Intermittent fasting (IF) is an eating pattern that focuses on when you eat rather than what you eat. It alternates between periods of eating and fasting, helping your body shift from burning calories from food to using stored fat for energy. Here's what you need to know:
- How it works: During fasting, your body burns glycogen first, then switches to fat for energy after about 12-24 hours. This process can boost metabolism and improve insulin sensitivity.
- Health benefits: IF can aid in weight loss, improve blood sugar control, reduce inflammation, and support heart and brain health.
- Popular methods:
- 16/8: Fast for 16 hours, eat within an 8-hour window.
- 14/10: Fast for 14 hours, eat within a 10-hour window (great for beginners).
- 5:2: Eat normally for 5 days, restrict calories (500–800) for 2 days.
- Alternate-day fasting: Alternate between normal eating and low-calorie days.
- Eat-Stop-Eat: Fast for 24 hours once or twice a week.
- Who should avoid it: Pregnant women, children, people with eating disorders, and those with certain medical conditions should consult a doctor before trying IF.
Quick Comparison of Fasting Methods
| Method | Fasting Window | Eating Window | Best For | Common Challenges |
|---|---|---|---|---|
| 16/8 | 16 hours | 8 hours | Daily routine | Initial hunger, meal timing |
| 14/10 | 14 hours | 10 hours | Beginners, women | Adjusting to new schedule |
| 5:2 Diet | 2 low-cal days | 5 normal days | Non-daily fasting | Managing low-calorie days |
| Alternate-Day | Every other day | Normal eating | Strict routines | Higher dropout rates |
| Eat-Stop-Eat | 24 hours | Normal eating | Experienced fasters | Extended fasting periods |
Getting started: Start small with a 12-hour fast, stay hydrated, and focus on whole, nutrient-rich foods during eating periods. Track your progress with apps or journals, and consult your doctor if you have health concerns.
Intermittent fasting is flexible and can be tailored to your lifestyle, but it requires consistency and patience to see results.
How Intermittent Fasting Works
The Body's Energy Switch
When you fast, your body starts by using glucose from your most recent meals. Once that supply runs out, it taps into glycogen stores - a backup energy source stored in your liver and muscles - to keep your blood sugar stable. These glycogen reserves provide energy during the early hours of fasting, helping you stay steady.
Around the 12-hour mark, glycogen levels begin to drop, and your body ramps up fat burning, especially between 16 and 24 hours. Michigan Medicine dietitian Sue Ryskamp explains:
"When our insulin levels go down far enough and for long enough, as they do during a fasting period, we're able to burn off fat."
This is why fasting methods like the 16:8 approach are structured to align with these natural shifts in energy use.
After roughly 24 hours, glycogen is mostly depleted, and your body turns to stored fat more aggressively. It converts triglycerides into free fatty acids and glycerol, which your liver transforms into ketone bodies and glucose. These serve as alternative energy sources, fueling your body and brain. This transition marks the beginning of a series of metabolic and hormonal adjustments that occur during fasting.
What Happens in Your Body During Fasting
Fasting isn’t just about burning fat - it triggers a range of changes that affect your entire body. For example, fasting can improve insulin sensitivity by 20–31% and significantly increase hormone levels. In men, human growth hormone levels can rise up to fivefold, supporting both muscle maintenance and fat burning.
Julius Fink from Juntendo University's Graduate School of Medicine highlights the wide-reaching effects of fasting:
"Fasting triggers a cascade of changes in the hormonal, microbiome and enzymatic environments, leading to shifted glucose and fat metabolisms."
Here’s a snapshot of how key hormones respond during fasting:
| Hormone | Change During Fasting |
|---|---|
| Insulin | Drops significantly |
| Glucagon | Increases to stimulate glucose release |
| Leptin | Decreases initially, then stabilizes |
| Ghrelin | Rises at first, then drops |
| Cortisol | Spikes briefly, but may decrease with extended fasting |
These hormonal shifts work together to optimize energy use. For instance, glucagon prompts your liver to release stored glucose, while lower leptin levels help regulate appetite and energy expenditure.
Interestingly, short-term fasting can actually increase your metabolism. A study found that a 3-day fast boosted metabolism by 14% in healthy men, challenging the idea that calorie restriction always slows metabolic rates.
Extended fasting also leads to higher ketone production. Ketones act as an alternative fuel, especially for your brain, which usually depends on glucose. This metabolic flexibility ensures your body keeps functioning even when food is scarce.
Fasting even benefits your gut. It promotes a healthier balance of bacteria in your digestive system, which can enhance testosterone levels, improve glucose uptake, and optimize glycogen storage.
Health Benefits of Intermittent Fasting
Physical Health Benefits
For many, weight loss is the primary motivation behind intermittent fasting, and science supports its effectiveness. Research shows that overweight individuals can lose between 4% and 10% of their body weight over 4 to 24 weeks of fasting. Even more encouraging, studies on young men practicing 16-hour fasting revealed fat loss without sacrificing muscle mass. This addresses a common worry about losing muscle along with fat.
The mechanism here is straightforward: during fasting, the body shifts from burning calories from food to using stored fat as energy.
Intermittent fasting also shows promise for improving blood sugar control. A 2023 study found that fasting three days a week enhanced insulin sensitivity, reducing the risk of type 2 diabetes. For individuals already managing diabetes, the results can be profound. In one case series, participants saw their HbA1C levels drop by 4% over seven months while shedding an average of 22 pounds.
Another major benefit is lowering inflammation. A review of 18 studies conducted in 2022 concluded that intermittent fasting significantly reduces C-reactive protein levels, a key marker of inflammation. This is important because chronic inflammation is linked to conditions like heart disease and arthritis.
Speaking of heart health, intermittent fasting has been shown to improve blood pressure, cholesterol, and triglyceride levels. For example, studies conducted during Ramadan fasting reported reductions in bad cholesterol (LDL) and fasting blood glucose levels in both men and women.
Lastly, fasting can dramatically increase basal human growth hormone levels, with some studies reporting a tenfold boost after fasting for 37.5 hours.
Mental Health and Brain Benefits
The benefits of intermittent fasting extend beyond the physical, offering notable advantages for mental health and brain function. Since the brain uses up to 20% of the body’s total energy, changes in eating patterns can have a significant impact on how we think and feel - often for the better.
A review of 11 studies involving over 1,400 participants found that fasting during Ramadan was associated with improvements in stress, anxiety, and depression symptoms. Many people report feeling more mentally clear, less stressed, and in a better mood when practicing intermittent fasting.
The cognitive benefits appear to grow with age. In a study of individuals aged 60 and older with mild cognitive impairment, nearly 25% of those who fasted regularly showed no cognitive impairment after three years. This compared to 14% in occasional fasters and less than 4% in non-fasters.
Dr. Ila Dayananda, chief medical officer of Oula Health, explains the nuanced effects of fasting on mental health:
"Fasting can have positive and negative impacts on mental health, depending on such factors as individual health status, duration of fasting, and frequency."
Fasting helps stabilize blood sugar levels, which can lead to a steadier mood throughout the day. It also encourages mindfulness around eating habits, giving individuals a stronger sense of self-discipline and control. Many practitioners find that fasting increases their awareness of their body’s needs and signals.
Animal studies suggest another fascinating possibility: intermittent fasting may promote the production of new brain cells. However, more human research is needed to confirm this benefit.
Benefits vs. Risks Comparison
Intermittent fasting offers numerous advantages, but it’s important to weigh these against potential downsides. Here’s a closer look:
| Benefit | Scientific Support | Practical Impact | Possible Concerns |
|---|---|---|---|
| Weight Loss | Studies confirm reductions in body weight and fat | Easier to maintain than continuous calorie restriction for some | Risk of losing lean muscle without resistance training |
| Improved Blood Sugar Control | Enhances insulin sensitivity and lowers insulin levels | Could reduce reliance on insulin therapy in type 2 diabetes | May cause low blood sugar in diabetics |
| Reduced Inflammation | Lowers inflammatory markers like C-reactive protein | Potentially beneficial for inflammatory conditions | Mixed results for specific markers in some studies |
| Heart Health | Improves blood pressure, cholesterol, and triglycerides | Can lower cardiovascular disease risk | Effects on good cholesterol (HDL) vary |
| Brain Function | Animal studies suggest protection for brain health | Could help prevent Alzheimer’s or Parkinson’s | More human studies are needed |
While side effects are generally mild, they shouldn’t be ignored. A 2019 study involving 1,422 people fasting for 4 to 21 days found that 15% experienced sleep disturbances. Other common issues include hunger, headaches, digestive discomfort, mood swings, and fatigue.
It often takes two to four weeks for the body to adjust to intermittent fasting. Starting gradually and paying attention to your body’s responses can make the transition smoother. By understanding both the benefits and the challenges, you can determine if intermittent fasting aligns with your lifestyle and health goals.
Types of Intermittent Fasting
Popular Fasting Methods
Intermittent fasting comes in various forms, making it flexible enough to suit different lifestyles and goals. Each method has specific fasting and eating windows, so you can choose the one that aligns best with your daily routine.
One of the most well-known approaches is the 16/8 method, where you fast for 16 hours and eat within an 8-hour window. For instance, you might eat from 12:00 PM to 8:00 PM and then fast until noon the next day. Since much of the fasting period happens while you’re asleep, it’s a practical choice for many.
For those new to fasting, the 14/10 method offers a slightly easier start. This involves fasting for 14 hours and eating during a 10-hour window. For example, you could eat between 10:00 AM and 8:00 PM. Some experts suggest women begin with this method before gradually extending their fasting period to 16 hours.
Another beginner-friendly option is the 12-hour fast, which often aligns with a typical overnight fasting period. You might eat between 7:00 AM and 7:00 PM, making it a simple and manageable introduction to fasting.
For those with more experience, the 18/6 method involves fasting for 18 hours and eating within a 6-hour window. This approach requires careful planning to ensure you meet your nutritional needs in a shorter timeframe.
The 5:2 diet takes a different route. Instead of daily fasting, you eat normally for five days a week and limit your calorie intake to 500–800 on two non-consecutive days.
Another method, alternate-day fasting, alternates between regular eating days and days where calories are restricted to around 500. However, it’s worth noting that studies show a higher dropout rate for this method compared to other approaches. For instance, 38% of participants in one study left the alternate-day fasting group within a year, compared to 29% in a calorie-restricted group.
Lastly, there’s the Eat-Stop-Eat method, which involves fasting for a full 24 hours once or twice a week. For example, if you finish dinner at 7:00 PM on Monday, you wouldn’t eat again until 7:00 PM Tuesday. While effective for some, this method can be challenging to sustain.
Julia Zumpano, RD, LD, a dietitian at Cleveland Clinic, highlights the importance of finding what works for you:
"If you want to give intermittent fasting a try, be prepared to figure out what works best for you. It might take some trial and error first."
Fasting Methods Comparison
The best intermittent fasting method for you depends on your lifestyle, schedule, and personal preferences. Below is a comparison of some popular methods, their fasting and eating windows, and potential challenges:
| Method | Fasting Window | Eating Window | Best For | Common Challenges |
|---|---|---|---|---|
| 12-Hour Fast | 12 hours | 12 hours | Beginners seeking an easy start | May not provide as many metabolic benefits |
| 14/10 | 14 hours | 10 hours | Newcomers, especially women | Adjusting to a new eating schedule |
| 16/8 | 16 hours | 8 hours | Those looking for a daily routine | Initial hunger and meal timing adjustments |
| 18/6 | 18 hours | 6 hours | Experienced fasters | Planning meals to meet nutritional needs |
| 5:2 Diet | 2 days (500–800 cal) | 5 days (normal eating) | People who prefer not to fast daily | Managing low-calorie days |
| Alternate-Day | Every other day (500 cal) | Every other day (normal eating) | Individuals comfortable with strict routines | Higher dropout rates |
| Eat-Stop-Eat | 24 hours (1–2×/week) | Remaining days (normal eating) | Seasoned fasters | Extended fasting periods can be difficult |
Time-restricted methods like 16/8 and 14/10 are popular because they integrate well into daily routines. On the other hand, calorie-restriction approaches such as the 5:2 diet and alternate-day fasting often focus more on weight loss but come with unique challenges. A recent meta-analysis from 2023 found that time-restricted eating, alternate-day fasting, and the 5:2 diet can lead to weight loss ranging from 1% to 13% over 2 to 52 weeks. However, it’s worth noting that alternate-day fasting has been linked to increased LDL cholesterol levels six months after stopping the diet.
Ultimately, the key is to choose a method that aligns with your lifestyle and can be sustained over the long term. As Keri Gans, M.S., R.D., author of The Small Change Diet, explains:
"The more flexible the intermittent fasting plan is, the more likely you are to stick with it. One must choose one that can fit into their lifestyle - otherwise they most likely will fail."
Pick the approach that works best for you as you explore how to incorporate intermittent fasting into your routine.
How to Start Intermittent Fasting
First Steps for Beginners
If you're ready to try intermittent fasting, take it slow and steady. Jumping in too quickly can overwhelm your body, so it's important to ease into the process.
Talk to your doctor first. Before starting any new diet, including intermittent fasting, check with a healthcare professional. This is especially important if you have medical conditions or take medications that could be affected by changes in your eating habits.
Start small and extend gradually. Instead of diving straight into a 16-hour fast, begin with a 12-hour fasting window. For example, you might eat between 7:00 AM and 7:00 PM, then gradually shorten your eating window as your body adjusts. Eliminating breakfast is a common first step since it naturally extends the overnight fast.
As Zane Griggs, a well-known advocate of intermittent fasting, explains:
"Generally speaking, our dependence on body fat for fuel starts to be elevated after about 12 hours of not taking in any sort of calories."
Pick a schedule that works for you. Your fasting plan should fit your daily routine. Whether you prefer eating earlier or later in the day, choose a schedule that feels sustainable.
Know your calorie needs. Use a Total Daily Energy Expenditure (TDEE) calculator to make sure you're getting enough nutrition during your eating window.
Stay hydrated. Drinking plenty of water is key during fasting periods. You can also enjoy herbal teas or black coffee without added sugar or cream to help curb hunger and keep your energy up.
Focus on whole, nutrient-rich foods. When you break your fast, prioritize lean proteins, fresh vegetables, fruits, whole grains, and healthy fats. Avoid processed snacks and sugary treats that can cause energy crashes.
Starting out might feel challenging, but with a bit of preparation and patience, you'll adapt to your new routine.
Common Problems and Solutions
Even with the best plans, intermittent fasting can come with its share of challenges, especially in the beginning. Here’s how to tackle some common hurdles:
Hunger pangs are one of the most frequent complaints. The good news? Hunger usually comes in waves and only lasts about 15–20 minutes. When it strikes, try drinking water or sparkling water to feel fuller. Sugar-free gum can also help, and staying occupied with hobbies or light exercise can take your mind off the cravings.
Low energy can be another issue, especially during the first few weeks. Staying active with light movement or engaging in social activities - like a quick phone call - can help boost your energy levels. If you use caffeine, enjoy it earlier in the day and in moderation to avoid disrupting your sleep.
Sleep quality is crucial when fasting. Poor sleep can make hunger and fatigue worse, so aim for 7–9 hours of rest each night. Limiting alcohol can also improve your sleep quality.
Plan for tough moments. Identify when you're most tempted to break your fast early - like during social events or late-night cravings - and have a strategy ready. This could mean scheduling activities to distract yourself or preparing a game plan for social situations.
Avoid food triggers. Steer clear of food-related content, such as cooking shows or social media posts about food, during your fasting periods.
Remember, your body needs time to adjust. Research shows that hunger often decreases over time, and you'll likely start feeling fuller after meals as your body adapts.
Tracking Your Progress
Once you've settled into a fasting routine, tracking your progress can help you stay consistent and motivated.
Use fasting apps for support. Many apps, like Growth Bundle's WeFast, act as timers to track your fasting periods and provide helpful reminders. Some even include meal plans and community features to keep you inspired.
Here are a few apps from Growth Bundle that can help monitor your progress:
- WeFast: Tracks fasting windows and offers healthy recipes.
- HealthView: Syncs with Apple Health to track overall health and fitness.
- Reflectly: Lets you log your mood and identify patterns related to your fasting routine.
- The Do App: Helps you build and maintain fasting habits over time.
Track your mood and energy levels. Many apps allow you to log how you're feeling, creating visual graphs that show how fasting impacts your well-being.
Keep a journal. Note changes in your sleep, exercise, stress, and hunger levels. This can help you identify what works best for you.
As dietitian Keri Gans puts it:
"The most important feature would be a timer that reminds you when to eat and when to fast. Everything else is just a bonus and should be selected based on your unique needs."
Share your progress with others. Many apps include social features that let you connect with friends or family, creating accountability and boosting consistency.
Measure more than just fasting hours. Tracking additional metrics like weight, body measurements, sleep quality, and energy levels provides a fuller picture of how fasting is benefiting you.
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Safety and Important Considerations
Who Should Not Try Intermittent Fasting?
Intermittent fasting isn't suitable for everyone. For some, it can pose serious health risks, making it essential to understand who should steer clear of this eating pattern.
Pregnant and breastfeeding women should avoid intermittent fasting. Pregnancy already demands higher energy and nutrient intake, and limiting eating times could deprive both mother and baby of vital nutrition.
Children and teens under 18 are also not good candidates for fasting. Their growing bodies need a steady supply of nutrients throughout the day to support proper development.
Individuals with Type 1 diabetes face significant risks. As Mark Mattson, a neuroscientist from Johns Hopkins, explains:
"Because those with type I diabetes take insulin, there is a concern that an intermittent fasting eating pattern may result in unsafe levels of hypoglycemia during the fasting period."
Low blood sugar levels in these cases can become life-threatening.
Those with a history of eating disorders should avoid intermittent fasting, as the restrictive nature of this approach could trigger unhealthy behaviors or thoughts about food.
People taking medications that require food or influence blood sugar, blood pressure, or heart conditions should be cautious. Skipping meals can interfere with how these medications work and may cause dangerous imbalances in sodium, potassium, and other minerals.
Individuals with heart disease may also need to avoid intermittent fasting. Some research has suggested potential cardiovascular risks linked to restricted eating windows.
If you are already underweight, fasting could lead to excessive weight loss, weakening your immune system, reducing energy, and even impacting bone health.
Before starting any intermittent fasting regimen, consult with your doctor or a registered dietitian to ensure it's safe for you.
Fitting Fasting Into Your Daily Life
If intermittent fasting is a safe option for you, tailoring it to your daily routine is key to making it work. A successful fasting plan should fit seamlessly into your lifestyle, balancing your personal habits, work schedule, and social commitments.
Align fasting with your schedule. For instance, if you work a standard 8:00 AM to 5:00 PM job, you might opt for a 16/8 fasting schedule, eating between 12:00 PM and 8:00 PM. This allows for lunch at work and dinner with your family. Night shift workers can adjust their eating windows to better match their sleep and activity patterns.
Adapt for holidays and special occasions. During holidays or celebrations, it’s okay to adjust your fasting schedule. Enjoy the festivities and return to your usual routine the next day.
Manage workplace culture. Many offices revolve around coffee breaks or lunch meetings. If your fasting window excludes morning meals, stick to black coffee or plain tea during coffee breaks. For lunch meetings, suggest scheduling them later in the day or focus on the conversation while others eat.
Coordinate with family meals. If your family eats dinner at 6:30 PM but your fasting window closes earlier, either shift your eating schedule or plan your final meal to coincide with family dinner. Family time doesn’t have to be compromised.
Plan for travel and time zones. When traveling, adjust your fasting window to the new time zone rather than forcing your body to stick to your usual schedule. Flexibility is key.
Stay hydrated and focus on nutrient-dense meals. During fasting periods, keep water, herbal teas, and black coffee on hand. During eating windows, prioritize foods like lean proteins, vegetables, whole grains, and healthy fats to keep you satisfied and energized.
The goal is to build a fasting routine that feels natural and sustainable. It’s not about perfection - it’s about finding a rhythm that fits your life and supports your well-being. Consistency will go much further than rigid adherence to a schedule.
Beginners Guide to Intermittent Fasting | Jason Fung
Key Takeaways
This section highlights the main points from our guide. Intermittent fasting is an eating pattern that focuses more on when you eat than what you eat, making it a practical option for many beginners aiming to improve their overall health and well-being.
Start small and ease into it. For those new to fasting, a good starting point is a 12-hour fasting window - such as fasting from 7 p.m. to 7 a.m. - since much of this time overlaps with sleep. Once you're comfortable, you can gradually extend the fasting window to 14 or even 16 hours, following the popular 16/8 method . This step-by-step approach helps you build lasting habits.
Align your fasting schedule with your daily life. As Emmi Carr from the University of Michigan School of Public Health recommends:
"If it is a healthy option for you, choose the most realistic and achievable style".
The idea is to pick a schedule that fits seamlessly into your routine, whether you're balancing office hours, family meals, or shift work.
Pay attention to your body's signals and adjust as needed. Dr. Rekha Kumar from Weill Cornell Medicine explains:
"For the people who can adhere to IF, it does work".
However, it's crucial to listen to your body. If you experience symptoms like dizziness, extreme hunger, or fatigue, consider adjusting your fasting plan. It's perfectly fine to take a break for social events or if you're feeling unwell - just get back to your routine the following day.
Prioritize quality during eating periods. When it's time to eat, focus on nutrient-rich foods such as lean proteins, vegetables, whole grains, and healthy fats . Staying hydrated is equally important - drink water, herbal teas, or black coffee during fasting hours .
Track your progress and stay mindful. Using a journal or tracking app can help you monitor your energy levels, mood, and overall health. Keep in mind that intermittent fasting is a lifestyle change, not a quick fix. As Kristin Stangl, Senior News + Features Editor at The Spruce Eats, shares:
"The biggest reward with intermittent fasting is that I am much more in tune with my body. Am I hungry or am I eating because I am 'supposed to' and 'it's breakfast time'?".
Lastly, consult your healthcare provider before starting intermittent fasting. For those who can safely practice it, starting small and staying consistent can lead to lasting benefits.
FAQs
How do I choose the best intermittent fasting method for my lifestyle and health goals?
Choosing the right intermittent fasting method depends on your lifestyle, eating habits, and health goals. Start by assessing your daily routine. For instance, if skipping breakfast feels natural to you, the 16/8 method - fasting for 16 hours and eating within an 8-hour window - might fit seamlessly into your schedule. On the other hand, if you prefer more flexibility, the 5:2 diet, where you eat normally for five days and reduce calorie intake on two non-consecutive days, could be a better match.
Your hunger patterns and personal preferences also play a big role. If you’re new to fasting, easing in with shorter fasting windows like 12:12 or 14:10 can make the transition smoother. For those with more experience or specific weight management goals, longer fasts such as 18:6 or occasional 24-hour fasts might be worth exploring. The key is to listen to your body, observe how it reacts, and tweak your approach to find a balance that works for you in the long run.
What challenges do beginners face with intermittent fasting, and how can they handle them?
Starting intermittent fasting can feel tough at first, especially during those initial weeks when your body is still adjusting. It’s not uncommon to experience hunger pangs, low energy levels, or even the occasional mood swing. The good news? These challenges are usually temporary.
To make the transition smoother, focus on staying hydrated. Sip on water, tea, or black coffee throughout the day to help curb hunger and keep your energy steady. Another helpful tip is to ease into fasting gradually - start with shorter fasting periods and slowly increase the duration as your body gets used to the routine. When it’s time to eat, aim for nutrient-packed meals filled with protein, healthy fats, and fiber. These foods can help you feel satisfied for longer stretches. And avoid diving into overly restrictive plans right away - it’s easier to stick with something that feels manageable.
Most importantly, be patient with yourself. Adjusting takes time, so listen to your body and focus on building a sustainable rhythm. Small, steady progress often leads to lasting success.
What should I eat during eating windows to get the most out of intermittent fasting?
To make intermittent fasting work effectively, focus on consuming nutrient-rich, whole foods during your eating windows. Include options like whole grains, fresh fruits, vegetables, lean proteins, and healthy fats. These choices not only promote overall well-being but also help maintain steady energy levels throughout the day.
When you're in a fasting period, stick to zero-calorie drinks such as water, herbal teas, or black coffee. Steer clear of processed foods and sugary treats during your eating windows, as they can counteract the advantages of fasting. Instead, aim for well-balanced meals that leave you feeling satisfied and energized.