Is Fasting Good for You? Here's What the Science Says

Key takeaways: 

• The most common types of fasting include intermittent fasting, alternate-day fasting, and prolonged fasting.
  
  

• Fasting may be good for your physical and mental health. Some science-backed benefits of fasting include weight loss, metabolic support and healthy aging.
  
  

• Risks and side effects of fasting range from common issues like fatigue and hunger to serious concerns like dehydration and nutrient deficiencies.
  
  

• People with certain health considerations should avoid fasting or proceed only under medical supervision.
  
  

• The Fasting Mimicking Diet (FMD) offers fasting benefits with less restriction.

From intermittent fasting to prolonged fasts, more and more people are turning to fasting as a tool for weight loss, metabolic support, healthy aging, and more. But this surge in popularity has many wondering: Is fasting actually good for you? 

The short answer is yes—fasting can be good for your physical and mental health, depending on your current health status, and the type of fasting involved. Here, we’ll break down the different types of fasting, examine the scientific studies on its benefits and highlight the potential risks—including who should not fast. We’ll also explain how the Fasting Mimicking Diet (FMD) may offer a more accessible option for those looking to gain the benefits of fasting without extreme restriction.

Guided by the latest scientific insights, let’s take a closer look at the risks and rewards of making fasting part of your health routine. 

What is fasting? Defining different types

Fasting is abstaining from food and any caloric drink for a specific period of time. It has been practiced for centuries for spiritual, cultural, and health-related reasons; today, fasting is often used for improving metabolic health, weight loss, and promoting cellular repair. There are several different types of fasting, each with its own approach, timing, and potential health benefits. While the core concept of fasting remains the same—refraining from caloric intake—the duration and frequency can vary greatly, allowing you to choose the method that best fits your goals and lifestyle. Let’s review some of the most common fasting types: 

1. Intermittent fasting (IF)
Intermittent fasting (IF) is a daily dietary approach, typically done within a 24 - 48 hour period, that focuses on when you eat rather than what you eat. This type of fasting involves alternating periods of eating and fasting, and there are several popular methods to choose from:

• 12:12 method: Most closely aligned with the longest living people, the 12:12 method involves 12 hours of fasting followed by a 12-hour eating window, often aligned with the natural circadian rhythm (such as eating from sunrise to sunset).
  
  

• 5:2 method: This weekly plan allows for normal eating five days each week while restricting calorie intake to roughly 20–30% of your usual intake on two non-consecutive days. Research indicates that this method may reduce oxidative stress and inflammation in the brain, potentially decreasing the risk of cognitive decline and supporting mental clarity. Products like Prolon’s 1-Day Reset can help simplify the adoption of this routine into your weekly schedule.
  
  

2. Alternate-day fasting (ADF)
Alternate-day fasting (ADF) involves switching between days of regular eating and days of fasting or consuming very few calories (typically under 500). It’s generally more intense than other IF methods and may lead to quicker weight loss results, but can be more difficult to maintain over the long term.

3. Prolonged fasting
Prolonged fasting refers to any fasting period lasting 72 hours or more. There are many known benefits to this type of fasting—it can stimulate the cellular renewal process known as autophagy, for example. However, prolonged fasting can be challenging in its traditional form as it typically requires complete abstention from food. One well-researched and more accessible approach to prolonged fasting is the Fasting Mimicking Diet (FMD). The program spans five days and includes plant-based, low-calorie meals that allow the body to receive precise nourishment while still maintaining a fasting state. Its patented technology is designed to help the body enter a fasting state and activate many of the same cellular benefits as water fasting—like cellular repair—without requiring total food abstinence. Three cycles of Prolon’s 5-Day FMD Program over three consecutive months have been shown to support healthy metabolic markers, reduced biological age and fat-focused weight loss while protecting muscle.

4. Calorie restriction (CR)
While not technically a form of fasting, calorie restriction (CR) is often mentioned alongside fasting due to its similar metabolic and longevity-related effects. CR involves consistently reducing daily caloric intake—typically by 20–40% daily. Unlike intermittent fasting, which focuses on when you eat, CR is all about how much you eat.

Research on CR has shown promising results in animal studies, where it’s been linked to extended lifespan and improved metabolic health. In humans, CR has been associated with benefits such as improved insulin sensitivity and cardiovascular support. However, practicing calorie restriction over an extended period may carry risks that have been linked to a shortened lifespan, including loss of muscle mass, nutrient deficiencies, and potential impacts on mental health and immune function. For most healthy individuals, a short- to moderate-term approach—ranging from several months to two years under medical supervision—may offer benefits while minimizing potential risks associated with long-term restriction.

The science-backed benefits of fasting

From metabolic support to cellular rejuvenation, here are the science-backed benefits of fasting—and the biological mechanisms that drive them.

1. Weight management and fat loss

One of the most commonly sought benefits of fasting is weight loss. During fasting periods, insulin levels drop, prompting the body to shift from using glucose as its primary energy source to burning stored fat. This metabolic switch—often referred to as ketosis—enhances fat oxidation and may increase the body’s ability to burn calories more efficiently.

Intermittent fasting has been shown to aid in reducing body weight and abdominal fat.
The Fasting Mimicking Diet may offer an added benefit: targeting deeper visceral fat and protecting lean muscle mass, supporting metabolism during weight loss efforts.

2. Insulin and blood sugar support 

Fasting may play a key role in reducing the risk of insulin resistance, a precursor to type 2 diabetes. When you fast, insulin levels drop, which gives the body a break from constant insulin stimulation.

Studies have found that intermittent fasting may lower both fasting blood glucose and insulin levels, potentially improving long-term glycemic control. These effects are particularly promising for people with prediabetes or metabolic syndrome, although more research is needed to determine the ideal fasting protocols for different individuals.

3. Metabolic support

By reducing insulin levels and promoting fat oxidation, fasting may help counteract metabolic slowdown that often accompanies aging or weight gain. Many experts also suggest that fasting may help "reset" the metabolism by encouraging the body to switch from glucose dependency to more flexible energy usage. Both intermittent and prolonged fasting may “reset” metabolic processes, but the deeper shifts in hormone signaling and cellular pathways are often more pronounced with prolonged fasting.

4. Cellular repair and autophagy

One of the most fascinating benefits of fasting is its effect on cellular maintenance and repair—particularly through a process called autophagy. Autophagy is the body’s natural mechanism for cleaning out damaged cells. Think of it as a biological recycling system that slows cellular aging and supports longevity. 

Prolonged fasts like Prolon’s 5-Day Program are known to activate autophagy, supporting longevity through cellular rejuvenation and detoxification. 

5. Brain health 

Fasting doesn’t just benefit the body—it may also support the brain. Some research suggests that intermittent fasting can support brain health by encouraging the growth of new neurons. For those over 65, studies suggest that intermittent fasting may reduce the risk of neurodegenerative diseases like Alzheimer's, potentially improving cognitive function and reducing cognitive decline. Research shows that FMD has also been shown to reduce neuroinflammation and cognitive decline in Alzheimer’s models.

6. Healthy aging & longevity 

Perhaps one of the most exciting potential benefits of fasting is healthy aging and longevity support. In a groundbreaking clinical study, researchers found that completing just three cycles of the Prolon FMD over three consecutive months led to an average reduction of 2.5 years in biological age. In other words, fasting for just 15 days out of 90 can have a measurable impact on cellular aging and longevity markers.

Based on this data and projected calculation, it could be possible  that following the FMD three times a year between the ages of 50 and 70 could reduce biological age by as much as 11 years. While longitudinal research is still needed, this could mean that well-timed fasting cycles may help you age more gracefully—not just on the outside, but at the cellular level. These benefits are unique to prolonged fasting, due to its deeper activation of regenerative pathways.

Emerging preclinical research, particularly in animal models, also suggests that fasting and calorie restriction may extend healthspan. These benefits appear to be linked to reduced metabolic stress, enhanced cellular repair mechanisms like autophagy and hormone regulation. 

While human data is still emerging, both intermittent and prolonged fasting appear to play roles in supporting a longer healthspan. 

Potential risks and side effects of fasting

While fasting may offer an array of health benefits, it’s important to acknowledge that it may also come with certain risks—especially when done improperly or without medical guidance. Understanding the potential downsides can help individuals make more informed and safe decisions.

Common fasting side effects 

During the initial stages of fasting—especially for those new to the practice—some discomfort is normal as the body adapts to a new eating pattern and use of energy sources. Common side effects may include:

• Hunger and food cravings
  
  

• Fatigue and low energy
  
  

• Headaches
  
  

• Dizziness or lightheadedness
  
  

• Irritability or mood changes
  
  

These symptoms often subside over time as the body becomes more metabolically flexible, but they can still pose challenges, particularly in the early days. 

“The way you prepare for a prolonged fast can set the tone for your entire experience. In the days leading up, focus on staying well-hydrated, reducing sugar and processed foods, and shifting toward lighter, plant-based meals to ease the metabolic transition,” explains Melanie Murphy Richter, a longevity dietitian. “Once the fast begins, it’s normal to feel some fatigue or brain fog—these are signs your body is entering a deeper state of fat-burning and cellular rejuvenation. During this time, rest when you can, avoid strenuous activity, and trust that these shifts are part of the transformation process.”

More serious risks

When water-only fasting is prolonged or not properly managed, more significant health risks may emerge. These risks include:

• Electrolyte imbalances: Extended water-only fasting without proper electrolyte intake can disrupt the body’s delicate mineral balance, potentially affecting heart and muscle function.
  
  

• Dehydration: Many people forget that food contributes to 20% of our daily hydration. When we reduce food intake, we also reduce food-based sources of hydration. Without intentional fluid intake, this can lead to dehydration.
  
  

• Nutrient deficiencies: If fasting is done too frequently or without adequate nutrition during eating windows, it may lead to deficiencies in essential vitamins and minerals over time.
  
  

Staying safe while fasting

To minimize risks, proper hydration and electrolyte balance are essential while fasting. Drinking plenty of water and, when appropriate, supplementing with potassium and magnesium may help the body maintain equilibrium. It’s also crucial to listen to your body—fatigue, dizziness, or persistent discomfort may signal that it’s time to adjust your fasting approach.

Who should exercise caution while fasting—or avoid it altogether?

Responsible fasting starts with knowing whether it’s right for your body and health background. Fasting is not appropriate, or safe, for everyone. Certain individuals should avoid fasting altogether or only attempt it under the guidance of a healthcare professional. Those who should exercise caution include: 

• Individuals with a history of eating disorders
  It’s possible that fasting may trigger harmful behaviors or thought patterns in those who have experienced anorexia, bulimia, or binge eating disorders. Even well-intentioned fasting regimens can risk relapse or worsen mental health symptoms. Always prioritize your mental health before starting a fasting protocol.
  
  

• Individuals with type 1 or type 2 diabetes
  Especially for those taking insulin or other glucose-lowering medications, fasting may increase the risk of hypoglycemia (dangerously low blood sugar). Any fasting plan should be discussed in detail with a healthcare provider to avoid complications. If you are diagnosed with a metabolic condition like type 2 diabetes or prediabetes and want to pursue a more comprehensive approach to your health with medical oversight, we urge you to explore L-Nutra Health’s personalized programs.
  
  

• Pregnant or breastfeeding women
  During pregnancy and lactation, nutritional needs are elevated. Fasting could interfere with the necessary intake of calories, protein, and micronutrients, potentially affecting both the mother’s and baby’s health.
  
  

• Individuals with certain medical conditions
  People with conditions such as advanced kidney disease, liver disorders, or heart problems should avoid fasting unless carefully supervised by a medical professional. Fasting may exacerbate symptoms or disrupt the balance of medications and electrolytes.
  
  

• Those taking specific medications
  Some medications require food for proper absorption or to avoid gastrointestinal side effects. Fasting without adjusting medication timing or dosage could reduce effectiveness or lead to adverse reactions.
  
  

• Older adults and frail individuals
  As we age, the body’s resilience to fasting may decrease. For individuals aged 65 or older, and those of any age who may be physically weak, it's best to speak with your physician first to determine if fasting is right for you.
  
  

• Children and adolescents
  Growing bodies require consistent nourishment. Fasting in this age group may hinder growth or development and is generally not recommended without medical supervision.

Always consult a qualified healthcare provider before beginning any fasting regimen—especially if you fall into one of the above categories. Personalized guidance can help ensure that fasting is both safe and effective.

How to fast safely and effectively

If you’re considering incorporating fasting into your health routine, safety should always come first. While fasting can offer a wide range of benefits, it’s not a one-size-fits-all approach, and taking a thoughtful, gradual path is key.

1. Start slowly, especially if you’re new to fasting. For example, ease into intermittent or time restricted fasting with shorter daily fasting windows before attempting longer fasts. This gives your body time to adjust and can help minimize common side effects like fatigue, headaches, or irritability.
   
   “If you’re new to fasting, easing in by aligning your meals with your circadian rhythm is one of the most sustainable approaches,” says Richter. “Start by finishing dinner at least three hours before bed to support digestion and metabolic balance. Then, reduce between-meal snacking and cut back on sugar and processed foods, which can disrupt hunger hormones and blood sugar. These gentle shifts help your body adapt naturally—without the stress of jumping into long fasts too quickly.”

2. Hydration is critical. Since fasting reduces fluid intake from food, it’s important to drink plenty of water and consider replenishing electrolytes, especially during longer fasts. Always pay close attention to how you’re feeling; signs like dizziness, persistent weakness, or confusion are indicators that it may be time to stop fasting and refuel.
   
   

3. Listen to your body. Fasting should never feel like punishment. If you’re constantly lightheaded, excessively hungry, or feeling unwell, it’s okay to pause or adjust your plan. Fasting is meant to support your health, not compromise it.
   
   

4. Consult a medical professional. Before starting any new fasting regimen—particularly prolonged fasts or if you have underlying health conditions like diabetes, heart issues, or are on medications—it’s strongly recommended to consult with and work alongside a healthcare professional.
   
   

5. Follow the science. For those looking for a structured, science-backed method with reduced risks, Prolon’s 5-Day FMD offers a clinically researched option – developed to mimic the effects of fasting while still providing essential nutrients. 
   
   

Is fasting good for YOU? Weighing the evidence

The benefits of fasting are supported by a growing body of scientific research. Studies suggest that various fasting methods may help with weight loss, metabolic function, cellular repair, and even contribute to healthy aging and longevity. Intermittent approaches and prolonged fasts like FMD offer flexibility and accessibility for many individuals looking to experience these benefits.

However, fasting is not universally safe or suitable. Certain groups, such as individuals with medical conditions, those on specific medications, or people with a history of eating disorders, should avoid fasting unless under professional supervision.

If you’re considering integrating fasting into your health routine, start slowly, prioritize hydration, listen to your body, and seek personalized medical advice. By taking a careful, informed approach, fasting can become a powerful and sustainable part of your longevity lifestyle. 

Sources:
MedicalNewsToday.com: Could intermittent fasting earlier in the day help reduce more abdominal fat
Nature.com: Fasting-mimicking diet causes hepatic and blood markers changes
NIH.gov: 2 years of calorie restriction and cardiometabolic risk (CALERIE): exploratory outcomes of a multicentre, phase 2, randomised controlled trial
NIH.gov: Calorie Restriction: What Recent Results Suggest for the Future of Aging Research
NIH.gov: Effects of Intermittent Fasting on Brain Metabolism
NIH.gov: Effects of intermittent fasting on cognitive health and Alzheimer's disease
NIH.gov: Effect of Intermittent Fasting Diet on Glucose and Lipid Metabolism and Insulin Resistance
NIH.gov: Fasting-mimicking diet cycles reduce neuroinflammation to attenuate cognitive decline in Alzheimer’s models
ScienceDirect.com: A fasting-mimicking diet programme reduces abdominal adipose tissue while preserving abdominal muscle mass