“Flexible Dieting” or IIFYM: What Is It & Does It Work?

A Balanced Look At The Idea Of ‘If It Fits Your Macros’ Or “Flexible Dieting”

A couple of years ago, the letters IIFYM wouldn’t have meant a great deal to even the most dedicated bodybuilders. But today, it’s hard to avoid them. The ‘If It Fits Your Macros’ diet, or IIFYM, has become incredibly popular over the last half decade. Its sudden rise in popularity is in no small part due to the number of YouTube personalities who advocate this diet as a way to build muscle while staying shredded all-year-round. 

As you are no doubt well aware, there are volumes and volumes written about IIFYM every single day. Just about every health and fitness blog will touch on the subject at some point, and there are entire blogs dedicated to nothing but the IIFYM diet. 

Yet it’s still hard to know what to think of the diet.

Many of its proponents sell eBooks related to the diet. Many more of them have YouTube channels, and with them, ad revenue on the line.

We aren’t doubting the sincerity of many IIFYM proponents, but it’s always best to get an impartial perspective on these things. 

It’s also just fundamentally hard to distill information when there is so much of it available, and when so many people are saying completely contradictory things. To listen to some bloggers, IIFYM is a ‘silver bullet’ for effortlessly maintaining a muscular, shredded physique. Others will tell you that it’s a shortcut to losing gains.

Everyone quotes studies and anecdotal data to support their claims, so deciding who to believe is always going to be a tough task. 

That’s why we’ve put together this article. Here, we’ll be discussing every aspect of the IIFYM diet; what it is; how it is supposed to work; the variations; the benefits, and the costs. 

We’ll also try to answer many of the questions raised by the many articles out there on this controversial dieting approach:

• Is it more effective than conventional caloric restriction?
• Can it help preserve muscle mass?
• How does it affect performance?
• Can you eat anything you want?
• Does it promote longevity? 

We’ll look at all of these questions and more in the sections below. As always, we will try to provide you with a balanced perspective, taking account of all the information we can find regarding the ‘If it fits your macros’ diet. 

Let’s start with the most basic question, but the one which it can actually be difficult to find an honest answer to: what is IIFYM and how is it supposed to work?

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What Is IIFYM?

IIFYM stands for ‘if it fits your macros’. 

One good thing about this diet is that it really is what it says on the tin.

The idea behind this diet is that you simply aim to hit (or more more usually stay within) your macro-nutrient targets each day.

To decide what these macro tragrets are, you work out what kind of ratio of macros you need, and work out the total amount of each which will bring you into a caloric deficit each day. 

So basically, you decide how much protein, fat, and carbohydrates you should be eating per day to achieve your particular goals. You then just make sure that you stay within those limits each day (or reach them if bulking). 

For example, if you’re going on a pre-competition cut, you might aim to drastically reduce the amount of carbs that you eat to bring you into a caloric deficit.

Say you evaluate your own needs and goals, and you decide that you should aim for 150g of protein, 180g of carbs, and 65g of fats.

If you are a pro-bodybuilder following the typical “chicken, rice and veggies” diet, then your day’s food is going to be roughly split between 5 meals (and maybe a shake).

If any of you have tried to diet by eating this way before, you’ll know how monotonous and soul-crushing it can be: “Oh boy, plain chicken, rice and broccoli for the third time today!” said nobody ever. 

The IIFYM diet plan isn’t like this. It allows for an almost unlimited amount of flexibility, hence why it is sometimes called “flexible dieting”. 

What you eat, how many times you eat, how large your meals are; these variables are largely up to you. The only important thing is that you choose your macro-nutrient targets wisely and that you stick to them each and every day.

Contrary to popular belief, this does not mean that you should simply get all of your calories in one epic sitting at KFC.

If you managed to hit your daily macro targets in one sitting at KFC, then you would technically be following the IIFYM protocol.

But anyone with half a brain knows that this is a one-way ticket to a fat stomach and eventually diabetes.

Quality Matters

IIFYM does not mean that you should stop giving consideration to the quality of what you eat. 

It does mean that you have a huge amount of flexibility in what you eat and when you eat it. 

While it doesn’t mean you should just start eating McDonald’s and Dunkin’ Donuts for every meal (so long as IIFYM), it does mean you can make room for the foods you enjoy without it being a disaster. 

For amateur bodybuilders and weekend powerlifters, this is a very attractive bonus of “flexible dieting” or IIFYM-type eating plans. 

For example, you might be bulking and you decide that you need 160g of protein, 400g of carbs, and 80g of fat (do not use these numbers as guidelines for you; everyone is different). 

So you might plan to have a small breakfast of some oats and a few eggs, an enormous lunch consisting of a steak sandiwch and cheese, and a nice lean fish meal for dinner. 

However, that morning you might learn that you are seeing old friends for dinner, and everyone is going to a BBQ joint. 

For anyone following IIFYM, that’s not a problem; just adjust your meals to allow for most of your protein and fat to come from your last meal of the day. So the enormous steak sandwich with cheese might become pasta with marinara sauce. You might drop your planned protein shake and have a flapjack instead. 

What you do exactly doesn’t matter; what matters is that the macro-nutrient tally at the end of each day is roughly the same.

Not drastically under, not drastically over, but on target.

That is in essence what we mean by an ‘if it fits your macros’ diet: a diet that allows a large degree of flexibility in how and when you get your protein carbs and fats, but very little flexibility in how much of each macronutrient you consume. 

Now let’s look at some of the supposed benefits of adopting the IIFYM approach to dieting.

We’ll take a look at what some of the most diehard proponents say, as well as the science used to back up their claims. We’ll then tell you what we think, before going over the arguments put forward by those who think IIFYM is a load of bull. 

The Benefits Of An IIFYM Approach To Dieting

The one big benefit that should be apparent to everyone before we even look at the diet’s effectiveness is the flexibility it permits.

That simple caloric restriction doesn’t really work very well for most people is now common knowledge. It’s usually what people try when they have no knowledge of how the human body works, and they just end up either starving themselves or drastically under-estimating how much they’re eating; neither outcome is desirable.

Diets which do not solely focus on caloric restriction but instead on a specific macro-nutrient or an eating schedule are what we usually see adopted by serious bodybuilders.

You know what we’re talking about: turkey, brown rice and cabbage, 6 times per day, thoroughly chewed, with nothing but water to wash it all down. No sauce, not much salt, not much flavor. 

Other diets utilized by bodybuilders, powerlifters and strongmen will focus more on when food is consumed.

Take carb-backloading for instance; this is when you consume most if not all of your daily carbs after training. 

We can vouch for the effectiveness of this one, but it doesn’t leave you with a lot of wiggle-room. 

If you find yourself out with friends in the day (and you train at night), you could end up being the guy or girl who has to turn down the pizza buffet because you are “saving your carbs for later”. If your friends aren’t real, serious fitness enthusiasts, this is unlikely to go down too well. 

If you adopt an IIFYM approach, timing doesn’t come into it. 

If you find yourself at the pizza buffet, you can go to town. You would then just have to alter your remaining meals accordingly (assuming you have any macros left in your daily “budget”). So the burger and wedges you had planned would have to become a protein shake and an apple.

But that’s fine, because you got to indulge when it mattered most. All that matters is that you stay to your plan and that your total macro targets keep you in a slight caloric deficit (or surplus if bulking). 

So what are the more substantive benefits? Physique and performance are my main goals; what does IIFYM offer me?

Obviously, any diet which puts you in a caloric deficit is going to lead to weight loss. 

Similarly, any diet which puts you in a caloric surplus is going to lead to weight gain. 

Now we all know that weight loss isn’t always a good thing. Weight loss can be mostly fat loss, it can be mostly muscle loss, or it can be both in equal amounts.

You achieve the former and avoid either of the other options by making your caloric deficit and your training conducive to preserving muscle mass.

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So how does “flexible dieting” compare to regular meal plans when it comes to body composition?

Well, you might be amazed to find out that many studies comparing these two dieting styles have found no observable difference between the two in terms of fat loss or lean body mass preservation. 

Take a look at this little experiment, conducted by staff writers at Bodybuilding.com. Though slightly flawed and not without bias, it is still interesting. 

We see that participants were split into two groups; those following a set meal plan, and those following a flexible dieting plan (or IIFYM bro). 

During the first phase, which can be thought of as the ‘cutting’ portion, the subjects following the IIFYM protocol lost more weight than the group following the rigid meal plan.

During the second “do whatever you like” re-feed portion of the experiment, participants who followed the IIFYM plan gained more weight, but only by 2 pounds. 

Clearly then, IIFYM can offer the same if not better results as traditional dieting approaches.

In fact, there is a lot of evidence suggesting that a flexible diet can help keep you on track for the period immediately following a cut, when you seek to re-enter a slight caloric surplus without adding fat mass back on. 

Does IIFYM Stop Yo-yo Effect?

If you have never personally experienced the yo-yo effect, then count yourself lucky. But we guarantee you know someone who has.

The yo-yo effect is where you diet and train really, really hard for months and finally get to the body fat level you want to be at, only to then start your re-feed phase and end up putting all of the weight back on in a month. 

The main reason this usually happens is because people end up punishing themselves during a diet so much that they can’t control themselves properly when it comes to re-had feeding time. 

They have spent weeks being hungry. They have dreamed of pizza and ice-cream for so long that they convince themselves they can afford to go a little crazy once the diet ends. 

After all, they look the best they ever have. A tiny tub of Ben & Jerry’s every night isn’t going to do any harm, is it? And what about some take-out; Indian food is healthy, right? That’s fine. And you promised yourself some shawarma once the diet was over.

There is scientific evidence suggesting that IIFYM helps prevent this kind of behavior.

Take this study for example, conducted in 1999 and published in the Journal Appetite.

The researchers here concluded: “The strongest canonical correlation (r=0.65) was the relationship between flexible dieting and the absence of overeating, lower body mass and lower levels of depression and anxiety. The second strongest canonical correlation (r=0.59) associated calorie counting and conscious dieting with overeating while alone and increased body mass.”

That doesn’t leave a lot of room for interpretation. 

These findings have been backed up in a separate study, published in 2011.

This study posited that the flexibility of flexible dieting helped attenuate food cravings, which the researchers linked closely to dietary success. They concluded: “Food cravings fully mediated the inverse relationship between rigid control and dieting success. Contrarily, flexible control predicted dieting success independently of food cravings, which were negatively associated with dieting success.”

Again, pretty unequivocal stuff.

So IIFYM seems to offer one big benefit over traditional diets when it comes to body composition goals: it can help prevent over-eating when you start to come off a cut and start to fill out again. 

None of us want to cut forever obviously, and very few of us will ever actually need to cut for cuttings sake; most of us will only do it to get down to a respectable body fat level before we start up the GAINZ TRAIN AGAIN BABY!

So the fact that IIFYM can help with the transition process from a cut to a maintenance or bulking period is a big benefit.

The Negatives Of “Flexible Dieting” – Why IIFYM May Not Be Right For You

Anything that has hardcore fans will have hardcore haters, and IIFYM is no exception.

Some people argue that all diets are in essence an IIFYM diet; even traditional meal plans plan macro-nutrient profiles, so they are in fact just macro diets.

This is really just pedantry. Yes, all diets should look at a macro-nutrient profile. No, not all diets are IIFYM, because they do not focus on macro-nutrient profile as their only variable. You cannot have 2 donuts and a soda while “clean eating”; you can’t treat yourself to a hotdog while intermittent fasting.

So what about more substantial criticisms of IIFYM-style dieting?

Well, we can’t find any better place to start than with Mr Olympia himself; “The Gift”, Phil Heath. Here is Phil sharing his thoughts on IIFYM. If the video doesn’t auto-skip, start at around 8.45:

To re-iterate one of the most striking lines from Phil in that entire interview: “When I was doing that (IIFYM), was I really getting better appearance-wise, or was I just getting watery, fat and stronger?”

He then goes on to say: “You know you’re not going to be doing that same type of diet during a prep…Show me a guy that does that, I’ll laugh.”

Phil then goes on to get to the heart of what he thinks of IIFYM-type diets; the desire to make things easier for the sake of results.

Indeed, this is just another way of phrasing what we described above as one of the major benefits of an IIFYM diet; the fact that it is so much more enjoyable than traditional diets with seemingly little sacrifice in the way of results.

But couched in these terms, it seems possible that people under-estimate what a difference this kind of dieting can make over the long-term.

Phil suggests that people following a “flexible diet” fail to appreciate the fact that all protein is not the same, and that all carbs are not the same. The big difference between eating the occasional cheat meal and eating “clean” 99.9% of the time is the effect this has on energy and training. Eating yourself into a “food coma” a few times per week can have a negative impact on your training. Although this would not be an enormous impact and relatively rare, this can add up over time.

We see similar thoughts here echoed by Ben Pakulski. While we disagree with BPak on quite a few things (and as ingratiating we find his “I can talk rings around you” demeanour), he certainly makes a very good point about IIFYM in this video:

One thing we can pull out of this video really ties together all of the sentiments expressed in both of these videos, as well as all the videos like them from pros talking about IIFYM: if you’re a pro, then you can’t afford to risk compromising performance even by a few tenths of a percentage. 

True enough, if you’re trying to be the best in the world, and you’re training 2-4 hours every single day, then exactly what you put into your body with every single meal becomes incredibly important. Nutrition may in fact be said to be the most important factor when it comes to reaching the top 0.1% in the world. 

But what if you aren’t trying to gain one of the best physiques in the world?

What if you’re just trying to look really good by the standards of your own local gym? 

What if you want to be a very competitive strength athlete, or a fairly successful boxer on your regional circuit?

In these cases, it seems that the compromise in performance for the extra flexibility in your diet would probably be worth it. That, of course, is totally for you to decide. 

But the performance and physique aspect is not the most serious criticism of IIFYM-type diets in our opinion. The most valid and universally-relevant criticism relates to health.

What About Micro-nutrients?

That IIFYM diets don’t place sufficient emphasis on health and longevity is, in our opinion, the most damning criticism of those types of diet plans. 

Most IIFYM diets should make a stipulation for fiber as a matter of course. 

However, IIFYM diets do not make any stipulation for micro-nutrient intake, nor for micro-nutrient intake frequency. 

Nor do they make any strict demands for water intake. 

If you know anything about health and fitness, you’ll know why this is such a big problem. If you don’t, we’ll give you a very quick run-down of some of the more serious problems relating to micro-nutrient deficiencies. 

The long-term consequences of a systemic Vitamin A deficiency are quite serious.

The immediate consequences of a Vitamin D deficiency include falling testosterone levels. This is serious if you care one iota about strength, fitness, and athleticism, not to mention overall health and well-being. Vitamin D has a close relationship with testosterone levels. This is why men with low testosterone levels are often advised to try Vitamin D supplementation to rectify the problem naturally before turning to HRT.

Don’t even get us started on salt.

Setting absolutely no sodium guidelines for the day is suicide if you want to look nice and shredded.

It will also make your life hell if you have a contest coming up that you need to make weight for; powerlifters and semi-professional fighters make great use of sodium/water manipulation to make weight and then to blow back up just before a meet/fight. 

These variables may sound like the kind of thing only Phil Heath or Floyd Mayweather need to worry about, but they really aren’t. These are the things that people need to take into account if they actually want the results they say they do.

The guy in your gym who looks totally jacked, rock hard and vascular all year round; he damn sure takes note of his salt intake, and he almost definitely looks after his vitamin intake, even if it’s just eye-balling things. 

If you want to look like that, then you need to take these things into account too.

What About Meal Composition? – A Serious IIFYM Flaw?

How a meal is composed will have a huge impact on your body composition. 

That meal composition affects body fat levels and overall health is a well-established fact.

We’ll just give you one example to illustrate this point: the relationship between insulin and fat storage. 

Carbohydrate consumption elicits a massive insulin response. Consuming things like whey protein and red meat also brings about an insulin spike, but the effects is most pronounced with foods such as simple sugars and simple carbohydrates. 

If you consume a meal very rich in fats and very rich in simple carbohydrates, then you will be releasing a lot of fat into your bloodstream while insulin levels are at their highest. This means that your body will be in “storage” overdrive while you have a lot of fat floating around your system. 

This is why it is established wisdom in the bodybuilding world to not consume huge amounts of fats and simple carbs together in the same meal. This is also why bodybuilders are recommended to eat mainly carbs and protein around their workouts; this allows them to make best use of the “storage” overdrive, forcing protein to their damaged muscles rather than forcing fat into their fat cells. 

That brings us onto a separate criticism of IIFYM: it ignores nutrient timing. 

Nutrient timing is a well-established practice in the fitness community.

There are heaps of scientific studies, clinical trials, and literature reviews attesting to the fact that proper nutrient timing can have a huge impact on results, be they aesthetics, strength, or athleticism. 

An IIFYM diet protocol makes no stipulations about nutrient timing or meal composition. Yet we know by looking at the available scientific data that both of these factors have a significant influence on results. 

Meal Size Affects Nutrient Absorption & Performance

If you are following a “flexible diet”, then one major benefit is that it allows you to “save” your macros for whatever meal you want. 

As stated above, this can be a major blessing if you are going out with friends later on in the day.

If you end up at an all-you-can-eat Chinese buffet, you can really enjoy yourself and just make up for it elsewhere. 

However, when this happens, people can tend to “save” a little too much for later on in the day.

Then when it comes to really chowing down, one of two things happens: they either can’t physically eat enough to get their macros in for the day without making themselves feel sick, or they massively over-eat because they have starved themselves all day long. 

This kind of “saving the goods for later” can have a dramatic effect on training. 

Imagine knowing that you are having a huge meal with your family later on this evening, but this morning is deadlifts and back.

You might be tempted to save your carb intake for your dinner out, but that will leave you short on nutrients in and around what is probably your most brutal training session of the week. 

Of course you can workout a meal schedule to avoid this, but we are talking here about IIFYM in general, and the lack of weight it gives to meal timing and frequency is an issue.

Conclusion – What To Make Of IIFYM

As with all things, nobody can really tell you that IIFYM is for you unless they know exactly what your goals are, how badly you want them, and how important other factors are for you. 

The main benefit of IIFYM is that it allows you to enjoy the foods that you like to eat without ruining your macro-nutrient intake for the day. Sneaking a Snickers bar on a diet of broccoli, chicken and rice might put you over your carb limit for the day. If following IIFYM, you can have a Snickers bar so long as it fits in with your macros for the day. 

IIFYM or “flexible dieting” seems to also prevent overeating once a diet ends. The yoyo effect is many lifters’ worst enemy, so allowing yourself some extra freedom during your diet to prevent cravings might be just what the doctor ordered here. 

However, it does have serious limitations. 

It doesn’t place any fundamental value on meal timing, meal frequency, or micronutrient breakdown. It doesn’t take into account that food quality is a thing; a calorie is not a calorie, and a carb is not a carb. 

All of these limitations will have an effect on the way you train, as well as he results you get out of training. You therefore need o decide where your priorities lie: dietary flexibility and “cheat” eating, or results. 

You also need to evaluate whether or not you can be trusted to plan your diet properly. 

If you know nothing about training or nutrition (i.e if you have read a few articles on Flex and now talk like you’re Jay Cutler), then you should probably stick to a more rigid diet laid out by someone who knows more than you. 

If you have a few years of training under your belt, you have experience dieting and you know how your body responds to certain foods, then IIFYM might be a good option for you to mix things up. 

Left to their own devices, a total noob could really see some terrible progress employing an IIFYM mentality. They could even find themselves getting ill because their micronutrient profile is all over the place. 

You also need the experience to measure your progress accurately and to know whether your current diet is working or not; if not, you need to know what to change and when to change it. If you can’t do this, then we would recommend you stick to a more traditional dieting approach until you have more experience in the gym and in the kitchen eating as an athlete. 

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