The Myths & Facts of Intermittent Fasting

“…Tony the Tiger tells us that breakfast is the most important meal of the day! It’s grrrrrreat!”

This adage about breakfast has become commonplace that it’s readily and unquestionably accepted as fact.

Well then, what’s with the growing popularity of Intermittent Fasting and SKIPPING breakfast?

(Tony just audibly gasped.)

In this Ultimate Guide to Intermitting Fasting, I’ll teach you everything about the science of fasting and what results you can expect:

Let’s dig in!

What is Intermittent Fasting?

Intermittent fasting is not a diet, but rather a dieting pattern:

Intermittent Fasting: making a conscious decision to skip certain meals, or avoid consuming food for a specific time period.

Let’s talk about why it’s become so popular: because it can work incredibly well for weight maintenance!

When you skip meals or only eat during certain hours, you’re more likely to eat fewer calories overall, which can result in a lower number on the scale!

(We dive deeper into the science on this in the next section.)

Let’s now talk about some popular Intermittent Fasting Strategies:

#1) INTERMITTENT FASTING 16/8 PLAN

What it is: Fasting for 16 hours and then eating within a specific 8-hour window. For example, only eating from noon-8 PM, essentially skipping breakfast.

Some people only eat in a 6-hour window, or even a 4-hour window. This is the “feasting” and “fasting” parts of your days and the most common form of Intermittent Fasting. It’s also my preferred method (5 years running).

Two examples: The top means you are skipping breakfast, the bottom means you are skipping dinner each day:

Adjust this window to make it work for your life:

If you start eating at: 7AM, stop eating and start fasting at 3pm.
If you start eating at: 11AM, stop eating and start fasting at 7pm.
If you start eating at: 2PM, stop eating and start fasting at 10pm.
If you start eating at: 6PM, stop eating and start fasting at 2AM.

#2) INTERMITTENT FASTING 24-HOUR PLAN

With this plan, you eat your normal 3 meals per day, and then occasionally pick a day to skip breakfast and lunch the next day.

Eat on a normal schedule (finishing dinner at 8PM) and then don’t eat until 8PM the following day.

If you can only do an 18 hour fast, or a 20 hour fast, or a 22 hour fast – that’s okay! Adjust with different time frames and see how your body responds.

Two examples: skipping breakfast and lunch one day of the week, and then another where you skip lunch and dinner one day, two days in a week.

These are just two popular strategies, though there are many variations of both that you can modify for yourself:

Some people eat in a 4-hour window, others do 6 or 8.

Some people do 20-hour fasts or 24-hour fasts.

Another strategy is to eat only one meal a day (OMAD).

You’ll need to experiment, adjust to work for your lifestyle and goals, and see how your body responds. If there’s one thing we’ve learned after Coaching over 15,000 1-on-1 clients: there is no “one-size-fits-all” approach to exercise and nutrition that works for everyone, all of the time.

Let’s first get into the science here behind Intermittent Fasting and if you should consider it!

How Does Intermittent Fasting Work?

Now, you might be thinking: “okay, so by skipping a meal, I will eat less than I normally eat on average (2 meals instead of 3), and thus I will lose weight, right?”

All things being equal, yes.

By cutting out an entire meal each day, you are consuming fewer calories per week – even if your two meals per day are slightly bigger than before. Overall, you’re still consuming fewer calories per day.

In this example, you’re eating LARGER lunches and dinners than you normally do, but by skipping breakfast you’ll consume 500 less calories per day.

And thus, weight loss!

However, that doesn’t mean this works 100% of the time, for everyone. By understanding the underlying mechanism of weight loss (i.e. calorie deficit), we can better understand how our dieting strategy is affecting our behavior. Then we know if this strategy will work for us or not.

In the case of intermittent fasting, by condensing the eating window, many people feel more full and naturally eat less (like in the example above.)

However, it’s also totally possible for somebody to be so hungry as a result of fasting that they overconsume calories during that same period, which would result in weight gain instead of weight loss.

If you were reliant on the idea that “Intermittent Fasting works for weight loss” – full stop – you could be easily discouraged if it didn’t work.

You might think: “Is my metabolism broken?” But because we know that intermittent fasting is one dieting strategy that CAN work for weight loss if it helps you stick to a caloric deficit more easily, you’re empowered to decide if this is a good fit for you. Eureka!

This is highlighted in a recent JAMA study ^[2] in which both calorie-restricted dieters and intermittent fasters lost similar amounts of weight over a year period.

You might be thinking: “Ok, ok, I get it. Caloric deficit. But what about the timing of meals – can’t that also influence how your body reacts?”

Yes, your body operates differently when “feasting” compared to when “fasting”. But it’s important to understand how this fits into the big picture.

When you eat a meal, your body spends a few hours processing that food, burning what it can from what you just consumed.

Because it has all of this readily available, easy-to-burn energy (thanks to the food you ate), your body will choose to use that as energy rather than the fat you have stored.

During the “fasted state” (the hours in which your body is not consuming or digesting any food) your body doesn’t have a recently consumed meal to use as energy.

Thus, it is more likely to pull from the fat stored in your body as it’s the only energy source readily available.

However, when we compare the differences in energy used from body fat over an entire day, that’s entirely dependent on the total calories consumed. So while your body is more likely to pull energy from a recent meal, and will rely on fat stores once that energy runs out, if you eat the same amount of calories throughout the day, the result is the same amount of energy pulled from fat. It all balances out in the end.

The same goes for working out in a “fasted” state.

Without a ready supply of glucose and glycogen to pull from (which has been depleted throughout your fasted state, and hasn’t yet been replenished with a pre-workout meal), your body is forced to adapt and pull from a source of energy that it does have available: the fat stored in your cells.

While many of us get excited about the idea of being in a “fat-burning mode”, the same principle holds. If we burn an equal amount of calories, whether fasted or not, the result is less total energy stored in our fat cells at the end of the day. (There’s even an argument for athletes whose sports require glycogen to be readily available to meet their energy demands – making sure these stores are never depleted is important so an athlete doesn’t ‘bonk’ in the middle of their competition.)

Hopefully, you can see how easy it is to take a true fact of our biology (we burn more fat from fat stores when in a fasted state) and extrapolate it to seem more than it is. The same goes for six meals a day!

The truth is, there isn’t a magic pill or solution that’s going to overcome the basics. The best reason to do Intermittent Fasting is because you like it and it fits within your lifestyle.

TL/DR: Fasting can help promote weight loss and muscle building when done properly ~ though it isn’t the ONLY method that works.

Should I Eat 6 Small Meals a Day?

There are a few main reasons why diet books recommend six small meals:

1) When you eat a meal, your body does have to burn extra calories ^[9] just to process that meal. So, the theory is that if you eat all day long with small meals, your body is constantly burning extra calories and your metabolism is firing at optimal capacity, right? Well, that’s not true.

Whether you eat 2000 calories spread out throughout the day, or 2000 calories in a small window, your body will burn the same number of calories processing the food ^[10].

So, the whole “keep your metabolism firing at optimum capacity by always eating” sounds good in principle, but reality tells a different story.

2) When you eat smaller meals, you might be less likely to overeat during your regular meals. I can definitely see some truth here, especially for people who struggle with portion control or don’t know how much food they should be eating.

However, once you educate yourself and take control of your eating, some might find that eating six times a day is very prohibitive and requires a lot of effort. I know I do.

Also, because you’re eating six small meals, I’d argue that you probably never feel “full,” and you might be MORE likely to eat extra calories during each snack.

This is why personal preference is so important when picking a diet strategy that works for you.

Although grounded in seemingly logical principles, the “six meals a day” doesn’t work for the reason you think it would (#1), and may feel prohibitive to prepare and eat 6 times a day (#2). Other people may find that 6 meals a day fits them perfectly. If you find what works for you, that rules!

If we think back to caveman days, we’d have been in serious trouble as a species if we had to eat every three hours. Do you think Joe Caveman pulled out his pocket sundial six times a day to consume his equally portioned meals?

Hell no! He ate when he could, endured and dealt with long periods of NOT eating (no refrigeration or food storage) and his body adapted to still function optimally enough to still go out and catch new food.

A recent study (written about in the NYT, highlighted by LeanGains) has done a great job of challenging the “six-meals-a-day” technique for weight loss ^[11]:

There were [no statistical] differences between the low- and high- [meal frequency] groups for adiposity indices, appetite measurements or gut peptides (peptide YY and ghrelin) either before or after the intervention. We conclude that increasing meal frequency does not promote greater body weight loss under the conditions described in the present study.

That’s why we made this:

Should I Try intermittent fasting? (4 Big Benefits)

Now that we’re through a lot of the science stuff, let’s get into the reality of the situation: why should you consider Intermittent Fasting?

#1) Because it can work for your goals. Although we know that not all calories are created equal, caloric restriction plays a central role in weight loss.

When you fast, you are potentially making it easier to restrict your total caloric intake over the course of the week, which can lead to consistent weight loss and maintenance.

#2) Because it simplifies your day. Rather than having to prepare, pack, eat, and time your meals every 2-3 hours, you simply skip a meal or two and only worry about eating food in your eating window.

It’s one less decision you have to make every day.

It could allow you to enjoy bigger portioned meals (thus making your tastebuds and stomach satiated) and STILL eat fewer calories on average.

It’s a point that Coach Matt makes in this video on intermittent fasting:

#3) It requires less time (and potentially less money). Rather than having to prepare or purchase three to six meals a day, you only need to prepare two meals.

Instead of stopping what you’re doing six times a day to eat, you simply only have to stop to eat twice. Rather than having to do the dishes six times, you only have to do them twice.

Rather than having to purchase six meals a day, you only need to purchase two.

#4) Plus, Wolverine does it:

If adamantium-clawed superheroes do Intermittent Fasting, it can probably work for you too, if you can make it work for your particular lifestyle and situation!

What Are the Negative Effects of intermittent fasting?

It’s important to understand Intermittent Fasting is NOT a cure-all panacea.

Let’s talk about some of the potential drawbacks of Intermittent Fasting:

#1) If you skip breakfast, you might be so hungry from this that you OVEREAT your other meals, which can lead to weight gain. The important thing here is that with an intermittent fasting plan, you’re eating fewer calories than normal because you’re skipping a meal every day (if your goal is weight loss.)

In other words, don’t delude yourself into thinking that if you skip breakfast and then eat 4,000 calories of candy bars for lunch and dinner that you will lose weight.

This is simply a math and behavior strategy for giving yourself fewer chances to overeat and put your body into a caloric surplus.

(If you struggle with portion control, figure out your calorie goals and track your calorie intake in your meals to make sure you’re not overeating.)

#2) Skipping meals can result in feelings of lethargy, hunger, and “hangriness”!

If you eat breakfast every morning, your body expects to wake up and eat food.

Once you retrain your body to NOT expect food all day every day (or first thing in the morning), it’s possible that these side effects become less of an issue. In addition, ghrelin (a hormone that makes you hungry ^[13]), is actually lowest in the mornings and decreases after a few hours of not eating.

#3) Intermittent Fasting can be more complex for people who have issues with blood sugar regulation, suffer from hypoglycemia, have diabetes, etc. If you fit into this category, check with your doctor or dietitian before adjusting your eating schedule.

#4) Intermittent Fasting can affect women differently: there’s a whole section dedicated to that here.

Can I Build Muscle and Gain Weight While Intermittent Fasting? (Pros and Cons)

It is possible to build muscle and strength while Intermittent Fasting, though it’s not inherently superior to building muscle and strength while following a traditional eating pattern.

(We have our big “how to build muscle” guide, in addition to a whole “Strength 101” series – and I’d recommend you read those if you’re looking for a place to start strength training.)

Personally, I did intermittent fasting from 2015 to 2020 while building muscle and strength:

Let’s talk first about the nutritional strategy first, and then we’ll get into the exercise portion.

Here’s how I built muscle while simultaneously fasting: In order to gain weight and build muscle, I had to be in a caloric surplus.

And because I was not eating for 16 hours, that meant I was cramming 3000 nutritious calories into an 8 hour window, which often left me feeling bloated and overly full.

11 AM Work out with heavy strength training in a fasted state.
12 PM Immediately consume 1/2 of my calories for the day (a regular whole-food meal, followed by a calorie-dense homemade protein shake).
7 PM Consume the second portion of my calories for the day in a big dinner.
8 PM – 12 PM the next day: Fast for 16 hours.

After following this strategy for a number of years, I gave up fasting in 2019, and instead consumed my calories throughout the day.

Here’s a different strategy for Fasted Muscle Building: my friend Nate Green packed on a crazy amount of muscle while fasting for a full 24 hours on Sundays. ^[15]

Let’s now talk about strength training!

If you want to build muscle while fasting, you need to work out. Specifically, by lifting heavy, so here are a few workout options!

#1) “Build Your Own Workout Routine” and get your hands dirty. Our guide will walk you through building a full-body exercise program in 10 simple steps.

#2) Follow our Strength Building Guide and start training today. You’ll want to do lots of heavy compounds lifts:

How to tie it all together: pick up heavy things, eat enough protein, and consume enough calories, and you’ll build muscle and strength.

Should You Do Intermittent Fasting and the Keto Diet?

We have a crazy extensive guide on the Keto Diet in case you’re not familiar with it, so here it is in a nutshell:

By only eating fat and protein, your body must adapt to run on fat for fuel instead of carbohydrates. In the absence of carbs/glucose, your body converts fats to ketones and uses them for fuel.

This process is called “ketosis,” and there are two ways for a body to enter ketosis:

Eating in a way that induces ketosis (very low carb, high fat).
Fasting…Hey, that’s what you’re reading about right now!

We actually have an amazing success story here on Nerd Fitness, Larry, who followed our strategies, went Keto and start intermittent fasting. He ended up losing weight, getting stronger, AND overcame the challenges of rheumatoid arthritis (click on the image for his story)!

Why Keto and Fasting can work: eating Keto can be really challenging. And every time you eat, it’s an opportunity to do it wrong and accidentally eat foods that knock you out of ketosis.

You’re also tempted to overeat.

So, by skipping a meal, you’re eliminating one meal, one decision, one chance to screw up.

Note: if you’re thinking “Steve, am I losing weight because I’m skipping 1/3rd of my meals for the day, AND eliminating an entire macronutrient?”, then you’d be right.

Both Keto and IF have secondary effects that could also be factoring in – physiological benefits which I explain in both articles.

Your value may vary!

You need to decide what works for you.

You probably won’t become “keto-adapted” (your body running on ketones) just skipping breakfast every day – your body will still have enough glucose stored from your carb-focused meals for lunch and dinner the day before.

In order to use fasting to enter ketosis, the fast needs to be long enough to deplete your carb/glucose stores, or you need to severely restrict carbohydrates from your meals in addition to IF in order to enter ketosis.

MORAL OF THE STORY: Experiment and try different strategies that will work for you.

By skipping a meal or minimizing carbohydrate intake, you’re more likely than not to lose weight:

You can do intermittent fasting without eating a Keto Diet and lose weight.
You can do a Keto Diet without intermittent Fasting and lose weight.
You can combine them and lose weight.
You can do neither and lose weight.

Sticking with Keto is BRUTALLY difficult, and probably not the right diet for 98% of the planet. Those people would be better off following our “Start Eating Healthy” guide with small changes.

Does Intermittent Fasting Have Different Effects on Men and Women?

The quick answer is: “yes, Intermittent Fasting can affect men and women differently.”

Anecdotally, we have many women in our online coaching program that swear by Intermittent Fasting, while others have had adverse effects.

Let’s dig into the science and studies.

A recent PubMed summary concluded that “fasting can be prescribed as a safe medical intervention as well as a lifestyle regimen which can improve women’s health in many folds ^[18].

Now, in that extract, many of the studies cited are focused on specifically calorie restriction (and not just fasting), and they also say that “future studies should address this gap by designing medically supervised fasting techniques to extract better evidence.”

Digging into the PubMed Archives brought me to the following conclusions ^[19]:

One small study (with 8 men and 8 women, all non-obese) resulted in the following: “Glucose response to a meal was slightly impaired in women after 3 weeks of treatment, but insulin response was unchanged. Men had no change in glucose response and a significant reduction in insulin response.”^[20]

Another small study (8 women) studied the effects on their menstrual cycles after a 72-hour fast – which is significantly longer than any fast recommended in this article: “in spite of profound metabolic changes, a 72-hour fast during the follicular phase does not affect the menstrual cycle of normal cycling women.” ^[21].

Yet another study tracked 11 women with 72-hour fasts (again, longer than we’d recommend) and it found that “Fasting in women elicited expected metabolic responses – included increased cortisol (a stress hormone) – and apparently advanced the central circadian clock (which can throw off sleeping patterns). ^[22]

Those studies above, in working with small sample sizes, and different types of fasting than recommended here, would lead me to believe that fasting affects men and women differently, and that many of the weight loss benefits associated with intermittent fasting (that affect insulin and glucose responses) work positively for men and negatively for women.

There are also a series of articles^[23] out there that dig into the potential reproductive health issues, stress challenges, induction of early-menopause ^[24] associated with fasting (and calorie restriction) for women.

Precision Nutrition – a great resource – recommends not attempting Intermittent Fasting as a woman if:

The challenge associated with all of this is that there aren’t enough long-term studies, with large enough sample sizes, specifically targeting female humans, with relation to the different types of Intermittent Fasting.

It does appear that men and women will have different experiences with intermittent fasting; we’re all unique snowflakes (yep, especially you), and your body will be affected by intermittent fasting differently than the person next to you.

There is enough evidence as cited in the articles and studies above that would give me pause to recommend Intermittent Fasting for women, especially if you are considering getting pregnant in the near term.

If you are looking to attempt fasting for weight loss reasons, my research has shown me that Intermittent Fasting could be less effective for women than men with regards to weight loss, and thus you would be wise to keep your efforts elsewhere:

Now, if you’ve read the above warnings, you are still curious about Intermittent Fasting, and you want to give it a try as a female, that is your choice!

You know your body best.

So, get blood work done, speak with your doctor and get a check-up.

Give intermittent fasting a shot, track your results, and see how your body/blood work changes as a result of Intermittent Fasting and decide if it’s right for you.

Your mileage may vary, so speak with a doctor or find a doctor versed in intermittent fasting plans and treat it like an experiment on yourself!

6 Tips and Tricks about Fasting

#1) Don’t freak out! Stop wondering: “can I fast 15 hours instead of 16?” or “what if I eat an apple during my fasted period, will that ruin everything?” Relax. Your body is a complex piece of machinery and learns to adapt. Everything is not as cut and dry as you think.

If you want to eat breakfast one day but not another, that’s okay. If you are going for optimal aesthetic or athletic performance, I can see the need to be more rigid in your discipline, but otherwise…freaking chill out and don’t stress over minutiae!

Don’t let perfect be the enemy of good when it comes to your intermittent fasting plan. It’s just one tool in your toolkit.

#2) Listen to your body during your strength training workouts. If you get lightheaded, make sure you are consuming enough water.

If you notice a significant drop in performance, make sure you are eating enough calories (especially fats and protein) during your feasting window.

And if you feel severely “off,” pause your workout. Give yourself permission to EASE into intermittent fasting and fasted workouts. This is especially true if you are an endurance athlete.

#3) Stay busy. If you are just sitting around thinking about how hungry you are, you’ll be more likely to struggle with this. For that reason, I time my fasting periods for maximum efficiency and minimal discomfort:

My first few hours of fasting come after consuming a MONSTER dinner, where the last thing I want to think about is eating.
When I’m sleeping: 8 of my 16 hours are occupied by sleeping. Tough to feel hungry when I’m dreaming about becoming a Jedi.
When I’m busy: After waking up, 12 hours of my fasting is already done. I spend three hours doing my best work (while drinking a cup of black coffee), and then comes my final hour of fasting: training.

#4) Zero-calorie beverages are okay. I drink green tea in the morning for my caffeine kick while writing. If you want to drink water, black coffee, or tea during your fasted period, that’s okay. Remember, don’t overthink it – keep things simple! Although be aware that Dr. Rhonda Patrick over at FoundMyFitness believes that a fast should stop at the first consumption of anything other than water, so experiment yourself and see how your body responds.

If you want to put milk in your coffee, or drink diet soda occasionally while fasting, I’m not going to stop you. Remember, we’re going for consistency and habit-building here – if milk or cream in your coffee makes life worth living, don’t deprive yourself.

There are MUCH bigger fish to fry with regards to getting healthy than a few calories here and there during a fast.

80% adherence that you stick with for a year is better than 100% adherence that you abandon after a month because it was too restrictive.

If you’re trying to get to a minimum bodyfat percentage, you’ll need to be more strict with overall calories – until then, however, do what allows you to stay compliant!

#5) Track your results, listen to your body:

Concerned about losing muscle mass? Keep track of your strength training routines and see if you are getting stronger.
Buy a cheap set of body fat calipers and keep track of your body fat composition.
Track your calories, and see how your body changes when eating the same amount of food, but condensed into a certain window.

#6) Don’t expect miracles. Yes, Intermittent Fasting can potentially help you lose weight, increase insulin sensitivity and growth hormone secretion (all good things), but it is only ONE factor in hundreds that will determine your body composition and overall health. Don’t expect to drop to 8% body fat and get ripped just by skipping breakfast.

You need to focus on building healthy habits, eating better foods, and getting stronger.

This is just one tool that can contribute to your success.

Getting Started with Intermittent Fasting: Next Steps

Intermittent fasting can potentially have some very positive benefits for somebody trying to lose weight or gain lean body mass.

Men and women will tend to have different results, just like each individual person will have different results. The ONLY way to find out is through a conversation with your doctor and self-experimentation.

There are multiple ways to “do” an Intermittent Fasting Plan:

Fast and feast regularly: Fast for a certain number of hours, then consume all calories within a certain number of hours.
Eat normally, then fast 1-2x a week: Consume your normal meals every day, then pick one or two days a week where you fast for 24 hours. Eat your last meal Sunday night, and then don’t eat again until dinner the following day.
Fast occasionally: probably the easiest method for the person who wants to do the least amount of work. Simply skip a meal whenever it’s convenient. On the road? Skip breakfast. Busy day at work? Skip lunch. Eat poorly all day Saturday? Make your first meal of the day dinner on Sunday.

After that, get started! Take photos, step on the scale, and track your progress for the next month.

See how your body responds.

See how your physique changes. See how your workouts change.

And then decide if it’s something you want to keep doing!

4 years later, I have no plans on going back to eating breakfast. Sorry General Mills and Dr. Kellogg!

Thanks for reading, and I hope we gave you ALL the information you wanted about Intermittent Fasting, it’s underlying mechanisms for success, and reasons it may (or may not) work for you!

-Steve

PS: Before you take off, grab our Intermittent Fasting Worksheet to help you start your fasting practice:

PPS: Make sure you check out the rest of our guides on losing weight:

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Photo Source:^[29]

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