You’ve heard it before: intermittent fasting. Maybe you overheard some gym bros talking about it near the squat rack. Or read a comment on social media after your cousin’s wedding:
“You look so good! How’d you get ready for your wedding?”
“Thanks love…I used intermittent fasting!!”
BING. You’ve got my attention. Now you want to learn more about intermittent fasting. What is it? How does it work? We’ve put together an ultimate guide on intermittent fast. In this article, we’ll cover everything you need to know from what it is to how to get started.
What Is Intermittent Fasting?
Intermittent fasting (IF) is a pattern of eating that cycles between periods of eating and not eating.
It’s a structured approach that optimizes your body’s fat burning capabilities.
No, fasting is not starving. Starvation is “suffering or death caused by a lack of food.” It’s involuntary.
Fasting on the other hand, is controlled.
It describes what we as humans have been doing since the beginning of time.
Hunter gatherers didn’t pour themselves a bowl of cereal at 7, order indian at 12, and eat dinner at at 6. They scavenged and hunted, eating only what they were able to grow or kill. If circumstances like the weather made it difficult to harvest or hunt, they starved for days. Weeks. As a result, humans learned how to function without food for long periods of time.
(I wonder what a caveman would think if he walked in on us having dinner?)
Will intermittent fasting put my body into starvation mode?
Do a quick google search and you’ll see them pop up: articles claiming intermittent fasting puts your body into starvation mode. They aren’t true. In fact, one study shows that fasting for up to 48 hours can actually boost metabolism by 3.6-14%. That’s right, fasting revs up your metabolism, causing you to burn more calories.
So intermittent fasting isn’t starving. Intermittent fasting is controlled. And it’s goal is to manipulate the physiology of your body to optimize fat burning.
A Very Powerful Weight Loss Tool, But How Does It Work?
Ok, enough with the vague fat burning claims. How exactly does it work? There are a ton of resources out there explaining intermittent fasting. But they get too scientific and full of jargon. We don’t like jargon. We like simple. So without too much jargon, this is how intermittent fasting works.
From Banana to Soccer Goal
When your body needs energy it breaks down food. Let’s say food is categorized as either a carbohydrate, fat, or protein. Obviously, the composition is a lot more complicated than that, but let’s put that aside for the time being.
Let’s use an imaginary person-Joe. Joe is a student on his university’s soccer team. Before this evening’s soccer practice, he eats a banana. Banana= carbohydrate.
Now there is a preferred order of how the body likes to breakdown food. Your body breaks down food in the following order:
Notice that carbohydrates are first. Boooo! We want our body to burn fat first. Wouldn’t that be nice? Well that’s not how the body works unfortunately.
See your body is like your lazy Uncle Pete sitting on a couch using a stick to turn the tv on when he can’t find the remote.
It wants to do the least amount of work possible.
So why are carbohydrates first? Because carbohydrates are readily available for energy use.
Well, technically carbohydrates aren’t readily available- glucose is.
Going back to the banana, your body doesn’t access this fruit directly for energy. Joe chews the banana then swallows it. It then goes through his digestive system, breaking down into smaller components-glucose.
But to be clear, glucose is a carbohydrate. It’s a carbohydrate in soluble form. Soluble means that it’s easy to dissolve in water.
And what vital transportation system in your body is made of water?
Your bloodstream. In fact, 92% of blood is water. Once it’s in the bloodstream, glucose pushes into your muscles for energy.
So the banana enters your body as a solid carbohydrate and gets disbursed to your muscles in glucose form.
Are you starting to see why carbs are readily available for energy?
So now the glucose is in Joe’s bloodstream. He goes to soccer practice with lots of energy. He blasts through defenders and kicks the ball toward the goalie with a powerful thrust of his hip.
From Banana To Belly Fat
Now let’s say Joe eats the banana but instead of going to practice, he stays in bed and binge watches Game of Thrones. What happens to the banana?
Well it still gets broken down into glucose. But since his body isn’t active, it stores the glucose away in his liver as glycogen.
Glycogen is a storage for extra glucose. Your liver acts as a main storehouse. They can be broken down again later into glucose for energy when necessary.
Glycogen in the liver is like a small cupboard in your kitchen. It’s a small storage for canned foods (glucose) that’s just an arm’s length away when you need them. You’re exerting a little bit of energy to retrieve them (reaching with your arm, using a can opener to open it) but not so much.
But what happens when the small cupboard is overflowing with canned food?
It has to move the spillover into a bigger, long term storage: fat deposits. And guess what? Unlike the small cupboard, your fat deposits have an unlimited amount of storage room.
Remember we said that fats are number two in the order of how food breaks down? That’s because fat deposits are more difficult to access.
They need your body to use more energy. Fat deposits are like rental storage. You have to drive to the storage, open it and get the stuff, then drive home and back into the kitchen. Your body would rather use up all the canned foods in the cupboard (glycogen) before using fat stores. Make sense?
From (No) Banana to Fat Burning Machine!
Back to Joe. Let’s say after a night of binge watching, he wakes up and realizes a paper is due that afternoon which he didn’t start. So he jumps out of bed, starts working, forgetting to eat breakfast, lunch, anything at all.
Since the last thing Joe ate was the banana the day before, 16 hours pass by the time he finishes and submits his paper. And whoops! Joe also has soccer practice tonight.
By this point, Joe has depleted his glucose and glycogen storage. So what will happen to Joe during practice? Will he faint?
Nope! His body has no choice but to tap into fat storages and use fat for energy. He’ll now rely on his fat stores during practice, turning his body into a fat burning machine.
What Are The Side Effects Of Intermittent Fasting?
As tantalizing and awesome as all that sounds, intermittent fasting can be rough on your body. As a result, it might be harder to stick to. This can work against your efforts toward progress.
I struggle with intermittent fasting. Within the first 4-5 hours of fasting, I get drained and feel low energy. When I did it in college, I’d be more likely to space out and lose concentration in class. Reading and studying was hard because I often got brain fog. But within a week, I was already seeing results.
All in all, it’s a trade off. If you can endure the hit to your energy levels and focus, you’ll reap the rewards of rapid results. Also, not everyone might experience these side effects. You might be able to fast all day and feel completely fine. I’ve seen this happen with clients.
Here are some of the side effects:
You’ll feel hungry. If you’ve never done keto or a low-carb diet, intermittent fasting may be a rude awakening. 3-4 hours into a fast and may feel your stomach grumble. Be warned- your sense of smell gets stronger! You’ll be able to sniff out a slice of pizza with a 20 mile radius. (A hidden superpower?)
Trainer Tip: Try to stay as busy as possible to distract yourself. Drink water, iced tea, or coffee. And if you’re REALLY feeling the pangs, use sugar-free drinks as a last resort.
You might get constipated. Yup. You’ll take a number 2 less often since less is going in.
You might get headaches. They’re common and tend to disappear after the first few rounds of fasting.
Tip: Increase your water intake.
You might feel tired. Expect to feel groggy since your body is running on less energy. Your blood sugar level is low and your body won’t be operating at an optimal state.
Tip: Get more sleep than usual. Take naps. And if you’re not opposed, increase your caffeine intake.
You might feel irritable. Why? “The same biochemistry that regulates mood also regulates appetite,” Steiger says. (i.e. HANGRY) The amount of food you eat affects the activity of neurotransmitters that play a role in anxiety and depression. When you fast, your body’s chemistry gets off-balance, causing your mood to get off-balance too.
What Are The Benefits Of Intermittent Fasting?
Ok, enough with all these side effects. What are the benefits?
It can help you lose weight and body fat. This is the obvious one and likely the main motivation you are trying intermittent fasting.
It optimizes your body’s fat burning abilities. Yup. You saw what happened to Joe. After a fasting window of 16+ hours, the body switches from carbohydrates to stored fat for energy.
It simplifies your life. Have you ever rushed at lunch, desperately searching for something healthy to eat? Or do you have set times in the week when you meal prep?
I use to meal prep all my meals for the week every Sunday afternoon. I planned everything from the macros to the calorie breakdown. Then during my week, I kept careful track of my time, making sure to eat every 2-4 hours.
But during an intermittent fast, all I have to do is look at my watch, set my alarm clock to ring in 16 hours, and go.
No need to worry about hunting down a healthy meal. No need to carry tupperware or protein shakes. You just don’t eat. Intermittent fasting makes your day so much simpler, not to mention cheaper. (Less groceries!)
It requires less time and money. If every meal takes you 20 minutes to eat and you eat 6 meals a day, you spend 120 minutes or two hours a day on just eating.
But if you’re fasting and only eat one or two meals a day, that time spent eating goes down to 20-40 minutes. That doesn’t even include the time you save from doing less dishes!
How Do I Get Started?
To successfully execute an intermittent fast, do some planning upfront. Seriously. This will make everything much easier when it’s time to execute. You’ll be able to expect roadblocks and plan for them. You don’t want to be stuck fasting during your little cousin’s ice cream cake party!
Step 1: Planning
There are many ways you can put into place intermittent fasting. It’s a very flexible technique that can be tailored to your specific needs. So before you choose one, it’s best to consider some factors in your schedule.
Factors to consider:
- Special events or outings with food (birthdays, dinners)
- Work (or school) schedule
- Your scheduled workout time
Special Events or Outings (w/food)
On your weekly calendar, note all time slots with a special event or an outing involving food. From catered weddings, to family dinners, note everything. You’ll want to plan it so your eating window starts right around these times. You don’t want to be stuck on the sidelines drinking water with a side of ice.
Work (or school) Schedule
Next, check out your work schedule. Do you work a 9-5? Graveyard shift? 12 hour swing shifts? Do you take morning or evening classes?
Do you work a demanding, physical job (construction worker, firefighter) ? A mentally demanding job (doctor, attorney, engineer)?
Fasting may affect your performance in a negative way. You might need to time your eating window so it lands on your most important work hours.
Your Scheduled Workout Time
You can schedule your eating window to coincide with your workout schedule… or not. It’s a matter of preference.
But remember, you’ll burn more fat working out in a fasted state (see: Joe from example above). Your body runs out of carbs and pulls energy from fat stores. But if it’s inconvenient to do so, or you get totally zapped when you workout on an empty stomach, use the eating window.
Working out in a fasted state may make you feel weaker in the gym. You might notice that you’re not able to lift the same amount of weights as usual. Don’t panic. This is normal. You might need to try working out with and without eating to see what works best for you.
2. Choose Your Intermittent Fasting Method
After considering all factors, pick an intermittent fasting method that fits your lifestyle.
Here are some of the most common ones:
The 16/8 method: The most common method. Choose a 16 hour fasting period (make sure to lock up all the candy bars). After the 16 hour period, immediately begin your 8 hour eating period.
Trainer Tip: You can count hours asleep towards your fasting hours. So if you sleep for 7 hours, then when you wake up you’ll only have to fast for 9 hours more.
The 5:2 diet: For five of seven days of the week, eat like normal- no fasting windows. Then, on any two non-consecutive days of that week, only consume 500 calories.
Dr. Fung says these calories can be consumed at any time during the day as small snacks or a single meal. This is a great way for beginner’s to test drive intermittent fasting to see if it’s right for them.
The 20/4 method: Choose a 20 hour fasting window for the day. Then, begin a 4 hour eating period immediately after. Because this eating window is so short, you’ll likely only be able to squeeze in one or two small meals.
The 24 hour fast: No eating for an entire 24 hour period. Pick a time during the day- say 9:00 am Monday- then begin your fast. You can start eating at 9:00 am Tuesday.
Trainer Tip: I like to do this once a week, usually on a rest day and when I don’t have to do much brain work. Fasting lowers my brain functionality and energy, so I try to avoid doing it on those days.
The 36-hour fast: Same procedure as the 24 hour fast, except longer. Not for the faint of heart!
You might be feeling a little overwhelmed about which one to choose. Well it doesn’t matter which one for, “many roads lead to Rome.” They’ll all get you to your fat burning destination. Consider all the relevant factors, then make a decision based on that.
Here’s a (realistic) example of a person’s week and how they’d put intermittent fasting in place:
Jen works a 9-5 at a tech start-up. From her calendar, she has the following commitments:
- a company dinner on Wednesday at 6:00 PM
- family brunch on Saturday at 11:00 AM and
- a friend’s wedding on Sunday at 2:00 PM.
- she likes to workout after work around 6 PM
Taking this information, she starts her fast at 10 PM on Sunday before the week starts. She breaks her fast at 2 PM, choosing to schedule her eating windows around her workout. Her eating schedule is as follows:
- 2 PM: Meal 1
- 4 PM: Meal 2
- 6PM: Workout
- AFTER WORKOUT: Protein shake
- FIRST HOUR AFTER WORKOUT: Meal 3
- BEFORE 10 PM (END OF EATING PERIOD): Meal 4 (optional)
From Monday to Thursday, everything looks pretty standard. She’s using the 16/8 method to cater to her workouts.
On Wednesday, she takes a rest day from the gym so she can go to the company dinner. It’s timed perfectly with her eating window. Smooth sailing there.
But take a look at Friday and onwards. She changes her eating window to4 hours. Why? Her family brunch is on 11 AM Saturday the next day. So she needs to manipulate her eating window to accommodate that. She seamlessly transitions to the 20/4 method!
But that’s not all! Look at what she’s done in preparation for the wedding. Anticipating that the wedding has a catered buffet, she uses a 24 hour fast immediately after her family brunch to sync up with the wedding. The fast ends right when the wedding begins. Pretty nifty right? Intermittent fasting is effective, but you’ll have to plan ahead and address all roadblocks to reap its benefits.
Frequently Asked Questions
But doesn’t fasting cause muscle loss?
Answer: Technically yes.
Actually, any kind of diet can cause muscle loss. When you lose weight through dieting (no exercise), the loss is a mix of fat and lean mass, according to Healthline.
So yes, fasting can cause muscle loss. But so can any other diet. But, lifting weights and keeping protein intake high helps reduce it.
Interesting enough, this study shows that intermittent fasting causes less muscle loss than other calorie restricting diets.
Is intermittent fasting for everyone?
Answer: No. Intermittent fasting isn’t for everyone.
If you’re underweight or have a history of eating disorders, consult a health professional first.
Does intermittent fasting have different effects on men and women?
Answer: There’s evidence showing that intermittent fasting may not be as beneficial for women as it is for men.
Studies on female rats (none on humans) found that intermittent fasting had the following negative effects:
- emaciating (becoming abnormally thin or weak)
- masculinization (development of male sexual characteristics)
- missed cycles
Some women even reported their menstrual period stopped during intermittent fasting. It only came back when they started eating normally.
Precision Nutrition recommends not attempting Intermittent Fasting as a woman if:
- you’re pregnant
- you have a history of disordered eating
- you are chronically stressed
- you don’t sleep well
- you’re new to diet and exercise
But keep in mind that these studies don’t speak for the unique genetic makeup of every single person. There are definitely women out there doing intermittent fasting who are doing just fine.
Take careful consideration before starting. Consult a physician. Follow guidelines like easing into it gradually and stopping at the first sign of a missed cycle.
Can I build muscle and gain weight while intermittent fasting?
Intermittent fasting is a more efficient way of gaining muscle and minimizing fat gain than the “bulk & cut” method.
Can I drink liquids during the fast?
Answer: Yes, and I recommend you drink plenty to help control your cravings. Make sure they’re non-caloric, like water, black coffee, and tea.
Can I eat anything during my eating windows?
Answer: You shouldn’t.
Others sources tend to disagree with this. They say you can eat anything you want during your windows. But from personal experience as a personal trainer, I have to disagree.
Food quality is still important here. Try to eat mostly whole, single ingredient foods. A quick way to figure out what to eat is this: “If it rots, eat it.” If you can leave it on a kitchen counter and it won’t go bad after a month, stay away from it. So foods with added preservatives like frozen ready-to-eat meals, pop tarts, etc. are out. Fruits, veggies, and fresh produce are in.
Do I have to track my calories during my eating periods?
Answer: Counting calories isn’t usually required when doing intermittent fasting.
Since the eating windows are limited and you only have so much time to eat, theoretically you should be consuming less calories.
But after a week if you find that you’re not seeing progress, then I’d suggest tracking calories. A fitness app like MyFitnessPal works great.
Can I do intermittent fasting even if I’m vegetarian, vegan, do paleo, etc?
Remember, intermittent fasting is just an eating style. So it can adapt to many lifestyles, whether you’re vegan or paleo.
Can I exercise during fasting?
But wont fasting slow down my metabolism?
Answer: For fast shorter than 48 hours, no. In fact, it actually boosts metabolism. But anything more than 48 hours, and you’re metabolism begins to drop.
Intermittent fasting doesn’t require you to fast for this long, so you should be fine.
Intermittent fasting is another tool in your toolbox to use to reach your fitness goals. Everyone should know how to properly use it. It can be the one thing that moves you toward your goals when everything else fails. We hope this guide was useful and clarified how intermittent fasting works in a very straightforward way.