The Science Behind Intermittent Fasting: Benefits and Risks Explained

Intermittent fasting is an eating pattern that cycles between periods of fasting and eating. Supporters claim a wide range of potential health benefits, like weight loss, improved metabolism, and even healthy aging and longevity. (1) 

But while many people have found success with intermittent fasting, it’s not for everyone and may not be more effective than other eating patterns. 

Here’s a deep dive into the science behind intermittent fasting. Learn its benefits, risks, and if you should try it. 

What Is Intermittent Fasting?

Intermittent fasting (IF) is a dietary approach that alternates between structured fasting and eating times, or windows. Unlike traditional diets, IF doesn’t necessarily harp on what you eat but rather when you eat. 

This flexibility is what has made IF a popular choice for those looking to manage their weight and improve health without eliminating specific food groups or drastically changing their diet.

There are several popular methods of intermittent fasting, and their structure varies drastically:

Expert Opinions & Perspectives

Intermittent fasting has received some positive feedback from experts in the nutrition field, though it’s not without its major caveats.

According to Joanna Gregg, a registered dietitian with MyFitnessPal, IF might be especially beneficial for individuals who struggle with late-night snacking or overeating during the day. By restricting eating to specific windows, some people naturally reduce their total calorie intake, which can lead to weight loss. 

Research also suggests that fasting for 8-16 hours may improve inflammatory markers and insulin sensitivity, which can help reduce the risk of chronic diseases like diabetes. (2) However, Gregg advises caution for individuals with certain health conditions, including Type 1 diabetes, or for those who are pregnant or breastfeeding (1). These individuals should check with their doctors. 

Daisy Mercer, another MyFitnessPal dietitian, emphasizes the fat-burning benefits of intermittent fasting. During fasting periods, the body depletes its stores of sugar (glycogen) and begins burning fat for energy, which might promote weight loss. (2) However, she notes that fasting isn’t suitable for everyone, especially those with diabetes. Fasting can cause blood sugar levels to drop dangerously low. (1) 

Lastly, Stephanie Nelson, MFP’s lead nutrition scientist and registered dietitian, reminds us that intermittent fasting isn’t a magic solution for weight loss. It works by helping people create a calorie deficit, which is the real driver of fat loss. Nelson warns against overly restrictive diets that are difficult to maintain in the long term. Sustainable, small calorie deficits can be just as effective. (3)

About the Experts:

Caroline Thomason is a registered dietitian and certified diabetes educator. With 12 years in the industry, she’s published in 40+ publications, a CPG consultant and advisor, a speaker, broadcast spokesperson, and recipe developer.

Joanna Gregg, RD, is a food data curator at MyFitnessPal. She earned her bachelor’s degree in Nutrition from Georgia State University and obtained her Master’s degree from The University of Nebraska.

Daisy Mercer, RD, is a Food Data Curator at MyFitnessPal. She graduated with her bachelors of Food Science and Dietetics from Colorado State University and completed her dietetic internship with the VA San Diego Healthcare system.

Stephanie Nelson is a Registered Dietitian and is MyFitnessPal’s in-house nutrition expert and nutrition scientist. Passionate for promoting healthy lifestyles, Stephanie graduated from San Diego State University with a focus on research and disease prevention.

The History and Evolution of Fasting

Fasting has been practiced for thousands of years for both spiritual and health reasons. Religious fasting practices, such as Ramadan in Islam and Yom Kippur in Judaism, emphasize the importance of abstaining from food for reflection and spiritual purification. (4)

Early humans also fasted unintentionally when food was scarce. And even today, you fast overnight while you sleep. Modern intermittent fasting has had a health focus and is backed by some research to support benefits like improved metabolism, longevity, and mental clarity. (1)

How Intermittent Fasting Works 

When you fast, the body first burns through stored energy in the form of glycogen. Once glycogen stores are depleted, typically after 12 to 16 hours of fasting, the body enters a state called ketosis, where it begins to burn fat for fuel. (5) 

Ketosis isn’t a magical trick for fat loss. You still must be in a calorie deficit over the course of the day to lose weight.

Here’s an overview of the most common intermittent fasting methods:

12/12 Method

This is the most beginner-friendly fasting approach. You fast for 12 hours and eat normally for 12 hours. For most people, this is as easy as setting a reasonable nighttime eating cutoff, such as after 7 pm.  

The MyFitnessPal in-app timer offers 3 choices: a 12 hour, 14 hour, and 16 hour fast. If you’re new to fasting, you might want to begin with a 12-hour fast.

16/8 Method

You fast for 16 hours and eat during an 8-hour window. Most MyFitnessPal members using the Intermittent Fasting Tracker feature choose this method.

It’s flexible enough to fit into most daily routines—many people simply skip breakfast and eat lunch and dinner within the 8-hour window. This method works for many folks because it gives your body time to burn fat in a fasted state while still allowing you to enjoy meals during a reasonable timeframe. 

However, it’s important to remember that no fasting method is a magic fix for weight loss, and even a plan like this requires nutritional balance and consistency to see results.

5:2 Diet

The 5:2 diet is another popular option where you eat normally for five days of the week, then restrict your calorie intake to around 500-600 calories on two non-consecutive days each week. 

This method allows for flexibility since you’re only calorie-restricting twice a week, but it’s important to stay mindful of your nutrition choices on fasting days to avoid feeling overly hungry or fatigued. 

When possible, choose protein and veggies for higher volume foods that are lower in calories. You likely will still experience some hunger, and if it feels unmanageable, scaling back on your fasting window can help.

While it can be effective for some, it’s not a quick fix—long-term success with any fasting plan depends on consistency and a balanced approach.

Eat-Stop-Eat

With Eat-Stop-Eat, you fast for 24 hours once or twice a week. It’s a more extreme method that can be effective for fat loss, but it’s also challenging since going an entire day without food can lead to intense hunger and low energy. 

If your lifestyle naturally allows for this type of eating pattern, or if you find that you don’t eat very much one day a week, this could be a fit for you. Overall, as a dietitian, this isn’t something I would recommend for most people.

This method can lead to negative side effects like fatigue, irritability, or overeating during non-fasting days. This is a more extreme way of fasting. It also may have an increased risk for binge eating as a result of fasting for too long and going into your next eating window starving. 

Alternate-Day Fasting

Alternate-day fasting involves fasting every other day, with around 500-600 calories consumed on fasting days. This method can lead to significant calorie reduction over time, but it’s one of the more challenging approaches to maintain due to frequent fasting. 

Some people may experience the mental fatigue of fasting every other day. On non-fasting days, it’s crucial to eat balanced meals to avoid overcompensating. While it can be effective for some, the restrictive nature of fasting every other day may cause side effects like low energy or nutrient deficiencies if not done carefully.

This type of fasting is likely not a good fit for most people as it can be quite restrictive and both mentally and physically draining.

OMAD (One Meal a Day)

OMAD, or One Meal a Day, is another extreme fasting method where you fast for 23 hours and eat all your daily calories in a single meal during a 1-hour window. 

While some people find it simplifies meal planning, it can be difficult to meet all your nutritional needs in just one sitting. This method can lead to overeating or digestive discomfort, and its restrictive nature may also cause unpleasant side effects like binge eating, fatigue, or nutrient imbalances. 

This might be a good fit for people who already like to eat once a day or have a shift work type of schedule. You’ll also want to consider what time you like to exercise and if you’ll have enough energy for movement throughout the day, with only one meal a day.

However, going for long periods of time without eating isn’t something we recommend for most people who want to eat healthier.

How Intermittent Fasting Affects the Body

Intermittent fasting may impact several physiological processes that can positively affect health:

Metabolism: Fasting shifts the body into ketosis, promoting fat burning as the main source of energy. (2)

Insulin levels: Fasting may improve insulin sensitivity, which helps regulate blood sugar levels. (2)

Cellular repair: Fasting activates autophagy, a cellular repair process where the body clears out damaged cells, potentially reducing the risk of diseases and promoting longevity. (6) However, it’s important to note that the process of cellular repair seems to occur in many different calorie deficit settings. Fasting isn’t the only way to achieve this health benefit. (6)

Hormones: Fasting may also regulate hunger hormones like ghrelin and leptin, helping control appetite due to taking longer breaks from food. (7)

Health Benefits of Intermittent Fasting

Weight Loss and Fat Loss

Intermittent fasting may be an effective tool for weight loss primarily because it restricts your eating times, which might reduce your overall caloric intake. When your body is in a fasted state, it starts to burn stored fat for energy, particularly visceral fat—the type stored around internal organs. (8) 

Visceral fat is a kind of fat associated with a higher risk of serious health conditions like heart disease and type 2 diabetes. Studies suggest that intermittent fasting may be especially helpful in targeting this stubborn fat while also preserving muscle mass, which can be challenging with traditional calorie-restricted diets. (8) 

Keep in mind, it’s important to pair intermittent fasting with balanced, nutrient-dense meals to ensure long-term success and prevent overeating during eating times.

Metabolic Health

Intermittent fasting may improve several markers of metabolic health, making it a potential strategy for preventing metabolic diseases such as type 2 diabetes and heart disease. (1) 

By reducing insulin levels and increasing insulin sensitivity, IF may help the body use glucose more efficiently and reduces the risk of insulin resistance, a precursor to diabetes. (9)

Plus, fasting may be associated with lower levels of inflammation, which is linked to many chronic diseases. By giving your digestive system a break and allowing your body to focus on repair processes, intermittent fasting may improve overall metabolic function and help lower blood pressure, cholesterol, and blood sugar levels. (10)

Longevity and Anti-Aging

One of the most exciting potential benefits of intermittent fasting is its potential link to longevity and anti-aging side effects. Some data have shown that fasting can increase lifespan and protect against age-related diseases by promoting a process called autophagy. (6) Autophagy is essentially your body’s way of cleaning up damaged cells and regenerating newer, healthier ones. (6) 

This cellular repair process not only helps to slow down aging but also protects against diseases like Alzheimer’s and cancer. (11) While human studies are still ongoing, the early results suggest that intermittent fasting may have anti-aging benefits by enhancing the body’s natural repair mechanisms. 

Cognitive Function and Brain Health

Fasting may also provide significant benefits for your brain. Intermittent fasting has been shown to increase the production of brain-derived neurotrophic factor (BDNF), a protein that supports the growth of new neurons and protects existing ones from damage. (12) 

BDNF plays a crucial role in learning, memory, and overall brain health. It may help reduce the risk of neurodegenerative diseases like Alzheimer’s and Parkinson’s. (13) Additionally, by reducing inflammation and oxidative stress, fasting helps protect brain cells and promotes better cognitive function over time. (13) 

We need more human research in this area to make more substantial claims, but the initial research is interesting. 

Risks and Drawbacks: Is Intermittent Fasting Safe for Everyone?

Despite its benefits, intermittent fasting isn’t healthy for everyone. Here are some potential risks for different populations.

Women and Intermittent Fasting

Women’s bodies are particularly sensitive to changes in energy intake, and intermittent fasting (IF) can have unique effects on their hormonal balance. Calorie restriction from fasting may lead to hormonal disruptions, particularly in reproductive hormones like estrogen and progesterone, which are responsible for regulating menstrual cycles. (14)

Prolonged fasting can cause irregular periods or, in extreme cases, amenorrhea (the absence of menstruation), which is a sign that the body isn’t getting enough energy. (14) 

Women who are pregnant or breastfeeding are often advised to avoid intermittent fasting unless explicitly recommended by a healthcare provider. Fasting can lead to nutrient deficiencies that may harm both the mother and child. 

For women in other life stages, shorter fasting windows (12-14 hours) might be a safer approach to avoid hormonal imbalances.

Fasting for Those with Medical Conditions

People with underlying medical conditions should be especially cautious when considering intermittent fasting. 

Individuals with diabetes need to be aware that fasting can lead to dangerous fluctuations in blood sugar levels. (1)

For those with heart disease, fasting could impact the cardiovascular system if it leads to rapid weight changes, blood pressure fluctuations, or dehydration. 

Anyone with a history of eating disorders should avoid fasting. Intermittent fasting may trigger unhealthy behaviors related to food control. (15) 

Consulting with a healthcare provider is essential before starting intermittent fasting to evaluate risks and ensure that it’s safe for your condition.

Common Side Effects

Intermittent fasting can trigger a range of other side effects even if you don’t have a health concern. 

Especially in the initial stages as the body adjusts to new eating patterns, some of the most common side effects include headaches, dehydration, fluctuations in blood sugar levels, fatigue, and dizziness. (16) Hunger during fasting periods may disrupt sleep cycles or cause insomnia. 

These side effects might improve once your body adapts to a fasting schedule. Watch your symptoms, stay hydrated, and eat nutrient-dense meals during eating windows to minimize these issues. (17)

Choosing the Right Fasting Method for You

Consider these parts of your lifestyle and food preferences before fasting and determine the right one for you.

Work schedule: Work to align fasting with your work schedule to stay sharp while working. Longer shifts may require shorter fasting windows to maintain energy and focus.

Lifestyle: Consider your social and daily activities. A flexible fasting plan will work better for busy or social lifestyles.

Exercise routine: Time your meals around workouts for enough energy. High-intensity workouts may require shorter fasting periods for proper recovery and energy.

Meal preferences: Choose a fasting method that suits your eating habits. Larger, less frequent meals may work better with fasting if that’s your natural tendency.

Binge tendencies: If you’re prone to binge eating, fasting may trigger overeating and might not be right for you. Opt for flexible eating windows to avoid restriction.

Sleep schedule: Avoid fasting too close to bedtime to prevent sleep disruptions. Adjust your fasting periods to fit your natural sleep patterns for a restful night.

Health history: If you have a history of health conditions like diabetes, heart disease, or if you are in a stage of life where you’re pregnant or breast-feeding, fasting is not a good fit for you.

Remember, fasting may not be right for you for a number of reasons, and that’s OK too. If you find yourself experiencing negative side effects, open up your eating window to improve your symptoms. 

What to Eat During Intermittent Fasting

While intermittent fasting focuses on when you eat, what you eat and drink matters. 

Nutrient-Dense Foods

Intermittent fasting may not tell you what to eat, but nutrition matters for any diet plan. 

Here are a few key nutritious foods to eat as part of a balanced diet while fasting. 

Lean proteins: Chicken, fish, turkey, eggs, tofu, yogurt, and legumes. (18)

Whole grains: Quinoa, brown rice, oats, and whole-wheat products. (18)

Healthy fats: Avocados, nuts, seeds, low fat cheese, and olive oil. (18)

Fiber-rich vegetables and fruits: Set a goal to include five different colors in your day to eat more fruits and veggies. (18)

Tracking your meals and snacks as well as your macros in the MyFitnessPal app can help keep you on track to meet your nutrition goals.

Hydration and Electrolytes

Dehydration is a potentially serious side effect of fasting. Just because you’re not eating doesn’t mean you should stop drinking water. Water, tea, coffee, and other no-calorie beverages will not break a fast, but they will help keep you hydrated. (19)

Also, maintaining electrolyte balance is important, especially during extended fasts. Consider adding electrolyte-rich foods during your eating windows, such as bananas, avocados, and leafy greens. (20)

Keep track of how much water you’re drinking in the MyFitnessPal app to make sure you stay hydrated.

Combining Intermittent Fasting with Other Diets

Combining fasting with specific diets can help you maximize benefits like fat loss, muscle maintenance, and improved overall health.

Keto and Intermittent Fasting

The ketogenic diet and intermittent fasting both encourage the body to enter ketosis, a metabolic state where fat is used as the primary energy source. When fasting, the body depletes its glycogen stores and switches to burning fat, which aligns with the goals of keto. (21)

This combination may accelerate fat burning and support weight loss. However, transitioning into ketosis may be more challenging during fasting, so it’s important to ensure your body is prepared for both dietary approaches. 

If you find this approach too restrictive or extreme, add in high-fiber carbohydrates and be more flexible with your fasting window. (22)

Intermittent Fasting and Plant-Based Diets

Pairing intermittent fasting with a plant-based diet offers a nutrient-dense approach while focusing on whole foods. Plant-based diets are naturally high in fiber, which promotes satiety and supports digestion during eating windows. (23)

When combined with intermittent fasting, this eating pattern may improve gut health, help control blood sugar levels, and provide essential nutrients like vitamins and minerals. 

Make sure your plant-based meals are rich in protein from sources like beans, legumes, and tofu to maintain muscle mass while fasting.

Low-Carb Diets and Fasting

A low carb diet generally keeps carbs low while focusing on protein, healthy fat, and lower carb fruits and veggies. 

Lower carb diets can support healthy blood sugar levels, and when combined with fasting might speed up weight loss. (24) However, many people find this combination challenging to stick to long term. 

Intermittent Fasting for Different Goals

Intermittent fasting can be adapted to fit various health and wellness goals. Whether you’re looking to lose weight, build muscle, or boost mental clarity, fasting can be a versatile tool.

Intermittent Fasting for Weight Loss

Intermittent fasting reduces the number of hours available for eating, so it tends to reduce calorie intake and create a calorie deficit. To stay on track, plan nutrient-dense meals during eating windows and create a routine that helps you avoid overeating, which can be a common pitfall for folks new to fasting. (1)

Intermittent Fasting for Muscle Gain

Intermittent can support muscle mass when combined with the right nutrition and exercise routine. (25) 

The most important nutrition factors for increasing muscle are eating enough calories and protein intake. Strength training during your eating window will also help preserve muscle mass, while fasting can increase growth hormone production, which supports muscle maintenance.  (25) 

Intermittent Fasting for Improved Mental Clarity

Some people report improved mental clarity and focus when fasting. Intermittent fasting might also help reduce brain fog by stabilizing blood sugar levels, leading to better concentration. (14) Additionally, fasting encourages the production of brain-derived neurotrophic factor (BDNF), a protein linked to cognitive improvement and neuroprotection. (14)

Common Challenges and How to Overcome Them

Intermittent fasting can present challenges, especially when first starting out. Fortunately, there are strategies to overcome common obstacles.

Dealing with Hunger and Cravings

Managing hunger is likely the top struggle for those who begin intermittent fasting. To combat this, focus on consuming high protein and fiber-rich foods during your eating windows, as they will help you feel full longer. (26) 

Drinking water, herbal teas, or black coffee during fasting hours can also help suppress hunger and prevent dehydration. And lastly, eating enough total calories during your eating window will help reduce symptoms of hunger and cravings. (26) 

Staying Consistent with Your Fasting Schedule

Maintaining consistency is key to success with intermittent fasting. To help with this, create a fasting schedule that aligns with your daily routine. 

Use tools like the MyFitnessPal Intermittent Fasting Timer to log your fasting hours and track your progress. Additionally, being flexible with your fasting window on busy or social days can help you stay consistent without feeling restricted.

Intermittent Fasting and Social Situations

Fasting can be challenging in social situations, such as dining out or attending events. To manage this, consider shifting your fasting window to accommodate your plans. For instance, if you have a dinner event, start your fasting period later in the day. Opt for lighter meals before or after the event, and stay mindful of your overall calorie intake.

Success Stories & Testimonials

Two of my personal clients have successfully integrated intermittent fasting into their lives and achieved impressive results. (I’ve changed their names for their privacy.) 

Jane, a 55-year-old counselor and social worker, used intermittent fasting to lose 20 pounds in about six months. She credits her success to having a list of foods she ate almost every day that kept her full and prevented sugar cravings at night while fasting.

Similarly, Mark, a 40-year-old lawyer, noticed improved mental clarity and sustained energy after incorporating intermittent fasting into his routine. He enjoyed not having to think about food on busy days at the office, and could focus on cooking a balanced meal at home.

Frequently Asked Questions About Intermittent Fasting

The Bottom Line

Intermittent fasting offers numerous potential benefits, from weight loss and metabolic health to improved brain function and cellular repair. However, it might not be the right nutrition approach for you.

It’s important to consider the potential risks, especially if you have certain medical conditions, and stop fasting if you experience unmanageable hunger or cravings.

At MyFitnessPal, we don’t endorse one type of diet, rather, we support each person finding what works for you. 

The post The Science Behind Intermittent Fasting: Benefits and Risks Explained appeared first on MyFitnessPal Blog.

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