food entirely.
  
  

• While fasting can be safe and effective for many, it’s not appropriate for everyone and should always be approached with individual health in mind.
  
  

• Tools like Prolon’s 5-Day FMD make fasting more accessible, helping users unlock cellular and metabolic benefits while minimizing discomfort.

Fasting has emerged as one of the most talked-about strategies for overall health in the nutrition and wellness spaces, but with growing popularity comes growing confusion. Is it just skipping meals? Is it safe? And how do you know what type is right for you?

In truth, fasting is far more than a trend. It’s an ancient practice grounded in both tradition and science, offering a wide range of benefits from metabolic health and weight management to cellular rejuvenation and healthy aging. But with so many fasting methods available, it can be hard to know where to start.

This comprehensive guide will break down everything you need to know about fasting, including the science, different types of fasting, health benefits, and safety considerations. 

What is fasting?

At its core, fasting is the voluntary abstention from food (and sometimes drink) for a set period of time. It’s not about deprivation; it’s about giving the body time to reset, repair and rebalance.

The roots of fasting

Fasting isn’t new. It has deep roots in spiritual and cultural rituals across the globe—from Ramadan to Yom Kippur to to the natural rhythms of feast and famine followed by early agrarian communities. Across traditions, fasting has long been used as a tool for reflection, renewal and resilience—both physical and spiritual.

For most of human history, food wasn’t available around the clock; our ancestors evolved in environments shaped by scarcity and seasonality. Rather than eating three balanced meals a day, they cycled through periods of feast and famine.

This intermittent approach to nutrition shaped human biology. Our metabolism learned to adapt, switching between glucose and fat as fuel. Our cells developed built-in repair mechanisms that activate in low-nutrient states. These biological rhythms are essential to health and vitality, but they can be tough to activate within the body in an environment where food is available 24/7. 

Today, many people eat frequently throughout the day with meals and snacks, keeping the body’s nutrient-sensing pathways (cellular signaling systems that detect and respond to the presence of nutrients) constantly active. Over time, this constant activation has the capacity to disrupt metabolic function, increase fat storage, and impair the body’s ability to self-regulate.

The science of fasting: how your body transforms

When done strategically, fasting - specifically prolonged fasting - can activate a series of deeply regenerative processes within the body that impact everything from energy metabolism to cellular health. As the body transitions away from constant food intake, it begins to optimize its internal systems in powerful ways, including: 

• Fuel shift (glucose to fat burning): Once glycogen (reserves of glucose inside the body) stores are depleted, the body switches to stored fat as its primary fuel source. This leads to the production of ketones 

“When glycogen stores are depleted, the body doesn’t jump straight into fat burning. First, it will break down amino acids—including those from muscle tissue—to meet its immediate energy needs. This is a typical pattern in water-only fasting or chronic calorie restriction, where the body lacks any external nourishment. The Fasting Mimicking Diet, however, is designed to intervene at this critical point. By providing a precise formulation of nutrients, it helps the body bypass excessive muscle breakdown and shift more efficiently into fat burning and ketone production—while still activating the cellular rejuvenation benefits of a prolonged fast. This helps the body burn fat while protecting metabolically active muscle.” - Melanie Murphy Richter, Longevity Dietitian, MS, RDN

• Autophagy: After around 72 hours, fasting may activate the body’s “self-cleaning” process, breaking down and recycling damaged cells
  
  

• Cellular rejuvenation: Extended fasting (fasts that go beyond 72 hours) promotes the repair of new, healthier cells, supporting tissue repair, healthy aging and longevity.
  
  

• Metabolic flexibility: Fasting has been shown to support insulin sensitivity and blood sugar regulation.

Types of fasting: finding your fit 

There are many approaches to fasting, so it’s important to understand the different methods in order to choose one that aligns with your health goals and feels sustainable for you. Here are some of the most common types of fasting: 

Intermittent fasting (IF)

Intermittent fasting (IF) is an eating pattern that alternates between periods of eating and fasting. Instead of concentrating on what you eat, IF highlights when you eat.

Below are some of the most common approaches to IF:

• Time-restricted eating (TRE) 
  

  16:8 Method: A more restrictive IF approach, this method involves fasting for 16 hours and eating during an eight-hour window each day (e.g., only eating between 12 p.m. and 8 p.m.).
  
  12:12: A less restrictive IF method , where individuals fast for 12 hours and then eat within a 12 hour window, generally in accordance with their circadian rhythm (from sunup to sun down). According to Melanie Murphy Richter, MS, RDN, “The 12:12 method reflects a kind of metabolic common sense—one echoed in the world’s longest-living communities. By aligning your meals with the sun, you’re not just giving your body a break—you’re working with your biology. Our organs are designed to metabolize, process, and repair most efficiently during daylight hours. This gentle, sustainable rhythm is less about restriction and more about returning to the way we’re meant to function.”
  
  

• The 5:2 method
  

  With this form of intermittent fasting, you eat your regular diet for five days of the week and significantly reduce your calorie intake—down to about 20–30% of your usual intake—on two non-consecutive days. Emerging studies suggest that the 5:2 method may help lower oxidative stress and brain inflammation, which could reduce the risk of cognitive decline and support long-term brain health. It has also been associated with improved insulin sensitivity, reduced visceral fat, and support for healthy cholesterol and blood pressure levels—key factors in protecting metabolic and cardiovascular health. Prolon’s 5:2 fasting program features twice-weekly 1-Day Resets, offering a structured and convenient way to follow this approach without the guesswork. Because the fasting days are non-consecutive, the 5:2 method is often easier to sustain long term, making it a more accessible and flexible option for many people.
  

Alternate-day fasting (ADF)

This method involves alternating between a day of regular eating and a day of either complete fasting or very low calorie intake (typically under 500 calories). ADF is generally more intensive than other forms of intermittent fasting and may result in faster weight loss, but it can also be more challenging to maintain over time. ADF can be practiced for varying durations, depending on individual goals and tolerance with some studies showing benefits after 8-12 weeks. 

Water fasting

Water fasting is a more intensive approach that involves abstaining from all food and consuming only water for a set period typically ranging from 24 hours to several days. While some practice it for health or spiritual purposes, longer water fasts carry risks and should only be done under medical supervision.

Prolonged fasting

Prolonged fasting refers to any fast lasting 72 hours or more and is typically practiced to trigger deeper physiological changes that shorter fasts may not achieve. During this extended fasting period, the body shifts fully into fat-burning mode, depletes glycogen stores and activates key cellular repair processes such as autophagy. These changes are associated with benefits like improved insulin sensitivity, mental clarity and cellular rejuvenation—mechanisms that may contribute to healthy aging and disease prevention. While prolonged fasting can offer powerful health benefits, it should be approached with caution and ideally under medical supervision, especially for those with underlying health conditions or limited fasting experience.

The Fasting Mimicking Diet (FMD)

The Fasting Mimicking Diet (FMD) offers a more approachable version of prolonged fasting, providing very low-calorie, plant-based meals over five days to mimic the effects of water fasting without full food deprivation. Clinical studies show that three consecutive cycles of Prolon’s 5-Day FMD Program over three months can support healthy metabolic markers, promote sustained fat loss with muscle protection, and reduce biological age by 2.5 years. Additionally, only 20 days of the FMD (that’s just four cycles) has shown to be as effective on heart-health as 120 continuous days of The Mediterranean Diet

Types of fasting: finding your fit 

Fasting, both intermittent and prolonged, may offer a wide range of science-backed benefits when done strategically and safely. Here are some of the most compelling ways fasting can support your health:

• Metabolic support: Fasting may enhance insulin sensitivity, regulate blood sugar, and may reduce the risk of Type 2 diabetes
  

• Cardiovascular support: Studies show that fasting may help lower blood pressure, triglycerides, and LDL cholesterol (notably seen with FMD and the 5:2 diet)
  

• Brain health support: Fasting has been shown to support cognitive performance and increases mental clarity through the production of ketones and activation of autophagy.

• Healthy aging support: Prolonged fasting like FMD has been shown to trigger autophagy and cellular rejuvenation, processes linked to extended lifespan in various research models.
  

• Gut health support: Fasting can give the digestive system time to rest and repair while supporting a diverse, balanced microbiome.
  

• Weight loss: Clinical research shows that both intermittent and periodic prolonged fasting methods like the FMD can lead to sustainable weight loss, particularly around the abdomen.

Fasting safely: is fasting right for everyone? 

Fasting can be a powerful tool for supporting health, but safety should always come first. Whether you’re trying intermittent fasting or a prolonged fast, taking the right precautions can help you avoid unwanted side effects and get the most from your experience.

For some, fasting can be beneficial; for others, it may not be appropriate without medical oversight - or at all. 

Proceed with caution: Medical guidance strongly recommended

These individuals may benefit from fasting under professional supervision, but should not attempt it on their own:

• Individuals with diabetes: People managing type 1 or type 2 diabetes, especially those on insulin or glucose-lowering medications, are at elevated risk of hypoglycemia while fasting. Medical oversight is critical. A structured, clinically supported program like those offered by L-Nutra Health may provide a safer path.
  
  

• People with certain medical conditions: If you are living with complex or chronic health issues such as kidney disease, liver dysfunction or cardiovascular disease, fasting may exacerbate your condition. Consult your physician before starting any fasting protocol.
  
  

• Anyone taking medications that require food: Some medications need to be taken with meals for proper absorption or to minimize side effects. Always speak to your doctor or pharmacist before changing your eating schedule.
  
  

• Older adults (especially over 65): Aging bodies may be less resilient to the stress of fasting. For those who are older, underweight or frail, fasting might increase the risk of dehydration, falls and other complications. Medical clearance is advised before beginning any prolonged fast.

Proceed with caution: Medical guidance strongly recommended

For the following groups, fasting is generally not recommended due to significant health risks:

• Those with a history of eating disorders: Fasting can trigger harmful behaviors or relapse in individuals with a history of anorexia, bulimia, or binge eating. For anyone navigating recovery, mental and emotional health must take precedence. A fasting routine should never compromise psychological well-being.
  
  

• Pregnant or breastfeeding women: During these life stages, the body requires additional nutrients to support maternal and infant health. Fasting may deprive both mother and baby of essential nourishment and is typically discouraged unless medically prescribed.
  
  

• Children and adolescents: Young bodies and brains are still developing and need consistent energy and nutrients to grow. Fasting is not appropriate for children or teens except as part of a carefully monitored medical treatment plan.

Remember, before beginning any fasting regimen, it’s crucial to consider your unique health needs. Consulting a qualified healthcare provider can help determine whether fasting is appropriate, and how to do it safely. With the right support and a personalized approach, fasting can be both effective and health-promoting.

Common side effects of fasting and how to manage them

Fasting may come with (often temporary) side effects, especially in the beginning. The good news is the onset of side effects is usually a sign that your body is adjusting to a new metabolic state, but it’s important to be mindful of how you are feeling and what is and isn’t typical. Here are some of the more common fasting side effects and how to manage symptoms as your body adapts:

• Hunger: This is the most common challenge early on, but it typically subsides as your body adapts to fasting. Staying hydrated and keeping busy can help distract from cravings. Sometimes hunger comes in waves—riding it out can build resilience over time.
  

• Headaches: These are often a result of dehydration or electrolyte imbalances. Make sure you’re drinking enough water and consider supplementing with electrolytes (free of sugar or added calories).
  

• Fatigue/dizziness: A temporary dip in energy may be due to lower blood sugar or low sodium levels. Rest when needed, continue to hydrate during your fast and avoid intense exercise.

• Mood changes: Irritability or emotional ups and downs are common when adjusting to a new eating rhythm. Establishing a consistent fasting schedule and easing in with shorter fasts can help your body (and mind) adapt more gently.
  

• Constipation: Fasting often involves eating less overall, which means both reduced fiber and less total food to stimulate bowel movements. To support digestion during IF, include fiber-rich, whole foods during your eating windows and drink plenty of fluids throughout the day.

How to get the most out of your fast

Fasting isn’t just about abstaining from food—it’s about creating the right environment for your body to repair, reset, and thrive. These strategies can help maximize the benefits of your fasting practice:

• Stay hydrated: Water is essential, but you can also include herbal teas for variety. Electrolytes that are free of added sugars or calories may also help maintain balance, especially during longer fasts. Aim for at least 2–3 liters (about 8–12 cups) of water per day during your fast.
  

• Prioritize quality sleep: Deep rest supports hormone regulation, appetite control, and cellular repair—all crucial during fasting. Aim for 7–9 hours of uninterrupted sleep per night.
  

• Start slow: If you’re new to fasting, begin with a 12:12 window (12 hours fasting, 12 hours eating) and gradually increase as your body adjusts. This helps build confidence and consistency.
  

• Understand hunger: Hunger comes in waves and often passes more quickly than expected. Learning to recognize true physical hunger versus emotional cravings can deepen your fasting experience.
  

• Break your fast gently: Your first meal after fasting should be easy to digest and rich in nutrients. Avoid ultra-processed foods, refined carbs, and heavy meals that could shock your system. 

“After several days without food, your digestive system is more sensitive—so eating too quickly or jumping into heavy, dense meals can lead to bloating, discomfort, or even diarrhea. I always recommend easing back in with warm liquids, veggie smoothies, broths, or gently cooked vegetables. Chew thoroughly, eat slowly, and listen to your body. After completing the FMD, most people feel ready to return to a more normal diet by day 7 or 8—but how you reintroduce food still matters. Think of it as a continuation of the reset, not the end of it.” —Melanie Murphy Richter, MS, RDN

• Eat nutrient-dense foods: During eating windows, prioritize whole foods like vegetables, healthy fats, clean proteins, and complex carbs. These support blood sugar stability and sustain you through your next fast.

Debunking common fasting myths

Fasting is often misunderstood, with lingering myths that can create unnecessary fear or confusion. Here’s a closer look at some of the most common misconceptions—and the facts that set the record straight:

Myth 1: Fasting slows metabolism
Fact: Contrary to popular belief, some fasts  may actually support metabolism. Studies show the FMD, for example, helps reset metabolism and enhance metabolic flexibility, training the body to efficiently switch between fuel sources - using glucose when available and burning fat when needed. 

Myth 2: Fasting causes muscle loss
Fact: Not all fasting leads to muscle loss. The FMD is scientifically formulated to help preserve lean muscle while targeting fat. Studies show that the FMD supports fat loss without compromising muscle mass, thanks to its precise nutrient composition that encourages the body to burn fat for fuel instead of muscle.

Myth 3: Fasting leads to binge eating
Fact: While some people may initially overeat after a fast, most find that structured fasting helps regulate appetite and recalibrate hunger cues. With time and consistency, many individuals experience fewer cravings and feel more in control of their eating habits, not less. With the FMD, specifically, users report a better relationship with food post-fast.

Myth 4: Fasting is only for weight loss
Fact: Weight loss is just one of many potential benefits. As previously mentioned, fasting has been associated with cellular repair (autophagy), healthy aging, metabolic support, brain health support and more. 

Prolon: The clinically tested pathway to fasting benefits 

If you want to experience the powerful health benefits of prolonged fasting while limiting the potential side effects or discomfort that may come with food deprivation, Prolon’s 5-Day FMD offers an innovative approach. Prolon is the only program designed and clinically tested to mimic the effects of a water-only fast while still allowing you to eat.

Developed over decades by longevity researchers at the University of Southern California, Prolon provides the precise formulation of plant-based nutrients that keep your body in a fasting state, helping you unlock many of the same cellular and metabolic benefits of a prolonged fast without total deprivation. Clinical studies on Prolon show:

• Autophagy and cellular rejuvenation activated around Day 3 and may go through Day 7 (two days after the fast ends)
  

• Metabolic health support
  

• Reduced hunger and fatigue commonly associated with prolonged fasting, helping users stay on track
  

• Fat-focused weight loss with preservation of lean muscle mass
  

• Reduced biological age by as much as 2.5 years when used over three consecutive monthly cycles

Your empowered fasting journey

Fasting is far more than just abstaining from food—it's a powerful, science-backed tool for supporting metabolic health, promoting longevity and reconnecting with your body’s natural rhythms. Combining daily intermittent fasting with periodic prolonged fasting like FMD can help you maximize results as you continue to build your longevity lifestyle. 

As with any wellness strategy, personalization and safety matter. By understanding the science, listening to your body and seeking guidance when needed, you can make fasting a sustainable and transformative part of your health journey.

Empower yourself with the ancient wisdom of fasting—refined by decades of research and made accessible through innovative tools like Prolon. Ready to experience the benefits firsthand? Start here!

Sources

Nature.com: Fasting-mimicking diet causes hepatic and blood markers changes
NIH.gov: A 5:2 Intermittent Fasting Meal Replacement Diet and Glycemic Control for Adults With Diabetes
NIH.gov: Cognitive, Sleep, and Autonomic Responses to Induction of a Ketogenic Diet in Military Personnel: A Pilot Study
NIH.gov: Effect of Intermittent Fasting Diet on Glucose and Lipid Metabolism and Insulin Resistance
NIH.gov: The Effect of Fasting on Human Metabolism and Psychological Health
NIH.gov: The Effects of Intermittent Fasting on Brain and Cognitive Function
NIH.gov: Effects of intermittent fasting on cognitive health and Alzheimer's disease
NIH.gov: Intermittent fasting and time-restricted eating role in dietary interventions and precision nutrition
NIH.gov: Fasting-mimicking diet and markers/risk factors for aging, diabetes, cancer, and cardiovascular disease
NIH.gov: Fasting-Mimicking Diet Modulates Microbiota and Promotes Intestinal Regeneration to Reduce Inflammatory Bowel Disease Pathology