Intermittent Fasting 101 & How It Works

I’ve been using intermittent fasting now as a lifestyle for over a year, and I’ve never been lean and strong. It also helps me to focus while working. In fact, I’m fasting right now and enjoying a black coffee as I write this.

However, intermittent fasting, as the research is finding, is no magic pill for fat-burning. It works well for specific circumstances—likely sedentary desk-workers. And it seems like it doesn’t work for every personality type as it can make some people more irritable.

We’re going to take a look at how intermittent fasting works, how it could help you achieve your goals, and when you would (and wouldn’t) want to use this technique.

Coffee & intermittent fasting are best friends

What does intermittent fasting mean?

Intermittent fasting is a catch-all term for:

  • alternate day fasting
  • whole day fasting
  • time-restricted eating like the popular 16/8 intermittent fasting

Alternate day fasting

The most studied type of intermittent fasting is alternate day fasting. In it’s simplist form, there’d be a 24 hour fasting period, and then there’d be a 24 eating period.

Let’s say you eat around 2400 calories daily. If you don’t eat for a whole day, that takes 2400 calories out of your weekly calories. The next day, most people can’t make up that loss of calories by eating 4800 calories in one day. They can often eat more, let’s say another 500 calories (it’s different for every person), but there’s still a gap.

So now you’re missing 1900 calories, and that missing energy will need to come from somewhere—your fat. And this is what studies show, that people were not able to eat enough on the next day to make up for all the missing calories from the day they fasted. Because not eating also means not having any protein, that meant they lost weight—both muscle and fat. This isn’t ideal if you’re trying to keep your strength, so, this type of fasting isn’t usually the best method for most people.

Whole day fasting

Whole day fasting is when you choose to not eat once (or sometimes twice) throughout the week for 24 hour hours. The rest of the week you eat normally. But because you missed a whole day of eating, you’ll get into a calorie deficit so you’ll lose weight.

In one study, comparing whole day fasting to a regular diet (daily calorie restriction), 70% of people hit their calorie goals with the whole day fasting compared to only 39% of those who were doing the regular diet. So this study suggests that it’s easier to hit your calorie goals when doing the whole day fast compared to regular dieting.

I tried the whole day fast after being inspired after reading an article about Dr. John Berardi’s experiments with fasting. The last meal I ate on Saturday was at 8 pm, and then I couldn’t eat again until Sunday at 8 pm. Sunday morning I had pretty intense focus, then in the afternoon I was becoming a bit foggy, and by 5 pm my mood and concentration were completely shot. But I could imagine that whole day fasting could become more comfortable as you get used to it.

Whole day fasting could work particularly well for someone who works a stressful job during the week, and they need to eat to perform well. They could still try fasting by doing a whole day fast on their day off work. That would help them work optimally, while still using fasting as a way to get into a calorie deficit to burn fat without needing to do a regular diet that tracks calories.

Time-restricted eating

Time-restricted eating is typically 16–20 hours of fasting and 4–8 hours of eating daily, and it’s one of the most popular methods of intermittent fasting today. Martin Berkhan popularized it with his LeanGains method of fasting which is fasting for 16 hours and then eating for 8 hours. (Martin Berkhan’s approach has a few more details like using a BCAA supplement and lifting weights while being fasted for even better fat loss and muscle maintenance.)

There have been a lot of studies on this time-restricted eating due to Ramadan fasting. Ramadan includes about a month of complete fasting (no water either) from sunrise to sunset. In studies, people lose weight—meaning they lose both lean mass (muscle, water, glycogen) and fat. But aside from Ramadan, not a lot of research has looked into it time-restricted eating until recently.

A 2016 study looked into this 16/8 type of time-restricted eating on men who have been lifting weights for at least five years. The study found that the 16/8 technique helped to get them into a minor calorie deficit, so they burned a bit of fat while keeping their strength. (You can read a study review by Stronger by Science). This isn’t without tradeoffs of course. The people doing 16/8 had their testosterone levels drop, and their cortisol levels (stress) increased. It was only an 8-week study, so it’s too short to say anything meaningful about the long-term effects.

Intermittent fasting, work, and coffee

So does intermittent fasting work?

To answer that question, we need to know what you’re trying to do.

Are you overweight and trying to burn fat while hanging onto your muscle?

In a 2015 review, researchers looked at 40 studies and found that intermittent fasting got the same results as regular dieting (calorie restriction.) There didn’t seem to be any differences in improving glucose or insulin sensitivity either, but they admitted that there wasn’t enough research in that area to draw any conclusions.

But the researchers did say that intermittent fasting was much better at controlling hunger. So intermittent fasting didn’t change the results; it just felt a bit better for those doing it.

In this 2018 study with obese men, those doing intermittent fasting lost more fat compared to a well-thought out diet protocol. But we can’t take too, too much from this study since these people weren’t working out.

In this 2018 review of 6 studies on overweight and obese people, intermittent fasting had no real difference between a regular diet (calorie restriction). They both got the job done.

In this January 2019 study, researchers looked at healthy but overweight women and wanted to see how intermittent fasting (whole day fasts, 3x a week) help with insulin sensitivity and metabolic problems. They told the women in the intermittent fasting group to eat enough calories to stay at maintenance. However, the women couldn’t eat enough. On the days they were trying to eat more, they still came up short by 240 calories.

This study demonstrates the main benefits of intermittent fasting. It’s just too hard to eat enough to make up the difference of all the calories missed while fasting.

So if you’re trying to burn fat, intermittent fasting could help you get into a calorie deficit without trying. And because you can eat to fullness for lunch and dinner, you don’t feel like you’re on a “diet” or that you can’t eat something. You can still eat it, you just get to eat it later.

So what’s the final word for those trying to get leaner? Well, intermittent fasting can:

  • help you deal with hunger by allowing you to eat full, satisfying meals. All you have to do is wait.
  • make it easier to naturally get into a calorie deficit to burn fat by removing meals. No more counting calories.

And if you decide to do time-restricted eating like 16/8 and combine it with lifting weights, you can keep your strength as you get leaner. You’d burn fat while you’re fasting, then ideally you’d work out and then break your fast with a big and satisfying meal full of protein (ideally 30-40g of protein). Lifting weights and eating lots of protein would help you maintain your muscle, so you shouldn’t get weaker as time goes on.

Are you skinny and trying to rapidly gain weight and build muscle?

If you’re a skinny guy or gal who’s trying to gain weight, you should not be doing intermittent fasting. That’s because intermittent fasting is a technique that naturally helps you get into a calorie deficit to lose weight. That’s the complete opposite of your goals.

Intermittent fasting means eating less meals, like cutting out breakfast. That means that it’ll be harder to reach your lofty calorie goals and you won’t stimulate muscle protein synthesis as often.

Eating more often is one of the best tricks a skinny person can use to gain weight, especially if they’ve got a small stomach size.

And for optimal muscle building, you’d want to stimulate muscle protein synthesis as often as possible. That means having a good amount of protein of aiming for around 40g every 3–5 hours. (study, study) (We boosted the recommendation since one study shows that even elite athlete’s underestimate their protein intake and because older people need more protein. There is no downside to getting more protein, and it is a good way of bulletproofing your gains. Check out our protein powder article here for more information.)

If you want to learn more about bulking and intermittent fasting, check out this article that Shane wrote on Bony to Beastly.

Are you skinny-fat and trying to both get leaner and build muscle?

You may want to try intermittent fasting. The reason why is because intermittent fasting will help you get into a calorie deficit, so you burn off that extra fat you have hanging around.

Now, to build muscle, it helps to have extra energy (a calorie surplus). But… you’ve already got an energy surplus—your extra fat. And if you don’t already have a lot of muscle and don’t work out, you’re further away from your genetic potential, so it’s way easier to build muscle (called newbie gains).

So you could do intermittent fasting to burn fat, and at the same time some of your fat will get used for energy to build muscle. Then, once you break your fast, aim for 0.8–1g of protein (boosted recommendation) and lift weights and you’ll begin to build muscle.

So if you’re skinny-fat, you can become leaner and stronger at the same time with intermittent fasting. After a few months, if all goes well, you should have a bit more muscle and be pretty lean. Maybe now you’re no longer skinny-fat, but you’re fit and lean but still a bit slender. That could be a time to stop doing intermittent fasting and gear into a slow bulk by adding a couple of hundred calories daily to keep your BMI moving up into the range you’re aiming for.

I’m already lifting weights, strong and lean, and I want to keep it that way

I’ve been enjoying time-restricted eating (doing 15/9) as a lifestyle because at the moment I don’t want to prioritize the energy to either 110% bulking mode or 110% cutting mode. With this type of intermittent fasting, I can make tiny muscle gains while burning some tiny amounts of fat. It’s slow on both fronts, but it’s effortless. So if you’re looking for a tweak to your lifestyle to maintain your strength and leanness, and maybe even make some small improvements, intermittent fasting could help.

There is one thing you could try to optimize intermittent fasting though. Because protein doesn’t have a way of really storing in our body (some carbs stored as glycogen, fats stored as body fat/adipose), when you eat protein, it mainly either goes through a process of muscle protein synthesis, or it gets burnt off.

But if you eat your meals spaced out by 3-5 hours, you can optimize muscle protein synthesis and be building muscle all day. Research shows that you can get better gains by spreading out your protein like this (study).

One way to potentially get the benefits of intermittent fasting while still maximizing your muscle protein synthesis window is by having a whey protein shake with water in the morning. This is what I do. While I don’t have the protein shake every day, I’ll have a whey isolate shake with water when I feel especially sore from yesterday’s work out, or I feel extra hungry in the morning. This quick protein shake removes a bit of the hunger feeling, and I can maximize muscle protein synthesis, while still getting the main benefits of intermittent fasting by naturally reducing calories (because there are minimal calories, carbs or fats.)

Breaking the fast with an amazing, satisfying meal

Good questions

What can you have during your fast?

We drink water, black coffee, and green tea.

Will intermittent fasting cause an eating disorder?

A study from 2015 looked at all-day fasting for eight weeks on people who were between 25 and 65 with no previous eating disorders. They lose 9 pounds—4.8 was fat and 4 pounds of lean mass. Depression and binge eating decreased, and so did their body concerns about their size and shape. So it seemed to improve well-being. The study has limitations, of course. There was no control group for example, and who knows what would happen beyond eight weeks. But from the evidence so far, intermittent fasting may help us have a better relationship with food, not a worse one.

How does intermittent fasting impact mood and focus?

In this 2016 study on women, they found that they became more irritable but also felt proud and accomplished of the control they had.

Which type of intermittent fasting is best?

All types of intermittent fasting have worked in research, so it all comes down to personal preference. It’s not a magic bullet, but it may be a great fit for your lifestyle. If you don’t want to track calories, and you can handle more stress, you might be a good person to give it a shot.

Just remember that if you’re trying to burn fat and stay strong, intermittent fasting won’t fix a poor foundation. Your results will still come down to doing the fundamentals consistently.

  • Eating mainly whole and quality food
  • Eating enough protein every day
  • Doing an intelligently designed weight training program
  • Sleeping 7–9 hours

From the research, it seems like the main benefit to intermittent fasting is just helping you to naturally get into that calorie deficit to start burning fat without needing to track calories.

My thoughts on intermittent fasting

The evidence so far says that intermittent fasting is similar to a regular diet in terms of results. And that may be true when you’re in a study participant and are required to keep a detailed food log.

But from my own experience in the real world, where my training and nutrition aren’t first priority (family always comes first) intermittent fasting is just a whole lot easier in terms of a realistic lifestyle. A regular diet has the stress of counting calories and you need willpower needed to stop eating when you aren’t totally full. I find it easier to just not eat at all and to distract myself, rather than eating a smaller meal and stopping.

Intermittent fasting removes the load on your brain from worrying about counting calories to get into a deficit, and it’s easier to deal with the hunger when you can just skip breakfast and enjoy lunch and dinner like a normal person and eat to fullness.

And as a calm person, I enjoy the extra cortisol stress that helps perk me up and give me mental focus in the morning. When I was gaining weight, one thing I’d hate doing is having a 1,000 calorie smoothie for breakfast and the sleepiness that an epic meal like that would bring shortly after.

Because intermittent fasting main benefit is limiting calories, and it’s the main downside when it comes to muscle is shrinking the muscle protein synthesis window, I don’t worry about breaking the fast with an ultra-lean whey isolate protein.

My Experience With Intermittent Fasting

Even though I was naturally skinny for most of my life, after focusing on getting bigger and stronger, I had gained over 56 pounds (186 was my heaviest) by lifting weights and eating better. Even with a great training plan and high-quality nutrition, not all of that weight I had gained was lean. I needed to burn some of that extra fat off while keeping my muscle before I could focus on getting even stronger.

My business partner Shane was also naturally skinny, and we had bulked up quite a lot together when we were roommates. We still worked out together at the gym, and we both decided it could be a fun time to switch up our training and gear into a “cut” where the goal was to burn off fat and maintain our muscle.

I did a lot of reading, stocked up on lots of protein powder, and began tracking my daily calories on my phone with MyFitnessPal (a free calorie tracking app). I was lifting weights three times a week, and aiming for 150–170g of protein (around the 0.8g per body weight pound recommendation from the literature.)

It was hard. Not only was I eating less, but I was spending my time at night counting calories and making tweaks for what I could improve the next day instead of relaxing. At the gym, I felt tired as I lifted weights, and I was struggling to hit the same reps that I usually did since I wasn’t eating as many carbs.

Over the next few weeks, as the pounds were coming off, my strength was falling too quickly. I decided to stop my cut earlier than I had planned, once I had become lean enough to go back to bulking (around 15% bodyfat). I didn’t like being so obsessive over counting calories and was struggling to maintain my strength. Truth be told, I made a lot of mistakes, such as thinking I was getting enough protein every day when I likely wasn’t (see our protein powder guide and why we often underestimate the amount of protein we get.)

But a few years later, I decided to try and get super lean again. This time I experimented with intermittent fasting. Without even trying, the fat started to melt off while also getting stronger in the gym.

Shane and I were still working out together, and I brought up how easy intermittent fasting was, and how helpful I was finding it to get lean. He said something casually like “Oh yeah, that’s how I did that rapid and super successful cut a few years ago.”

To say that I was pissed is an understatement. Why keep this amazing fat-burning technique as a secret? Shane to this day remains adamant that he told me that he was going to do intermittent fasting during that time when we were both “cutting” and that I had dismissed his recommendation. He’s a pretty believable guy, so I became upset with my past self that I didn’t heed his advice.


  • Intermittent fasting works well to help burn fat, just like a regular diet where you restrict calories. It mainly helps with consistency and lifestyle by making it easier to get into a calorie deficit and by making hunger more manageable. (2015 review, 2016 review, 2018 review)
  • Intermittent fasting increases cortisol (stress), so if you have a stressful job, you may want to avoid intermittent fasting. If you still want to try it, you may do best with a whole day fast on your day off.
  • If you’re a calm person like me, you may enjoy the increase in cortisol and the mental focus it’ll bring.
  • Overweight, skinny-fat, and people who are already big enough will do best with intermittent fasting.
  • Skinny people who want to gain weight to improve their BMI and rapidly get bigger and stronger should not be doing intermittent fasting as they’ll be making it harder to reach their calorie and protein goals and cutting off one muscle protein synthesis window.
  • Intermittent fasting isn’t a silver bullet. You’ll still need to focus on the fundamentals for intermittent fasting to work well:
    • Lift weights
    • Eat enough protein
    • Eat a lot of whole, quality food
    • Get enough quality sleep
Burn Stubborn Fat, Gain Muscle