Intermittent fasting (IF) has quickly become one of the most popular diet and fitness trends on the planet. Instead of focusing on what you should eat, intermittent fasting builds healthy habits around when you should eat, and reconfigures your body’s natural processes at the same time.

Some are jumping on this trend to lose body weight while others are looking to heal health issues or increase their performance. Intermittent fasting can be a bit overwhelming, as there are so many reasons why one could decide to make this lifestyle change and the physiological responses behind the diet that makes it so effective. This guide covers what IF is, how IF affects your body, and a few IF fasting schedules to get you started.

The Big Picture of Intermittent Fasting

Simply put, intermittent fasting is a specific pattern of eating where you alternate between specific fasting periods and eating windows every day (or every other day). IF doesn’t have a list of things you can or can’t eat. Instead, the big idea is more about restricting your overall food intake throughout the day. Though you would be better off pairing it with a nutritious, whole food diet.

IF is basically scheduling when you eat and when you don’t. In a way, doing some sort of IF helps you get the most out of the meal(s) that you do end up eating, since each meal you eat needs to makeup the 3-6 meals you were getting before.

The effectiveness of IF differs between people depending on the purpose of the fast, what the desired fasting outcomes are, and what your individual health goals are. Different fasting durations and schedules, how frequently you fast during the week, and the amount of caloric restrictions you set for yourself are all factors that vary from person to person.

How Intermittent Fasting Affects Your Body

As mentioned before, the success of IF depends on a variety of interconnected factors, and research done on IF so far has shown the same. However, it is safe to say that there are a ton of areas fasting does help your body. There are plenty of studies that have been done to prove its efficacy, such as this one by Dr. Jason Fung, the only problem is that there are so many different types of IF. Not only does the eating period change between one IF’er and another, but the types of food and the diet they are using with (paleo, keto, vegan, etc.) can all have an impact on their results. Here, we’ll explore a few scientific studies on IF and what results are the most important to your health.

Overall Reduced Caloric Intake

At its core, fasting restricts your caloric intake for an extended amount of time. If you are someone who currently has 3 meals a day with snacks in between, switching to a simple 16:8 fasting schedule (we’ll cover this schedule in-depth later) will cut out around a meal’s worth of calories from your daily intake. 

There's a good chance you’ve heard that in order to lose weight, your daily caloric intake should be lower than how many calories you burn. IF is designed to help you restrict your caloric intake while not leaving you feeling starved. Think about it. If you eat three 1000 calorie meals a day usually, but now you switch to two 1300 calorie meals a day, you will be in a 400 calorie deficit, but your two meals will leave you feeling more satisfied than your three 1000 calorie meals. If you take it even more extreme and eat only one meal a day, by taking down 2200 calories in one sitting you would be running an 800 calorie deficit. This means that over time, the potential to lose weight is promising due to your overall caloric restriction. 

Insulin

When you eat, the food you ingest is broken down by enzymes in your body. The broken down food then travels as molecules into your bloodstream. Foods like carbohydrates, and especially carbs like white flour, sugar, or rice, are further broken down into sugar your cells use for energy. If your body doesn’t use this fuel, it stores the excess energy as fat cells. 

When fasting, your body will automatically begin decreasing insulin levels in your blood. If your body’s insulin levels are too high, your ability to lose weight is more difficult. This is because insulin is responsible for bringing the broken down sugars into the fat cells, while also helping to keep it there. Reducing insulin with IF makes it easier to lose fat over time because the fat cells can release the stored sugar more easily than before. 

You’re probably starting to put the pieces together as to why fasting is important to lose weight: it lets our body’s insulin levels go down to a level where we can burn fat effortlessly. 

Increased Metabolism

Research shows that not eating for extended periods of time could actually increase your metabolic rate by 3.6 - 14% total. Your metabolism is responsible for converting food into energy, using food to build proteins and lipids, and also eliminating nitrogen waste in your body. 

Think of it this way: fasting helps increase your resting energy expenditure. When fasting, norepinephrine increases in your body, since there is a decline in your body’s glucose supply when you restrict your food intake. Norepinephrine is the hormone that tells fat cells to release fatty acids, making more fat available to be burned.

Autophagy: A Real Natural Cleanse

Fasting also allows your body to focus on cellular repair and cleansing. Research shows that restricting food intake in the short-term leads to a size-able increase in the number of autophagosomes in your body.

Autophagosomes are cell membranes that hunt down dead, diseased, or old cells. Once these cells are detected, they are “eaten” up by the autophagosomes, which will strip down these bad cells and use the resulting molecules to make new cell parts. 

When fasting, more autophagosomes are created in your body, creating an all natural cleansing and recycling process that occurs without you even knowing it!

Potential Cons of Intermittent Fasting

While intermittent fasting does introduce several important changes to your body, it does come with it’s fair share of symptoms, especially for beginners. 

• Low Energy - IF in general puts you under a daily recommended caloric intake. This means that in the first few weeks of IF, you could feel a lack of energy as you go about your day. You might feel your productivity decline, and also find yourself losing focus.
• Binge Eating - As your brain gets used to yourself being hungry for longer, sometimes the urge to eat is too powerful to resist. IF might cause binge eating during eating windows, simply because your natural instinct to eat kicks in after you’ve been hungry for too long. A good way to prepare for this is by eating healthy snacks throughout your eating window.
• Risky For Some - IF might actually work against you if you have low blood pressure, are currently taking beta-blockers, or need diabetic medication. Of course, IF is also not recommended for pregnant women, children, or anyone with an existing eating disorder.
• Eating is Less Social - Those who are on a strict IF schedule may find themselves in a difficult situation when invited to a meal outside of their eating window. If you find yourself getting spontaneously invited to grab a meal while fasting, the first step is to think about how much you’ve already eaten that day, and whether or not you’ll make healthy choices should you choose to have a cheat day!

Types of Fasting Schedules

There are many popular styles of IF schedules you can set up for yourself, which we’ll cover down below. 

16:8 Intermittent Schedule

A 16:8 intermittent schedule is perhaps the most popular IF schedule out there. A few celebrities have also spoken positively about a 16:8 schedule, including actors Hugh Jackman and Dwayne “The Rock” Johnson. 

During this schedule, your goal is to eat at or under your daily calorie restriction within an 8-hour eating window, and fast for 16 hours afterwards. When you’re in the thick of your 16 hour fast, we recommend sticking to zero-calorie beverages like water or black coffee. This way, you’ll stay hydrated, feel full, and not worry about potentially consuming calories during your fasting period.

If you already skip “the most important meal of the day” and prefer to eat lunch past noon, you’re probably already fasting on a 16:8 schedule! If you can skip breakfast and an go about your day without eating as soon as you wake up, its safe to say your body has already been conditioned to this routine. This is what makes a 16:8 schedule easy to maintain in the long term. One of our favorite benefits is the ability to just get up and start your day. You don’t have to worry about cooking or figuring out where you can grab a healthy breakfast from!

Customizing Your 16:8 Schedule

For many people looking to start on their IF journey, a 16:8 schedule is a strong starting point. Even cutting out one meal a day like breakfast can help you shed weight over time. Even better, if you’re used to eating sugary, carb-heavy breakfast foods like pastries or cereal then you’ll make an even bigger impact by cutting breakfast out of the equation.

If you’re thinking about getting serious with IF, you can use a 16:8 schedule as the minimum amount of time for fasting and eating. But if you think you can handle fasting for longer periods of time, these other popular timing breakdowns also exist:

• 18:6 Intermittent Schedule - A tad more restrictive than a 16:8 schedule. This schedule could look something like a 12:30 p.m. lunch, a 3 p.m. snack, and a 6:30 p.m. dinner. Try this fast if you find yourself eating early dinners on a 16:8, or if you are a few months in to your IF regimen and want to progress past a weight loss plateau.
• 20:4 Intermittent Schedule - Often referred to as the Warrior Diet, this schedule was originally geared towards eating in the evenings within a 4 hour eating window. The extended fasting period allows your body’s insulin levels to stay lower for longer, which we’ve mentioned can assist with weight loss. 
• Circadian Rhythm Schedule - In this style of IF, your eating and fasting schedule attempts to emulate a natural, healthy circadian rhythm. This means you will stop eating from roughly when the sun sets to when you wake up in the morning. This schedule is a great way to cut off late night or evening snacking!

Pro Tip: I always follow 16:8 at the minimum, but if I have a long work day at the office and can’t get out till 8pm then that day is basically a 20:4 day. Sometimes, I’ll even go the whole day without eating and go with alternate day fasting. By being flexible, I’m able to say yes to breakfast sometimes or a late dinner with friends. Your eating window is NOT the end all be all. Have some fun!

Alternate Day Fasting / 5:2 Diet

Alternate day fasting is another schedule for more experienced individuals who have fasted for a while already. Essentially, this schedule sets apart 1 to 2 days per week where you’re fasting for 23 hours. In practice, this would look something like a 12:30 p.m. Saturday lunch and a 12:30 p.m. Sunday lunch while you fast in between those times. Overall, with alternate day fasting your goal is to maintain a 23:1 ratio for your fasting period and eating window.

Another form of alternate day fasting is eating three meals on one day, and then eating nothing at all the next day, followed by eating three meals the next day. Basically, you switch off fasting and feeding everyday!

The logic behind alternate day fasting is that as a whole, you design your entire week’s caloric intake to be at a deficit but without the negative hormonal effects consistent under-eating can have on your body. It’s actually better to not eat for a day rather than eat everyday but at a deficit, because your body will switch to burning fat and not think it is starving.

An attractive aspect of alternate day fasting is the ability to still eat what you like on your non-fasting days. Since you’re only worrying about 2 fasting days a week maximum, you can easily fit this into your schedule on a weekday or weekend. If you’re still really feeling hungry on this schedule, alternate day fasting can allow you to eat at least one meal each day of the week.

If you think you’ll be hungry and an alternate day fast seems challenging, a 5:2 diet follows the same broad strokes as alternate day fasting with slightly more freedom. In this diet, you eat normally for 5 days of the week, but restrict your calorie intake to 500-600 calories for 2 days of the week. 

Intermittent Fasting in a Nutshell

Now that you’re familiar with what intermittent fasting is, what it hopes to accomplish, and the different ways you can incorporate it into your lifestyle, the next step is getting started. Research has shown the physiological benefits of fasting, and for many out there the results have spoken for themselves.

If you’re looking for a way to build a healthy habit and reap the benefits of a healthy lifestyle quickly, intermittent fasting is an amazing way to begin! But remember, allow your body to slowly adjust. If you are used to a high carb, 6-meal-a-day diet then I wouldn’t recommend going straight to a 23:1 fasting period.