The Complete Intermittent Fasting Guide for Beginners
- Intermittent fasting involves alternating between abstaining from consuming food or caloric beverages and eating during certain time periods.
- Intermittent fasting is most commonly linked to healthy weight management. However, research supports other benefits—promoting healthy energy, brain power and cellular health.
- Intermittent fasting can be customized to your needs, as it is a concept rather than a single specific plan.
Intermittent fasting, also called IF, involves a cycle of fasting and then eating within a predetermined period of time. Humans naturally fast when we sleep and between meals and snacks. Intermittent fasting is based on research that supports extending those natural periods of fasting to promote certain health benefits.
Keep reading to learn how intermittent fasting works, how it could transform your eating habits and how you can get started.
What is intermittent fasting?
When you fast intermittently[1], you eat within a shortened time window—most commonly 8-10 hours. Here’s what that might look like over the course of a day:
- You wake up at your usual time, but you don’t eat breakfast.
- Around noon, you break your fast and have your first meal.
- In the evening, you eat dinner, which could include dessert.
- You stop eating by 8 p.m.
- You repeat this schedule the next day.
Why would people go long periods of time without eating? Although weight management has been considered one of the benefits of intermittent fasting, that isn’t the primary goal. Rather, IF is an eating schedule that may help regulate your insulin levels and support your overall health. Maintaining a healthy weight is one aspect.
This approach to eating might contradict traditional guidance about when to eat and how to measure hunger levels. Skipping a meal doesn’t shift your metabolism into “starvation mode” or inhibit your fat loss goals. Continue reading to learn about the potential benefits of giving your digestive system an extended break between meals.
How does intermittent fasting work?
Here is what happens in your body as you eat a meal:[2]
- Depending on what you’ve eaten, your blood glucose (sugar) levels may rise.
- In response, your pancreas produces the hormone insulin.
- Insulin signals to your cells that they need to increase their glucose supplies, while sending signals to keep your fat stores within your adipose tissue.
- Hormones like cholecystokinin (CCK) and leptin, which signal when you’re full, are also released.
When a meal contains larger quantities of carbohydrates (including sugars and starches), it slows down feelings of satiety, as this macronutrient doesn’t exert as much influence over CCK as protein and fats do. This delay can lead to eating too frequently or past the point of satiety. At that point, your pancreas has to work harder to release insulin, and the surplus glucose from the carbohydrates gets stored as body fat.[3]
Over time, those factors can influence your chances of gaining weight or developing insulin resistance (which can lead to type 2 diabetes). This may also influence cellular health and turnover.
Intermittent fasting allows your body to rest between meals, by diverting energy that would be used for digestion toward natural detoxification and repair processes that support other aspects of overall health. Taking a longer break between meals can allow your glucose levels to remain stable and your insulin levels to drop as your body focuses on cleaning and repairing—which can promote weight management and longevity.[4]
Types of intermittent fasting schedules
Intermittent fasting is flexible, with no strict rules or schedule. Some people fast daily, while others alternate days. Some people fast for 12 hours a day; others prefer longer stretches.
Here is a look at some ways IF can be implemented.
16/8
All calories are consumed during an 8-hour window, followed by 16 hours of fasting. This fasting schedule may appeal to those who are not hungry early in the day.
Fasting for 12 hours
The day is divided into equal 12-hour shifts for eating and fasting. Since about two-thirds of the “fasting” period would be spent sleeping, this plan is often appealing to beginners.
Eat-stop-eat
Developed by Brad Pilon, this intermittent fasting schedule involves fasting for a full 24-hour period on one or two non-consecutive days per week.[5]
5:2 diet
Fasting happens only two days a week. Caloric intake remains the same for five days, but the other two limit intake to 500-600 calories per day.
Alternate-day eating
You fast every other day, and eat normally on non-fasting days.
Weekly 24 hour fast
This approach involves fasting for longer periods once a week.
Meal skipping
This flexible approach simply involves shifting to the meals that work for your lifestyle. You might skip breakfast because you are not hungry until midday, or you can skip lunch if that is not a meal you usually need.
Warrior Diet
Similar to OMAD (one meal a day), the Warrior Diet involves fasting for 20 hours a day and consuming a large meal in the evenings. Small amounts of certain foods (including fruits, vegetables and broth) are permitted during the intermittent fasting hours.
Who should avoid intermittent fasting?
Before making any changes to your diet or exercise routine, including adopting an intermittent fasting routine, consult with a qualified healthcare professional.
If you are pregnant, breastfeeding, over age 65, have a history of disordered eating, have a medical condition or are taking medication, speak with your health-care practitioner about whether to engage in intermittent fasting.
More Diet Tips From Bulletproof
How to get started with fasting
Once you have spoken with a qualified healthcare provider, getting started with intermittent fasting is largely up to you. Choose one of the approaches above that works with your lifestyle.
You can follow the same eating pattern every day, or you can fast just a few times a week. You can even customize the amount of time you spend between meals—which is good news if you only occasionally want to follow a fasting schedule.
Choosing a time window
Adjusting to new habits and routines takes time. As you adjust to a fasting schedule, pay attention to how you feel and don’t be afraid to experiment with different fasting schedules.
For example, if you don’t want to skip breakfast, shift your fasting period. Eat a full meal around 8 a.m., and stop eating around 4 p.m. to allow for a 16-hour fast.
Choosing a meal plan
Intermittent fasting can be combined with any type of diet you choose to follow. However, intermittent fasting does support ketosis, by encouraging the body’s metabolic switch to this fat-burning states. Many IF followers choose to eat low-carb, high-fat foods when they break their fast. People report having fewer cravings and a reduced appetite on keto, so it pairs well with an intermittent fasting diet.[6]
During your eating window, if you get hungry between meals, try snacking on keto-friendly foods to avoid sharp drops in blood sugar, which can impair energy levels—try hard-boiled eggs, avocados, or fat-fueled coffee.
One note on calorie intake: Even if weight management is not your goal, you might naturally put yourself in a caloric deficit. In the short-term, cutting excess calories from your diet may help you stay at a healthy weight. But long-term calorie restriction can interfere with your metabolic rate and hormone function. That’s why you don’t want to stay in a caloric deficit for too long. If you experience fatigue, sluggishness or shifts in energy or mood, talk to your health-care provider and consider using a food tracker app to make sure you’re eating enough calories for your daily needs.
Tips for maintaining intermittent fasting
How can you implement and stick to intermittent fasting? Here are a few tips that can help:
- Start your fast after dinner, so a significant portion of your fasting period occurs during sleep.
- Eat satiating meals that contain quality fats and protein.
- Drink coffee to help manage your appetite and give you a caffeine boost
- Choose a fasting schedule that aligns with your lifestyle.
- Stay active and engaged so you’re not thinking about food.
What can you eat while intermittent fasting?
During a fast, you want to stay as close to zero calories as possible. Fasting purists only drink water during a fast. By not consuming any calories, you’re helping ensure your body stays in a fasted state and activates autophagy.
Most experts agree that drinking a cup of tea or coffee during intermittent fasting won’t significantly impact your results. In fact, the caffeine may help suppress your appetite, while delivering other benefits.[7] However, adding milk (dairy or plant-based) or sweeteners would break your fast.
How intermittent fasting affects your cells and hormones
Intermittent fasting can impact your cells, hormones and bodily systems. Here’s how:
HGH
Research shows that fasting results in increased peak HGH (human growth hormone) concentrations in sleep.[8] HGH is naturally produced by the pituitary gland and plays a key role in cell regeneration and growth.
Insulin
Implementing IF can be beneficial for those with insulin resistance. Research shows an IF regimen can reduce fasting insulin and insulin resistance in overweight and obese adults.[9]
Cellular repair
Over time, your cells naturally accumulate damaged components and waste. This debris can interfere with cellular function. In rodent studies, intermittent fasting has been shown to promote a process called autophagy, which is what happens when your body clears out the cellular debris so your cells can work even better.[10][11]
Gene expression
In studies, early time-restricted feeding increased the expression of the sirt1 gene, a molecule that promotes healthy aging, as well as LC3A, an autophagy gene.[12]
Fasting for weight loss
If you’re trying to lose weight, fasting can be an effective tool. It’s an easy way to control the amount of food you eat, and you’ll have more time to be active. Plus, combining fasting with a high fat, low-carb diet can further enhance your efforts to reduce your body fat levels and reach your body composition goal.
Benefits of intermittent fasting
Some people think of intermittent fasting for weight loss. It’s not that simple—you need to eat! But when you restrict your eating window, you naturally give your body a break from digestion. That translates to a few key intermittent fasting benefits:
Weight loss
Body weight is impacted by numerous factors, from your hormones to your blood sugar. In human studies, intermittent fasting has been shown to help prevent insulin resistance and leptin resistance[13], which may assist with weight management.[14]
Furthermore, a review on intermittent fasting programs determined both alternate-day fasting and whole-day fasting trials were effective at reducing body weight, body fat and total triglycerides.[15]
Inflammation
Research suggests intermittent fasting can be beneficial for inflammation. A study conducted on 110 volunteers celebrating Ramadan—a holy month for Muslims when no food or drink is ingested from dawn to dusk for 30 days in a row—found that prolonged intermittent fasting has a positive effect on the inflammatory response of the body.[16]
Heart health
A study conducted on 101 overweight and obese adults with prediabetes found that three weeks of either time-restricted fasting (16/8) or alternate-day fasting reduced triglyceride and glucose levels.[17] A separate review of alternate-day fasting trials from 3-12 weeks long found that body weight, body fat, total cholesterol and triglycerides were lowered in normal-weight, overweight and obese humans.[18]
Brain health
A rodent study found that intermittent fasting improved brain functions and structures.[19][20]
Anti-aging
In animal studies, intermittent fasting has been shown to promote healthy aging, support the brain and protect against disease.[21][22]
Potential side effects of intermittent fasting
What are some of the potential downsides of intermittent fasting? Hunger is one of the most common side effects of going prolonged periods without eating. In fact, a year-long study conducted on 112 participants found that the fasting group reported higher hunger scores than those who consumed a low-calorie diet with continuous calorie restriction.[23]
In addition, if you’re used to eating throughout the day, changing your schedule may be a bit of a mental challenge. Don’t get discouraged. Like any lifestyle change, it takes time to adjust and adapt.
The bottom line: Your individual dietary needs are based on factors like activity level and body composition. Your body will alert you if you need to adjust your intermittent fasting plan, and this eating pattern isn’t for everyone. What matters most is that you’re doing what helps you feel your best, day in and day out.
Ready to fuel up after your latest fast? These 19 tasty recipes provide the quality fats and protein your body needs to thrive in a single meal.
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This is an updated version of an article originally published December 2019.