How Does Intermittent Fasting Work?

Intermittent Fasting is all the rage these days, gaining significant traction in a lot of circles the past few years with big names around the industry touting its benefits and providing their own unique spin offs. The claim of IF is a simple one.  Burn fat and build muscle at the same time.  Also known as the holy grail of body recomposition, and a concept many believe to not be possible.

Traditional diet advice dictates that growth requires a caloric surplus and loss requires a caloric deficit, and you can’t have both a surplus and deficit at the same time. So, along came IF and it’s disciples to offer up a way around that.

If you’re looking for science or data as to why IF does what it does, you won’t find it here.  This entry is purely one of anecdote and empirical evidence from two people who decided to undergo it for 6 – 12 weeks. If you want a dedicated resource then please feel free to turn to these four IF resources in the industry:

There is an abundance of resources on all four sites and books.  If you are a looking for one resource, go with the Renegade Diet by Jason Ferruggia, but all four have the following common denominator. There is a fasting window greater than 12 hours. There is a feeding window less than 12 hours. You are not eating more than you are eating.

The 16/8 Fast

We elected on the 16/8 fast as promoted by Martin Berkhan and Jason Ferruggia, which breaks down into the following easy to follow guidelines.

    • 16 Hour Fasting Window
    • 8 Hour Feeding Window

The time of day you train will help dictate when you start the clock.  The key takeaway here is to not get super caught up in the details of when you train.  IF can be accomplished for any training schedule, but it’s all about sticking to your schedule so your body can be adequately trained. Here’s how your IF can work depending on when you train. Morning 

  • Feeding Window: 8 am – 4 pm (adjust based on time of class you attend, have first meal immediately after)
  • Fasting Window: 4 pm – 8 am


  • Feeding Window: 1 pm – 9 pm
  • Fasting Window: 9 pm – 1 pm


You’ll want to have your first meal prior to training.

  • Feeding Window: 2 pm – 10 pm, have your first meal be fairly carb exclusive
  • Fasting Window: 1o pm – 2 pm

A few key details in both our plans.

    • We both had our workout around 1 pm, so our feeding window was 1 – 9 pm.  Fasted during all other time.
    • We did not count calories.
    • We did not eat too gluttonous levels at many meals.  Just ate until we were full.
    • Carbs were crushed post workout in any and all varieties.
    • We both strayed a bit on the weekends.
    • We ate big meals at night, and they were carb heavy.

Lastly, IF is not recommended if you are over 15% fat for men or have more than 15 pounds to lose for a female.  You’re better off following a lower carb diet at first, as outlined in the 360Nutrition manual, and then IF can be transitioned to for any weight loss under those parameters if you desire.   IF is most suitable for folks who have stubborn weight to lose or who want to go from lean to really lean.

Here are the personal accounts from both of us.

Pritz’s Experience

by Bryan Pritz

I was first drawn to IF due to the claims that you didn’t have to be so strict with what you ate, but more importantly when you ate. For someone who, in a perfect world, would want to eat Baked Bear for breakfast, Chipotle for lunch and 2 Baked Bear ice cream sandwiches for dinner I wanted to know more about these claims. This idea that I could enjoy my food and life more and still get positive results was very appealing.

One thing I hate about many nutritional programs is the amount of details you have to pay attention to. I don’t want to have to worry about eating my first 502 calories before my left nipple is at it’s smallest for the day and then overloading protein based on 2.5x of my body weight, converted into Mexican Pesos, based on the future value of a 50 year bond.

Fuck that. I need simplicity.

So the notion that all I had to do was focus on eating during an 8 hour period and that’s it was another motivating factor.  Dave and I usually work 12 – 16 hour days so this fit in perfectly with our busy schedule, available time to train and requirements for getting results while still running the gym.

My Goals with IF

I decided that I would do this entirely as research. I had zero expectations, other than 7-11 would have a drastic increase in donut and candy sales. I have a serious sweet tooth. When I was a kid, I stashed pillow cases around the neighborhood on Halloween night so that when I filled one pillowcase, there was another close by. My athletic abilities tripled when the night started, as I sprinted from house to house, dodging kids, foregoing sidewalks and staircases to get to the front door quicker. I’d pull in a solid 4-5 large  pillowcases of candy by the end of the night. My mom had to buy them from me on a strictly negotiated $$/lb transaction in order for me to give it all up. Of course I had 15 hiding spots throughout the house that I hid my fair share.

Wait what is this post about? Oh yeah, being healthy.

What I hoped would happen was that I would keep my muscle mass that I had just put on in the previous 8 weeks and continue to lean out to a healthy degree. My specific physical goals are not to be like this.  But like this.

I also wanted to keep my strength progression moving forward, something I was a bit worried about with IF so I was watching the numbers rather closely.

My Strategy:

  • IF Duration: 10 Weeks and counting
  • Daily Fast: 16 Hours (9pm – 1pm)
  • Training Frequency: 4-5x/week (2-3 Daily Challenges, 1-2 Outdoor Challenges, 1 day to work on my own)
  • Daily Training Time: 1pm most days, 6:15am 1-2x/week
  • First Meal: 2pm – Immediately after workout.
  • Second Meal: 3pm – 4 pm – Typically I’d just have a snack in the afternoon. Fruit. Jerky. 7-11 donuts. Maybe some Peanut M&M’s, Twix, Kit Kat or maybe all 3.
  • Third Meal: 8pm – 9 pm – Steak with veggies, no sauce, no salt/pepper or no flavor as people tell me. Grass-fed burgers and veggies. Chicken & veggies. You get the idea.

This first meal was where I really customized the results for my personal goals and my desired outcome. Every Monday, when I’d wake up and take my morning selfie #nomakeup I’d determine how my week would be used for my goals. If it was a fat weekend, for the ensuing week my first meal post workout would be pretty clean with an egg scramble and a King’s Hawaiian Sweet Baby Jesus roll. Kept my meals clean about 80% of the time for the week. If I ate mostly clean during the weekend, I’d use my post workout meals to fuel my muscles and strength workouts with a Chipotle Quesarito’s #ThanksTimP

Operations/General Notes:

  • While I followed the plan most of the time, I didn’t follow it all the time aside from always eating after 1pm Monday – Friday.
  • I did consume alcohol and didn’t change anything about what I drank while out. I did drink more often during these 10 weeks than I had previously, due to my increased social status from some really good Facebook and Instagram posts.  One of my posts got 70+ likes.  NBD.
  • Most of my carbs came from tortillas, King’s Hawaiian Sweet rolls, and some sweets. Most of my carb intake came post workout from 2pm – 4pm.
  • I rarely ate desert, or binged for dinner. I asked people to go to Baked Bear almost nightly, but had no takers.

My Results: Down 10 Pounds, 3% Body Fat

To date, I am down roughly 9-10 pounds and have PR’d in deadlift, squat, jerk, hang clean and box jump over a 10 week period.


Overall, I am extremely happy with my results. So much to the point that it has developed into my daily lifestyle moving forward. I leaned out a solid amount. My muscle mass feels about the same, and my strength numbers all increased. Most importantly for me, I was able to eat more of what I really wanted and craved, without having to pay Tubbs McGee a visit. Some other notes:

  • My energy levels are higher now than any point in my adult life. This was the biggest shocker for me. I’ve never felt so good in my life, to be quite honest.
  • When I followed P360 Nutrition to a 90% adherence rate during this IF, weight dropped dramatically and quickly. When I ate like a fatty, I maintained. When 70%-80% of my meals were clean, I lost weight slowly.
  • 9-10lbs is pretty significant for me since I began IF already quite lean.
  • Had I been strict, I would expect those numbers to be around 15 lbs lost for a 10-week period.

My primary belief is that life should be enjoyed and experienced. That Health/Fitness/Nutrition should be a complement to your life and not an obsession. For me, IF was a perfect fit for this and I recommend it.

Dave’s Experience

by Dave Thomas

I am a huge skeptic by nature when it comes to diet books and advice.  Unless it’s a ghost story or an exorcism movie, I probably don’t believe it and I definitely don’t want to hear you talk about it.  Additionally, most nutrition experts in the world of fitness also happen to have phenomenal genetic tendencies for both fat loss and muscle growth and it tends to skew the results of whatever they elect to push a bit more favorably.  Being big on Instagram typically just means you’re a narcissist with great genetics, not a wellness expert.

Also, most B&A photo strategies go like this.

Before advice:  First, don’t let the sunlight hit your skin.  Ever.  Then, push your gut out and slouch your shoulders like someone just dumped popcorn on your head.  Next,  don’t shave.  Or comb your hair.  Listen to Dashboard Confessional on repeat all day before showing up.  Good, now think of your dog dying.  Okay smile you pathetic sack.

After advice: Go tanning.  Buy Axe styling paste.  Smile like Jennifer Lawrence just invited you over for a glass of Merlot, flex and say Cheeeeeeeeeese!

This all nets out to my taking any and all diet advice with a HANDFUL of salt.

Additionally, IF goes against a lot of my dieting instincts.  It skips meals, it requires mental work and worst of all, it strips me of my beloved breakfast.  Waking up knowing I couldn’t get my morning fix was a tremendous adjustment for me.  I was beyond terrified of the notion that skipping breakfast would help body recomposition.  Don’t you need calories for muscle??  Don’t you need to eat or your metabolism will shut down?

So I read.  A lot.  And then I read some more.  Until I realized that IF is backed by some very smart people and some very proven science.  And every expert on this topic assured me that was not the case. Check out Martin Berkhan’s section on breakfast at the bottom of this article.

“Muscle catabolism doesn’t occur in short-term fasting for up to 24 hours. If you’re still paranoid about this (I am), make sure to eat some slowly absorbed protein before bed, such as cottage cheese, egg white protein or meat with veggies (the extra fiber will slow absorption as meat is generally considered a “fast” protein). Another thing to keep in mind is the very slow absorption rate of whole food proteins. We’re talking a few grams per hour, which means that a mixed meal with 40-50 grams of protein will maintain a steady level of amino acids in your bloodstream well through the night and into the next day.  The belief that a few hours without food will cause muscle catabolism is absurd.”

Berkhan is probably the leading resource on IF and it doesn’t hurt his street cred with me that he can deadlift 600 pounds.

So, with my fear of wasting away subsided and armed with some motivation after seeing this work for Pritz, I was ready to go all in.   It can sometimes be hard to notice physical change in someone when you see them daily, and to say we see each other daily is an understatement.  But, even while spending 10 – 14 hours a day together I could see the difference in how he was put together, and I wanted to be on him.

Further, I saw this as a great educational ride if nothing else.  IF experimentation has been on my list for four years, and this was the kick in the ass I needed to finally go through with it and see what’s what.

My Goals with IF

I didn’t really go into this setting lofty goals because I feared it would skew how I ate.  I don’t get too geeked out over body composition and trying to look a certain way.  For me, performance is health.  The longer I can perform at a high level, the greater my life longevity.  Further, I train to enjoy life and sometimes that includes straying from my diet and Scrooge McDucking into a pile of cookies.  For this reason, I typically always hover between 10-12% body fat, and I am fine with that.

For the sake of this experiment, I simply wanted to follow the plan and watch what happened.  If I gained muscle, great.  If I lost fat, great.

I’m up about 14 pounds of lean body mass in general over a two and a half year span since opening the gym.  I did NOT want to slash weight too fast.  Anything that claims a ten pound weight loss in a week sends me running for the hills, as with that slashed weight goes my strength.  That’s who I am.  Some can maintain strength while leaning, I cannot.

My Strategy

    • IF Duration: 6 weeks and counting (beginner phase)
    • Daily Fast: 16 Hours (from 9 pm to 1 pm)
    • Training Frequency: 4-5x/week (2 strength sessions, 2 fat loss sessions)
    • Daily Training Time: 12:30 pm
    • Wake Up, 7:30 am: Extra large glass of ice water to wake up my system.  2 cups coffee with heavy cream.  I’ve ditched the cream the last week in favor of black coffee and it’s made a difference.
    • First Meal: 1 pm – Typically a four egg scramble, with hash browns and fruit.  Sometimes a side of toast if I went hard.  If not this scramble, a burrito bowl from Chipotle with double meat, rice, veggies, cheese & guacamole.
    • Second Meal: 4 pm – Chicken wrap with a side of macaroni salad.  Sometimes a scone and PM coffee with heavy cream.  Often times would not get to my second meal due to sheer work volume.
    • Third Meal: 9 pm – Steak with sweet potatoes and a green veggie.  Things of this nature.  Sometimes I’d walk down to Oscar’s and get three tacos in corn tortillas and a ceviche.  Other times I’d take a bubble bath in ice cream.

Here are some bullet points to how I’ve been operating.

    • I have been pretty liberal in my diet once the feeding window hurts.  I think initially, this was because I was mentally afraid I’d turn into Justin Bieber so I’d eat some calorie dense foods to make up for the periods of fasting.
    • On the weekends, I have actually kept to my fast until 1, but once that dial hits 1 I have been pretty non discriminant about what I eat.
    • In the very rare occasion I surfed or had a morning workout during these weeks, I’d eat thereafter and kind of punt the day as far as fasting goes.
    • I made the mistake of starting my IF experiment in the middle of wedding and bachelor party season.  This has certainly cost me some progress.
    • I haven’t used BCAA as directed by every expert.  I’ve never noticed any difference with BCAA when I did take them. I have no idea if they work or are placebo but if I needed a supplement to permanently operate with IF, it wasn’t for me anyways.  It’s not that I don’t believe in them, it’s just that I don’t like having to always take them.
    • The majority of my carbs have been from white potatoes, bread, sweet potatoes and corn tortillas.

My Results: Down 8 Pounds, 3% Body Fat

I am pleased with what’s happened so far.  I read a bit too much literature that said my carbs can be liberal, and I think had I tightened up on the weekends and cleaned up the scones, etc. I believe I would have done better off the bat.   I also discovered I have some major food intolerances that caused havoc with my digestion. Maybe some can get away with junkier carbs and still cut serious weight.  Not me.  I can maintain my body fat levels with liberal diet, but I cannot cut with it.

I’m down eight pounds in roughly 6 weeks, however with IF it’s always tough to tell because unless you are doing some serious measurements it’s hard to tell how much fat you’ve really lost since some muscle gain is probably occurring. (Editor’s update: this will now be fixed due to our body analysis machine at P360).

This was a crappy cell phone picture.  Is the light favorable?  Of course.   They make bathroom lighting favorable on purpose so you don’t hate yourself.  I’m also flexing so hard my brain might pop.  No one just has abs. But aside from that, no editing or anything like that.


I don’t think I have gained any muscle, but I also think my post workout plan was too scattered.  Some days I ate small (following the advice to save the feast for late in the day), and some days I ate huge.  I was a victim of too many opinions in my brain and too much work to have this be my entire focus.  So remember that grain of salt thing?  Take my report with a bit of it.

As far as strength goes, here is why I have mixed feelings on IF.  I PR’d on my Turkish Get-Up and overhead squat, and matched my split jerk PR, but my 3R squat is down 30 pounds, my 3R hang clean down 15 pounds and I just feel weaker.  I am strong on overhead stuff, so the fact I hit PRs in those movements could have nothing to do with diet.  Much the fact my squat was down 30 pounds was an alarm for me. This could be my personal ebb and flow as I fluctuate up and down a ton on most all lifts regardless of diet, but it could also be the reduced calories.  I’m just not certain on that end. I’m really excited to commit to this full bore and tighten it up in some areas for this at a later date.

What We Recommend

If you have some stubborn fat to get rid of, go for it.  The literature states this is not ideal for those with a lot of weight to lose.  Keep it cleaner than we did, and monitor your strength gains.  Even though Pritz had some favorable results with strength, Dave’s was a mixed bag so if strength is by far your number one goal then tread carefully and re-test lifts frequently.

    • Find your comfortable 16/8 schedule around your class times.
    • Keep your carbs as clean as possible, but make sure you eat them.  The absolute worst thing you can do is IF and no-carb.
    • Have as little alcohol and junk as possible on the weekends.
    • If your goal is more muscle growth, eat more carbs and more calories than usual during your feeding window.
    • If your goal is more fat loss, eat less carbs and your regular day of calories during the feeding window.
    • Either way, get 16 hours of fasting accomplished before eating.
    • Go in with a buddy.  It will help monitor your progress and motivate one another.


Use the resources we have provided above if you wish to read up on the literature and science of it all.  It will help you better commit if you know there is a reason and where it comes from.

Dave and Bryan are owners of Performance360 in Pacific Beach, San Diego.