How To Fast And Still Build Muscle

My chosen strategies for using fasting in a bodybuilding programme

This is a realm that hasn’t really been given enough thought. We know fasting is an incredibly potent weapon for fat loss. That much should undisputed. Just look at the boxes that get ticked off during a fasting programme.

  • A calorie deficit – You’re not eating
  • Insulin sensitivity – There’s no insulin
  • Metabolic flexibility – You switch to using fat for fuel

Then we have all the extra luggage of friendly features

  • Gut health – Time off for the digestive organs
  • Better sleep – Time off to focus on autophagy
  • Less stress – Lowered inflammation

Just because a diet is flying high on Instagram, doesn’t mean it can be written off as a fad. Fasting may be popular, and often misused, but it nevertheless manages to ceaselessly churn out incredible transformations. 

But moving past the usual features of fasting, there’s the topic of muscle-building. And that’s the question for this article.

Does a fasting diet produce the best platform for muscle and strength gain? 

There isn’t a clear-cut answer. In case you haven’t read part one, I’d recommend taking a quick detour, just so you can get to grips with the dilemma.

Does Fasting Build Muscle? – Part One


Starting With The Caveats – The Downsides Of Fasting For Bodybuilders

Is fasting ok for bodybuilding?

To give you the cliff-notes of the previous article; here’s the issue. Fasting is the defining part of the AMPK pathway, the switch that gets thrown when the body becomes catabolic. This isn’t at all inherently bad, as that’s exactly how fat loss takes place. The same is true for training. Skipping breakfast is a completely natural phenomenon. You’re just practicing something that the body is designed for. Three square meals a day is a modern invention, and I’d like to frame that as more of an observation than a criticism. 

But while fasting is all-natural and a potent tool, we need to look at its relationship with muscle-building. And it’s here that we need to accept that getting jacked beyond the brim with mirror muscles isn’t quite as natural. It’s absolutely fair game, but you’ll always have to put in the extra work to make sure you don’t lose all those gains.

So as a bodybuilder, and we’re not restricted to competitors here, there always needs to be a strong force of anabolism during a programme. Even when it’s weight loss we’re targetting. Without MTOR, the anabolic trigger, protein synthesis gets dominated by protein breakdown. That’s compounded with hormonal cascades that accompany an AMPK-fuelled (catabolic) environment. 

In the plainest of English, all this can translate to excessive muscle loss. 


So Fasting Shouldn’t Be Used In A Strength Programme?

does fasting cause muscle loss?

Not so fast…

And that would be a fair assumption, except now the shoe is on the other foot. During a strength phase, where you should absolutely be gaining weight, MTOR can then be overactivated. Just like the problems that rise from pressing too hard on AMPK, too much focus on anabolism is an open-invitation for fat gain and subsequent hormonal issues. Insulin resistance being a big one. That’s always one to watch for, as that hormonal dysfunction isn’t necessarily going to show up around your belly. At least not to begin with. 

Your body could happily be prediabetic for a number of years, taking damage from the inside, digging out a ditch that’s going to take considerable effort to crawl out of. At some point you’ll inevitably just get fat, or become a lethargic wreck. I’m simplifying, but this is the expected conclusion. The sort of damage that might not be fixed over a summer shred. It’s a strong argument for lean bulking and a cautionary tale against the typical dreamer bulk that stretches out over years. 

To make things worse, your rates of muscle-gain and fat-gain are correlated to your bodyfat percentage. The more fat you carry, the more your mass-gaining will be made of fat rather than lean muscle. To put things even more simply, fat attracts fat. Once you lose sight of your abs, things literally go pear-shaped in a hurry. 

Even during a bulk, it’s imperative to include famine to balance out the feasting. AMPK to contrast MTOR. Since fasting is an extremely obvious way to spike AMPK, it’s still a potential weapon for your massing toolbelt. 

Here’s the general formula.

  • Gaining weight inevitably increases bodyfat and builds up some insulin resistance
  • Insulin resistance amplifies fat gain and raises inflammation, which reduces recovery and performance
  • Fasting is the lever that breaks down that insulin resistance and prevents fat gain
  • Fat gain during a bulk ends up being insignificant using this method. 

On the other hand, if you skip the fasting and just try to add as much weight as possible, you’ll be left needing as much as a year to get down to your summer dreams. It’s the tale as old as time, people constantly underestimate how much fat they’ve put on across a bulk. 

The Verdict – Fasting Can Still Be Used For Strength


What About Fasting For A Fat Loss Programme?

how to fast for fat loss

This side is much easier to argue. Fasting will cause significant fat loss. You’re taking out at least one meal’s worth of calories, reducing hunger, and stopping insulin from blocking fat loss. The dilemma here, for muscle-crazy lifters, is whether it’s too much stress on the system. Here’s where bodybuilders will deviate in a big way from the usual weight loss programmes. Weight loss isn’t the ultimate goal, fat loss is. 

Whereas moving the scale is often the be-all and end-all of a general population diet, bodybuilding is the realm of targetted fat loss. Which is going to demand some synergy between weight loss and muscle retention. You will end up losing some muscle, that’s inevitable, but you’re going to want to minimise that as much as possible. This is actually the best approach in any diet, as the metabolism falls in correlation to lean mass. Drop too much muscle, and life on a deficit is about to get a whole lot harder.

As I described in Part One, fasting can cause a drop in lean mass, at least when it’s pushed excessively. It stimulates cortisol, potentially reduces protein synthesis, and negates insulin, the biggest anabolic trigger. So much like the case of a strength programme, it should be used with a dose of strategy. And it certainly has its uses.

  • Insulin resistance acts as a brake on fat mobilisation
  • Fasting drops insulin and reduces insulin resistance
  • Fat loss, especially around the stomach and thighs, are now easier to shift

Perhaps as a bigger advantage, is the fact that fasting should reduce appetite, curb cravings, making the diet much easier to cope with. I say should, because it still has to be done properly in order for things to work out. Fasting can easily spiral into a daily cycle of binging. 

Now for the actual strategies that will allow you to get the best out of fasting, depending on your goals.


Use Fasting For Muscle Gain If…

You have signs of insulin resistance (belly-fat, constant bloating, tiredness after meals)

Restrict the fasts to 14-16 hours.

This leaves you with plenty of time for three large protein-filled meals. Using whole-foods as the base for your meals will enable MPS to be stretched out across the day. Protein shakes and mass-gainers won’t work nearly as well, as protein synthesis will be sharper and shorter. 

Your first meal is a crucial one. It should be made mostly of protein and fats, in order to keep insulin low and improve digestion of carbs later on in the day. You’d be getting some of the perks of a ketogenic diet, without having to drop foods off your plate.

Don’t Use Fasting For Muscle Gain If…

You’re a typical hard-gainer, as getting in the calories over the entire day will be challenging as it is. But you should still fast for at least 12 hours, just to avoid digestion overlapping with the sleep. That would be a great recipe for insomnia and terrible recovery. Remember, the actual process of muscle building is made during the recovery phase. 


The Best Ways To Fast For Fat Loss

The Lean Fast

If you’re under 8% bodyfat (15% for women)…

  • Fast for 14-16 hours
  • At 14-16 hours, have a coffee with a tablespoon of saturated fat (butter, tallow) or MCT oil
  • Have your first real meal at 16-18 hours, with only protein and fats
  • The second and third meals can be the usual mix of macros

Going further than 14-16 with no calories just puts you in a state of limbo, where your body can’t pull out energy from fat stores quick enough. A shortened fast like this will still help reduce glycogen levels and bring about a mild state of ketosis, without causing you to tank and encounter rabid hunger.

The bulletproof coffee will provide a strong spike in energy, with just enough calories to manage for a few more hours. There’s no insulin spike, so you’ll remain in a fat-burning state until the first meal.

MCT might be the best choice if you’re not yet fat-adapted, as it’s made of shortened fatty-acids that get fast-tracked through the energy process. It’s the quickest fuel that’s not a carb.

The Keto Fast

If you’re on a low-carb diet…

  • Fast for 20-22 hours
  • Use at least 5 grams of salt across the day, especially in meals
  • The bulletproof coffee isn’t necessary, but can be used to push the fast for another two hours
  • Use dextrose for training, at 10-40g
  • Have a carb-up day every 2-4 days, at 2-3g/kg bodyweight

Take advantage of the fat-adaptation that’s already in effect. Low carbs puts you in the realm of ketosis, so there’s no awkward transition from feeding to fasting. Both low carb and fasting share metabolic states. The fuel essentially remains the same throughout the day, and blood glucose levels barely take a bump. This is quite possibly the ideal format for fasting, and it’s certainly the easiest.

There is a risk of electrolyte loss, as both of these diet strategies can cause the body to flush excessive amounts of sodium. But that only manages to be an issue if you don’t take the opportunity to add liberal amounts of salt to your plates.

In terms of muscle-protection, there is a risk of pushing too hard on glycogen depletion. Despite there being studies suggesting low carb-adapted athletes have no differences in glycogen retention, it’s hard not to see some downsides. Especially when we accept that most of us are not keto-adapted, and consistently train above the anaerobic threshold. 

Adding carbs to training will alleviate a lot of those concerns, and has been proven to improve lean muscle on a low-carb diet. But because you’re still operating on a deficit, glycogen depletion will still end up outstripping glycogen synthesis. So staging one or two carb-up days in the week might be a sensible solution. 

Any disruption in ketosis from the refeeds will be easily negated by subsequent fasts, training sessions, and the low-carb diet itself. Practically, a moderate carb-up day seems to be the best way to blend in keto with bodybuilding.

The Fat-Burning Fast

On a standard diet…

  • Fast for 18-20 hours
  • Use bulletproof coffee to improve fat-adaptation
  • Use dextrose for training while fasted
  • Take 1-2 days in the week off fasting, at maintenance calories

Finally we have the standard mix of carbs, proteins, and fats. This is ideal if you’re not keto-adapted, and don’t want to sacrifice performance and muscle while getting there. Here I’d recommend using the bulletproof coffee as you would on the lean fasting diet. The carbs in your meals will bounce you back into a glucose-dominant fuel state, and that will end up cause spikes in hunger over the following morning. A small dose of fat will go a long way towards managing any crisis you fall into.

Because the fuel states of fasting and feeding are going to clash somewhat in this variation, it would be beneficial to have a few days on a normal schedule. This is what’s called a refeed. The carbs go up, you’ll likely experience a boost in energy, and can step it up a gear in the gym. This is prime territory for a strength-focused day, where you put in the intensity necessary for holding onto muscle mass.

Training completely fasted has been a little overhyped, where people seem to think it’s the only way of targeting stubborn belly fat. There’s no evidence to really point that out, and fasted training can end up being a problem if you’re dropping down the weight-stack because you’re all out of gas. So if you do train fasted, it’s better done as just a cardio session.


Wrapping Up – There’s plenty of perks for working fasting into your bodybuilding programme

should i fast for muscle decision chart

I’ve barely touched on the metabolic machinery that takes place during fasting programmes, but there’s already enough positives going for it. Fasting can be a tool for balancing your hormones, enhancing fat loss, even just as a way to develop a strong routine where you can take out the guesswork and get your head down. And just as long as you’re able to use it carefully and strategically, fasting can also be perfectly suited for a muscle-building programme.


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