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Intermittent fasting is a dietary approach which has experienced a sudden rise in interest over the last few years in the fitness industry, but is actually not a new approach at all. Intermittent Fasting (IF) is an approach which at it’s core is very simple – eat sometimes, and don’t eat other times. While most humans do this by default when they sleep every night, IF advocates extend this period for a number of reasons.

In this article I’m going to focus on IF in the context of fitness, performance and body composition. There are some data which suggests that IF could improve certain health markers including those correlated with late onset diabetes and IF could even potentially reduce likelihood of developing chronic conditions such as Alzheimers and some cancers. This looks promising but there is insufficient human evidence to give a strong backing right now because we can’t really determine whether it was IF or another factor such as overall calorie restriction which made the difference, and therefore I’ll leave you to do your own research and make up your own mind on that topic.

So that’s the health side of things – let’s talk getting big, strong and lean. What I’ll do is give a general overview of the ways in which IF is reported to potentially aid body composition improvements and then I’m going to weigh up the evidence for each point. To sum up I’ll give my recommendations of who IF could be used with.

Before I start I’m going to briefly outline the various kinds of IF – as there are many – which are popular at the minute. This is by no means an exhaustive list as IF means nothing more than NOT eating for a certain pre determined time, and therefore there are almost limitless variations.

Eat Stop Eat by Brad Pilon advises one or two 24 hour fasts per week. He advises you eat dinner one day and then fast until dinner the next day, thus leaving a 24 hour gap but not actually spending any one day not eating. This is the simplest form of IF, and one which can be highly effective for those who spend a lot of time travelling or who are very busy, but it can have serious repercussions for athletes.

LeanGains by Martin Berkhan involves a 16 hour fast followed by an 8 hour feeding window (or 14/10 in women, the reasoning for this being that there is a small amount of evidence that extended fasting can cause adverse effects for the female hormonal system). This is probably the most popular version of IF which has a number of different approaches within it – all of which can be checked out on the LeanGains website. It’s the most apt for athletes of all of the different approaches, but still falls afoul of the same pitfalls that I’ll mention later.

The Warrior Diet by Ori Hoflmekler advocates a 20 hour fast followed by a 4 hour eating window. The theory behind this one is that we humans are ‘nocturnal eaters’ and that this fits in with our circadian rhythm. While this isn’t true, it can be seen as a more extreme version of LeanGains possibly useful for those who are elderly and therefore don’t have much appetite during the day or otherwise highly inactive for most of the day, and who do not train intensely.

Similar to the Eat Stop Eat is the recently popularised 5:2 diet which is, again, 1-2 fasting days per week, but rather than fasting for 24 hours dinner to dinner, you eat normally one day, go to sleep and then the following day consume 400 or 600 calories depending on sex throughout the whole day. This, much like Eat Stop Eat can be good for those who are busy but because of the extreme morning-night restriction it’s closer to the Warrior Diet for being useful for those who aren’t all that active. While this isn’t strictly IF, it’s usually referred to as such and so I’ve mentioned it here.

Of course, there are outliers and some highly active people do very well on any of the above approach, building muscle, getting lean and kicking ass in the gym – but these are exceptions to the rule. Most folks who try the above as part of a muscle building or strength gaining approach make SOME progress, but find that they stall quickly.

Everything works. For a while. Until it stops working.

IF and Fat Loss

Proponents of intermittent fasting claim that it is a tool which can be used in order to speed up fat loss or indeed get someone training for fat loss past a plateau without changing their food intake. So don’t change what you eat or how much, just change when and lose fat. This seems plausible as fasting has one distinct action to increase fat loss – it increases the secretion of ‘stress hormones’ cortisol and adrenaline.

These two hormones play a number of different roles but in this specific context they upregulate lipolysis – meaning they help fat cells release fatty acids into the bloodstream for use as fuel. More fatty acids released and used, faster fat loss – right?

Not quite – because the fasting window is followed by a feeding window.

An IF proponent will eat far less (read: no) food during their fasting period, but then they will eat MORE than usual to fit their normal intake in a smaller timeframe. This causes more rapid fat gain in this period to counterbalance the aforementioned fat loss resulting in a net zero.

Summed up, your fat cells store and burn fat all of the time simultaneously (sometimes one is shut off for a short period, but this isn’t worth mentioning as it’s not relevant here) and it’s the balance between these two processes over an extended period of time which dictates changes in body composition. This state of storage and burning is similar to a rotating door on a bar. It lets people in and out all of the time, sometimes more come than leave and sometimes more leave than arrive, and the amount of people left in the building at closing is dictated by the balance between the two – not the sudden rushes or the extended quiet periods.

So it doesn’t matter if you burn more while fasting because you store more while eating and the final result is dependent on caloric intake. This DOES NOT mean, though, that IF doesn’t improve fat loss…

Intermittent fasting can have PROFOUND effects on a person’s ability to adhere to a diet. The two main things which result in people failing in their fat loss goals are hunger and cravings, and IF can help solve both of those.

Hunger is largely habitual, meaning you get hungry at the times you usually eat each day rather than when you specifically need food. Your body likes routine and cycles, and this is one example of many – the hormone grehlin is what ‘tells’ you that you’re hungry, and it can be manipulated intentionally by altering your eating pattern.

If you always eat at 8 and 11 am, you will always be hungry at 8 and 11 am because grehlin will be released then in order to make sure you eat, and this can make fat loss a bit of a pain. If you decide to start skipping these two meals then after a period of around two weeks you will find that the hunger you used to experience is no longer there. Suddenly you don’t feel like you need food in the morning and you’ll be able to reduce calories easily.

Cravings happen for loads of reasons during a dieting phase (when not due to genuine hunger) but one prime reason that cravings get worse when your calorie intake is lower is because some nice foods which you enjoy can be difficult to ‘fit in’ to a reduced total intake. Think about it – if you have 1800 calories to play with and eat 4 meals per day, then that is two 500 calorie and two 400 calorie meals – which are hardly going to touch the sides.

If you decide to drop this number to two meals, you can suddenly eat two 900 calorie feasts which will be highly satiating and which will allow you a little more freedom with food choices. Feasting every day on foods you like, while not being hungry, can make dieting feel like a weight gain phase for some people – and that’s a good place to be.

Secondary to this, a lot of people find the clear “Eat now”, “Don’t eat now” rules of IF make cravings disappear as they simply forget about food at all. If you’re focusing on other things, staying busy and not hungry, it’s much easier to avoid high calorie things that will easily eradicate a calorie deficit.

Because of the above, Intermittent fasting can be considered a powerful tool in fat loss, even if it doesn’t really help you lose fat directly.

For completeness, though, I need to mention the downsides to this aspect of IF. After going a full day (or even a full morning) without eating, some people will find hedonistic tendencies are amplified. Overeating after a period of complete food abstinence is very common and must be considered. If you or a client don’t deal well with hunger and tend to find it makes you overeat (or want to) then IF might not be the best idea for you.

IF and performance

So what about performance? IF might not directly help or hinder fat loss (though it can improve it indirectly) but can it make you a better athlete? Some people think so.

First of all, I’ll say that fasting shouldn’t directly impact training at all, provided caloric intake is kept equal. If you’re training during your eating window you should be able to give your body all of the fuel it needs in order to perform maximally, that is pretty obvious - but this is not always the case – so let’s talk fasted training.

Fasted training should, by way of logic, impair performance. You’re training on an empty stomach so you have no ‘fuel in the tank’, and SURELY your blood sugar ends up crashing, right?

As another viewpoint, fasted training is purported to be great for performance as it effectively upregulates the ‘fight or flight’ response as mentioned earlier. This sudden release of adrenaline brings with it a stimulant-like energy which can feel euphoric – this can indeed improve focus, concentration and creativity - but this is NOT the same as increasing physical performance.

There is actually a pretty substantial amount of evidence in this area thanks to those practicing Ramadan. Now, there may be no specific data on other IF protocols, but bear in mind that those adhering to Ramadan are abstaining from food AND water, and they are generally at least somewhat sleep deprived so it’s safe to say that this extreme can be extrapolated to our comfortable fasts involving water and coffee. What we notice is that there is no impact on training…at all. In fact there was one study done (1) which showed that even after a three and a half day fast from food there was zero impact on performance.

What we can take from this is that, at least from a resistance training perspective, overnight fasting (and even longer) probably won’t kill gym performance. Of course endurance training will be affected, but because resistance training doesn’t typically deplete that much glycogen anyway, you can rest assured that so long as you eat well during the eating window, fasted training will not impair performance (at least it won’t as soon as you get used to hunger pangs).

So performance might not be impaired, but what about making gains?

IF and muscle gain

Staying with fasted training – there are some who say that doing this will inevitably lead to your muscles wasting away, and then there are those who state that fasted training will increase the anabolic effect of your next meal thus actually INCREASING muscle growth. They are both correct, and yet they are both mistaken.

Remember what I said about fat gain and loss being two opposing states which act alongside each other? Roughly, muscle protein synthesis (growth) and muscle protein breakdown work in the same manner.

So, if you train in a fasted state you will indeed cause muscle breakdown to accelerate, but then the post workout meal will create an amplified anabolic effect. All of this sounds scary and then cool, but in reality it leaves us again at a net zero. This process and the fat gain/loss processes are perfect examples of ways that, simply, your body is smarter than you.

The only real exception to this rule may be fasting before AND after training. There is a small amount of evidence that after around 2-3 hours of fasting after a fasted training session, stress hormones such as cortisol have been raised so high that muscle loss exceeds what the subsequent meal can counteract. Because of this one of the only hard pieces of advice I’m going to give in this article is to eat as soon as is convenient after a fasted training session.

Fasted training, then, is something that can be considered if it makes your schedule a little easier, or if you enjoy it because of the euphoric feeling you may get (let’s face it, pre workout caffeine ‘hits you’ harder with a lower dose, too) – but it can be safely said that nothing will happen which is different to what would happen if you’d eaten beforehand (provided you feel good enough to perform well psychologically).

So that’s fasted training covered, what about IF in general – does this hamper or help muscle gain?

Well above I state “so long as you eat well during the eating window’’ and I do this for a specific reason. Depending on your protocol of choice, eating enough food to GAIN weight and therefore muscle is going to be hard for most. Sure, in a beginner it is possible to gain some muscle at maintenance caloric intake, but sooner or later you’re going to need a surplus, and that means that you’re going to have to EAT.

If you are restricting food intake to 4 or even in some cases 8 hours, you are going to struggle to maintain optimal caloric intake whilst also eating the kinds of foods which will promote optimal health. Anecdotally people find it hard to consume enough fibre and wholefood protein in this time without being bloated and uncomfortable – especially with 1-2 meal approaches.

But let’s assume you’re able to eat enough – does fasting for a certain period reduce anabolism directly?

Well, it might. There is a small but growing amount of data which suggests that distribution of protein matters (2). Not as much as total intake of course, so it’s not make or break, but there appears to be a clear difference between 3-5 “pulses” of protein per day and 1-2 bolus doses. This is because, in simple terms, you can only stimulate so much muscle growth with a meal, and it’s only stimulated for a short time regardless of how much you eat. That means that once muscle growth is maximally stimulated, eating a bigger meal doesn’t do any more for you in terms of muscle growth signalling.

If you eat in a 4 hour window, you’re only stimulating muscle growth for that time and maybe 3 hours afterwards, but if you spread the same intake out over a 16 hour waking period you’re getting far more growth stimuli. This may add up over time (though it’s not conclusive yet).

Summary

So there you have it. In short, IF doesn’t really have much of an effect on anything to do with strength, muscle gain, fat loss OR strength training performance directly. In some people it may make a diet easier or harder to adhere to. In some people it may impact performance psychologically, but not everyone, and in most people there will be no impact on how much muscle you can gain.

If you are considering trying Intermittent Fasting, ask yourself the following questions:

  • Do you deal well with hunger?
  • Do you enjoy large meals, or grazing?
  • Do you enjoy breakfast?
  • Do you perform endurance training, or indeed do you perform high volume resistance training for a long period?
  • Are you prone to overeating following hunger?

If the above questions throw up no red flags, consider one of the approaches mentioned safe in the knowledge that aside from potentially making your schedule easier, or your diet more convenient and easy to adhere to – IF isn’t magic and not much will happen.