Blog Post

Flexible Dieting: Everything You Need to Know

Flexible Dieting Guide

Back in 2011, I first went onto and started learning about calories and macros.

I am not necessarily proud of that starting point. But it was my introduction to a concept called If It Fits Your Macros (IIFYM) which basically meant that it did not really matter what you ate as long as you meet your macronutrient targets. From a body composition perspective, while it is obviously a bit more nuanced than that, the outcomes will be mostly the same.

Flexible dieting is a mostly interchangeable term with IIFYM. But I prefer the terminology flexible dieting. IIFYM has a bit of a negative vibe to it since so many people have taken it to an extreme. Instead of trying to have mostly nutrient-rich foods to make up the bulk of the diet, a lot of people take it as a challenge to literally eat anything as long as it hits their macros.


I view flexible dieting as more than that. From one perspective, it can be viewed as tracking calories and macros while having flexibility with food choices within those macros. So, it is strict in that you will hit your numbers and get great results, but flexible in that you do not need a rigid meal plan and you can literally adjust based on whatever life throws at you.

From another perspective though, if you have a rigid set of numbers that you NEED to hit super closely every day, is that truly flexible?

Back in the day, I used to think it was. I viewed it as a way that you can get optimal body composition results while being more flexible.

But now I would say my version of flexible dieting also involves even more flexibility. Whatever that means to you. It could even just be having a day here and there for special occasions where you do not even come close to your macros. There are a lot of options.

Personally, although the gold standard layout of flexible dieting I will refer to below involves tracking macros, I would also make the argument that you can be flexible dieting without tracking macros.

I am not a stickler for definitions, so that is not a hill I am willing to die on. But the point I am making is that if you have tracked macros for a while, implementing flexible dieting principles, there is no reason you cannot just go and continue implementing those principles without physically tracking macros in the future. It might not be “optimal”, but you could still get the results you want from that.

If you want to skip a lot of the info about why flexible dieting works and just want to know how to do it, feel free to go to the bottom of this article where I will give a simple step-by-step guide.

What is The First Step?

Energy Balance

The first step is to figure out your calorie needs.

There are countless options for how to do this, but my recommendation is to go onto our calorie calculator and use that as a starting point.

Calorie Calculator Ideal Nutrition

That is a shameless plug, I know.

But a key point I want to highlight is that this is an ESTIMATE. People message me on Instagram all the time being like “your calorie calculator gave me a number that is 100kcal different to another one I used. Which one is right?”

And to be honest, the answer is probably that neither is 100% accurate.

But they are probably a better ballpark estimate than you would expect. Particularly when it comes to measuring Basal Metabolic Rate, which is a large portion of your total daily energy expenditure.

There was one study done on people who had reported being “diet-resistant.” Every participant reported having tried low calories, amongst other options, and not losing weight on that.

These are the people we would commonly assume have “slow metabolisms.” But when their BMR was measured and compared to what a formula would predict based on body composition, the average was within 5%.

This is crazy because that group is what we would most likely assume are going to be more likely to be outliers. But even among them, there were no real outliers with a significantly slower than expected metabolism.

Ben Carpenter 1200kcal per day
Image via Ben Carpenter on Instagram

On average they underreported their food intake in terms of calories by 47% though. And one person underestimated by as much as ~80%. But that is a separate topic.

I am not using that to suggest “outliers do not exist; everybody is within 20% of a formula.” I am more using it to point out that outliers are rare when it comes to BMR.

How Do You Turn That Calorie Estimate into Useful Information?

I am not overly concerned with being ridiculously accurate with that starting number, because it really does not matter too much. What matters is how your body responds. As much as I said outliers are rare, we also care about how YOU respond to whatever calories you are consuming.

The gold standard approach would be to use your estimated calorie expenditure and then consume that amount every day. This would involve tracking all your food through an app like MyFitnessPal and aiming for that number of calories.


To save time I will just link to an article describing HOW to track macros using MyFitnessPal.

It does not really matter what app you use. I just recommend MyFitnessPal because it has such a large database of foods.

The other thing to consider though is that it takes time to get “good” at tracking macros. MyFitnessPal has a lot of products that are inaccurately labelled. It takes time to be able to identify those.

I wanted to mention that because I have seen a lot of clients who were adamant that they were eating X calories when they were really eating something like X + 400kcal due to tracking mistakes.

It is also worth being aware that foods are not 100% accurately guaranteed to be the same calories their label says in the real world either. There is about a 20% margin of error on either side just due to the variability and logistics.

Following the gold standard approach, you would consume the number of calories the calorie calculator recommended for maintenance, for 2-3 weeks.

If you remain weight stable (within reason, there will always be fluctuations that naturally occur), then those are your maintenance calories. To rephrase, that is roughly the number of calories required to maintain your weight on average.

If your weight dropped, that was a calorie deficit. If your weight increased, it was a calorie surplus. To find your true maintenance calories, you basically adjust from there as needed.

What I mean by that is that if your weight dropped, you would add calories until it is no longer dropping. Or vice versa if it increased.

How to Go Straight into A Deficit/Surplus?

That is the trick though. You do not actually NEED to find maintenance first.

You can choose to jump straight into a deficit or a surplus.

As a general rule:

500kcal deficit = 1/2kg per week weight loss.

1000kcal deficit = 1kg per week weight loss.

Double the deficit, double the weight loss. The same numbers apply but in reverse for weight gain.

This is overly simplified maths though. It is just a good guide that is incredibly helpful in practice even if it is a slight bending of the truth. There are so many variables, and I will mention a few below, that impact this.

You also need to be aware that short term weight fluctuations (such as water weight) are influenced by things like food, sodium and carb intake.

If you decrease your calories, you probably consume less of those things. A 500kcal deficit might be 1/2kg per week weight loss on average, but your weight will likely drop by more in the first 1-2 weeks because of this. Vice versa for if you enter a surplus or were to have a “cheat meal” for example.

Glycogen and Water Weight Tweet

Basically, the same logic applies as finding maintenance calories. If you want to lose 1/2kg per week, you will find your estimated maintenance calories and take away 500kcal.

Then you would follow that for 2-3 weeks and adjust as needed. Heck, you could adjust sooner if you would like, it just probably takes 2-3 weeks to build up the data to make a decent decision.

How to Track Weight

Woman Standing on Scales

You do not need to track your weight.

But if you are trying to figure out the size of the deficit or surplus you are in, it is helpful information.

For those without a menstrual cycle, the gold standard is to take the weekly average of your morning weight, after going to the bathroom.

For those with a menstrual cycle, due to hormones influencing water weight changes, it makes more sense to use monthly averages. This takes time, which is why early on it can make sense to make decisions without having enough data, but eventually, the monthly averages become far more informative than weekly data does.

Why Are Calories Important?

Calorie’s control what we weigh.

Some people make a “calories vs hormones” debate. But they really go hand in hand to a certain degree.

Calorie intake and expenditure influence hormones, and hormones influence calorie intake and expenditure.

To be clear though, our bodies burn a certain number of calories each day.

This number changes on a day-to-day basis, based on a variety of factors such as resting metabolic rate, thermic effect of food, formal exercise and incidental movement.

TDEE Components
Image via Alex Thomas from Sports Nutrition Association

Calories only come from one external source: food. Since as a fact, a certain number of calories has been burnt, if we consume LESS calories than that, there is a DEFICT.

That energy (calories) must come from somewhere. If it has not come from food, it must come from our internal stores. And our internal storage forms are mainly fat and muscle.

If you eat below your calorie expenditure for one day, technically you would be in a deficit and contributing towards weight loss.

But one day does not really move the needle. 1kg of fat has ~7500kcal therefore it typically takes multiple days/weeks/months to see the results you want. It is not exactly 7500kcal, it is a broad range, but this is close enough for the point I want to make.

Therefore, a calorie deficit over time leads to weight loss. Alternatively, this is also why if you consume more calories than you burn, it gets stored in your body and leads to weight gain.

Other factors such as training, sleep, stress, protein intake etc contribute to influencing what proportion is coming in the form of muscle vs fat.

But the calories are the main factor in what our overall weight is.

Obviously, this gets messy because there are other things that influence weight over the short term such as water fluctuations etc. But this is the main principle as to how calories control our weight in the long term.

You cannot be in a calorie deficit for an extended period without losing weight, excluding outlier cases like large increases in water weight.

It is also worth being aware that what we eat influences our calorie expenditure too. I will talk about this in a later section though.


Protein rich foods

Protein is obviously important for a variety of reasons. From a body composition perspective though, it will help with muscle gain/retention.

The gold standard for people who are already kind of lean and looking to optimise muscle growth is 1.6-2.2g/kg/day protein. So that would be 160-220g protein for somebody who was 100kg.

If you are looking to undertake a large calorie deficit, or get particularly lean, the evidence-based range changes to 2.3-3.1g/kg/day of fat-free mass. Note that this range is based on fat-free mass (as in your weight, minus the amount of body fat you have) and the previous range was total body weight.

With the 1.6-2.2g/kg/day range, my guide is that if you are exceptionally lean or in a large deficit, I would go slightly higher. If you have more body fat (say >15% for men and >25% for women – but that is not a specific number) you might want to reduce the range slightly.

This is because it is really based on how much muscle mass you have. People with lower body fat percentages have more muscle per kg of total body weight.

Going higher than these numbers does not have a noticeable negative impact on body composition, but it also does not seem to provide any additional benefit for muscle growth or fat loss.

Also, keep in mind that these are the numbers for optimising body composition. You can always go slightly lower if you like. If you are just slightly below these numbers, it might suit your lifestyle more without significantly compromising results. Going significantly below these numbers will likely make it harder to gain/retain muscle though.


Dietary Fat Sources

If you hit your calories and protein, your body composition results are actually likely to be very similar regardless of your fat/carb breakdown.

One example of how this might not be the case though is that going exceptionally low fat seems to impact testosterone levels in men. This likely would carry over to a reduced ability to gain muscle.

There is no specific cut-off, but if you keep fat >15% of total calories, you will not even need to think about it. There also does not seem to be further hormonal benefits that are significant from going noticeably above this amount either, after this need has been met.

At the other end of the spectrum, while in a calorie deficit, super high fat diets such as ketogenic diets seem to get similar results to low fat diets.

It is harder to say for in a calorie surplus because although theoretically, it seems like lower fat diets might have some benefits, the extremes have not really been tested on a large scale with keto. This is partly because it is genuinely hard to get a group of people to consume sufficient calories on keto to get into a calorie surplus to optimise muscle gain.

Why Keto Might Not Be Ideal For Muscle Growth

Solid advice would be to aim for 15-30% of calories coming from fat. Arguably saving, the rest of your calories for more carbohydrates could carry over to improved exercise performance which could further improve outcomes. But you can likely still get similar results on high fat diets.  

Dietary Fat Intake for Muscle Gain or fat loss


dietary carbohydrate sources

Because protein and fat contain calories, and you have already set those targets, you just need to fill out the rest of your calories with carbohydrates.

Protein = 4kcal/g

Fat = 9kcal/g

Carbs = 4kcal/g

Calories are literally made up of macronutrients.

To keep things simple, since there is more nuance, I will use a simplified example.

Say you wanted to have 2000kcal.

And let’s say you wanted 150g of protein and 60g of fat.

Protein would be 150×4 = 600kcal

Fat would be 70 x 9 = 630kcal

Therefore, you have used 1230kcal (630+600) of your 2000kcal. There is 770kcal spare.

Carbs have 4kcal/g, therefore 770/4 = 192g of carbs.

The macros could be 150g protein, 70g fat and 192g of carbs, totalling 2000kcal.

What About Protein Quality?

Protein quality matters, but it also is not the top priority.

Total protein intake for the day is the most important factor.

Distributing it across the days is also beneficial for muscle gain. And it is also a good idea to time some of your protein within 3-5 hours around the time of your workout.

protein prioritisation pyramid

When it comes to protein quality, there are things that matter such as the biological value of the protein and the leucine content. This becomes even more important for those on a plant-based diet where it can be even more difficult to get enough leucine.

Flexible dieting is numbers focused and the emphasis is on the total protein intake. It does not necessarily mean you should ignore these other factors. That would be silly.

The concepts of flexible dieting basically acknowledge that total protein intake is far more important for body composition than all the other details.

This means that while you should still prioritise these other factors if you happen to be in a situation where you cannot implement perfect timing or have the perfect quality protein source, you can still focus on hitting the total protein intake and get great results regardless.

optimising protein intake for athletes

What About the Types of Fat?

The type of fat does not seem to influence body composition much.

There are obviously other health benefits associated with different types of fat though.

Flexible dieting really is a body composition (muscle gain and/or fat loss) focused concept though. You CAN apply health-focused concepts within flexible dieting and that is strongly encouraged.

But to be specific to body composition, whether the fat is polyunsaturated, monounsaturated, or saturated does not seem to matter.

Some people reading this might have read research counter to this claim.

I have also seen a few random studies here and there indicating it might matter.

For example, a 2014 study comparing large amounts of palm oil to sunflower oil in a matched 750kcal surplus showed that while the weight gain was equal, the sunflower oil group gained 3x as much lean mass, and therefore far less fat as well.

This seems to be the most commonly cited study on the topic, from what I can see.

But the total change in body mass over the duration was ~1.5kg over 7 weeks. It is hard to make strong claims based on a study like that unless it happens to align with the rest of the body of research.

So, it is certainly an area I am keeping an eye on. But for now, I think it is best to think more about the types of fats for the other benefits, and just focus on the total amount more so in relation to body composition.

What About the Types of Carbohydrates?

This is the part that confuses most people. It was the part that shocked me the most back in the day too.

The types of carbohydrates also really do not impact body composition differently either.

I am sure you can take the concept to extremes and find loopholes in that idea.

But if you are implementing flexible dieting well and >80% of your calories are coming from nutrient-rich, minimally processed foods anyway, there is no way you can exploit these loopholes anyway.

Most people would assume that sugar would make you gain more fat (or lose less fat) than other forms of carbohydrate.

But that is not really the case.

The most famous study on this topic is a 1997 study where they compared a diet containing 43% of total calories from sugar vs one containing 4% from sugar. Both diets were calorie deficits and resulted in pretty much the same weight loss, fat loss and muscle retention.

There is a tonne more research showing similar things. But this study is mentioned heaps because the difference between 4% and 43% is insane.

surwit et all 1997 sugar and body composition

The whole point of this is to highlight that you probably should not stress if you consume a small/moderate amount of sugar within an appropriate number of calories and macros.

Once again, from a health perspective, this can play more of a role. Every calorie you spend on added sugar is a calorie that you are spending on something that is not bringing in additional micronutrients and other beneficial components.

Obviously, sugar also has implications for those with health conditions such as diabetes and PCOS too. But as always, flexible dieting concepts can and should be adjusted as needed for individual situations.

If That Is True, Why Does Sugar Typically Lead to Weight Gain?

Table Sugar

People often discuss insulin and the impact it has. As discussed earlier, calories and hormones go hand-in-hand. It is not one or the other.

To start off with, the overall premise of the carbohydrate-insulin model of obesity has been disproven.

The below references are great examples of that:

To cut to the chase though, the short answer is that sugar makes it easier to eat more calories.

It makes things taste more appealing, which encourages consuming more calories. It also is far less satiating than other forms of calories too.

Adding some sugar to a product often does making it noticeably more satiating, while making it easier to eat more, and directly adding additional calories.

Volume Eating Example


Even early in the IIFYM days, fibre was always mentioned. People still encouraged aiming to get enough fibre. This helps from a satiation perspective and has a tonne of other health benefits.

To keep it brief, I do not have much advice beyond finding an appropriate amount of fibre for your personal needs/preferences.

The recommended amounts are typically 25g per day for women and 30g for men.

But there is an argument to be made for higher fibre intake too.

Another common recommendation is 14g of fibre per 1000kcal. This recommendation can fall apart at extremes of really high/low calorie intake. But I like it as a bit of a good ballpark figure that is a semi-individualised recommendation too.

Some people also thrive on lower fibre intake as well though, so I do not want to discount that either.

Basically, it makes sense to factor fibre into your overall thinking. A mistake people often make is to completely ignore it and just focus on the macros.

Other Aspects Including Micronutrients

Micronutrient rich foods

As a brief history of IIFYM, it was originally invented on to answer questions easily.

People would ask questions like “does it matter if I eat steak or chicken?” And eventually, people got tired of answering in long-winded ways, to basically bring it back to “as long as it fits your macros, the difference between those two options does not really matter.”

This eventually just got shortened to the acronym “IIFYM”.

But one of the things that was referenced even back then is that it probably takes ~80% of the average fit and healthy person’s maintenance calories to reach all the micronutrient recommended daily intakes.

This means that if 80% of your foods are coming from “healthy” foods, with decent variety, there is a solid chance you have met most/all micronutrient targets. It could take even less than this if you happened to have a lot of vegetables or random nutrient-dense foods like liver.

At maintenance calories, this would mean you could fill the last 20% with micronutrient poor foods and not worry that you are not getting enough micronutrients.

It is obviously more nuanced than that though. In some cases, there may or may not be benefits to going beyond the recommended daily intake.

It is also worth considering, what if you are in a 20% calorie deficit. Does it now mean you need 100% of your food to be “healthy” to reach that target?

Potentially a multivitamin could cover the gaps.

Even the multivitamin aspect is a slippery slope. There is merit to it. But then you could apply that thinking and try use one to cover all/most micronutrients, and only think about macros, calories and fibre.

Nutrition is more complex than that. There are components of food that cannot easily be replaced through supplementation.

My thought process is along the lines of the 80/20 principle. Aim to have 80% of your calories coming from whole, minimally processed, nutrient-rich foods, and the last 20% can come from whatever you would like.

How to Adjust Over Time

Over time, as your weight and body composition change, or your activity changes, you will need to adjust your calories.

Even just thinking about it simply, that makes sense. Like if you were 90kg and used that calorie calculator, it would give you a different number than if you were 80kg.

But it is even more complex than that. There is a concept called metabolic adaptation, which explains why calorie needs change over the course of a diet by more than a formula would predict.

Metabolic Adaptation Example

Basically, while in a calorie deficit, the body adjusts. It tries to conserve energy by downregulating certain processes.

One example of this is that people on low calories literally tend to move less.

They fidget less. Their heart rate slows down. They even blink slower. This is sub-conscious, but it reduces calorie expenditure.

It is not a massive deal, but it is worth being aware of. For the record, the opposite happens when you increase your calories too.

This is more to highlight that you need to adjust your calories over time.

That means you also need to adjust your macros over time.

Protein is easy. If you happen to can multiple kg of muscle (luck you) you should now re-calculate your protein needs.

For fat and carbohydrates, you just adjust as desired, so long as you do not take fat excessively low.

Benefits of Flexible Dieting

There are a tonne of benefits to flexible dieting.

The main one is that it allows you to eat based on your preferences. You can also adjust on the fly and still make great progress while in situations you might not be able to stick to a rigid diet.

It also can help encourage a step away from orthorexia from some people. Acknowledging that you do not need to eat super “clean” all the time to get great results can be helpful for some people.

Another underrated aspect of it is that if you are tracking, you are going to be more accurate than another option which has “rules” but no tracking or even weighing/measuring of food.

For example, most of the time when people do keto, they will lose body fat. But what if you do not? What if you still eat too many calories and do not see more progress?

Without understanding calorie balance, the solution could seem like you just “need to keto harder” so to speak. Try to reduce carbs further.

Or even if you do understand calorie balance, you can try and reduce your intake, but it is a little bit more of a guessing game. The more inconsistent your intake is, the harder it becomes too.

But with tracking macros, you know almost exactly what you are consuming, if you are tracking well. If you wanted to lose fat and it was not working, you can just reduce your calories further or increase calorie expenditure through exercise.

Downsides of Flexible Dieting

I have got another blog post on the downsides of flexible dieting in more detail.

While flexible dieting has a lot of benefits, it is not for everybody.

Calories in vs calories out IS a concept that applies to everybody. That is not a debatable concept.

But flexible dieting does not need to be utilised. A brief list of downsides are:

  • Tracking calories/macros takes time.
  • You still need to think and plan out your intake to a certain degree.
  • Tracking calories/macros can increase the risk of an eating disorder developing
  • There is not necessarily an emphasis on nutrient rich foods.
  • Processed foods vs unprocessed foods can lead to slightly different energy expenditure and absorption amounts, which actually influences the calories in vs calories out equation.
  • Some people do better with more rules or guidelines. It can be too loose a concept for some people.
  • Being too flexible can lead to overconsuming low-satiety foods, which makes it more difficult to sustain a calorie deficit.
MyFitnessPal and Male Eating Disorders Statistics

Quick Step-By-Step Guide for How to Implement Flexible Dieting:

  1. Find an estimate of your calorie needs for your goals through this calorie calculator.
  2. Aim for 1.6-2.2g/kg/day protein. Reduce the range slightly lower if you have a lot of body fat, increase it slightly more if you are particularly lean.
  3. Fill out the rest of your calories with fat and carbs. A good guide is 15-30% of calories coming from fat. Once you have added your protein, and your fat, you just fill out the rest of the calories with carbs.
  4. Track your calorie and macronutrient intake using an app like MyFitnessPal. Following the plan set up with the above steps for 2-3 weeks. If after that timeframe you are:
    – Looking to lose weight and it is happening too slowly – reduce calories.
    – Looking to lose weight and it is happening at the rate you desire – keep calories stable.
    – Looking to lose weight and it is happening too quickly or you are too hungry/fatigued – increase calories.

If you are looking to gain size, do the opposite of the above.

5. Aim for ~14g fibre per 1000kcal. But this is a flexible number and should be adjusted based on your needs/preferences.

6. Do not overthink meal timing or anything like that. It is still a good idea to aim to distribute your protein across the day. And it is a good idea to eat in a way that makes you feel good when you exercise. Feel free to apply any other principles you think are relevant.

7. Aim to have >80% of your calories coming from nutrient-rich, whole, minimally processed foods, with a decent variety. Then fill out the last 20% with whatever foods you would prefer.

By Aidan Muir

Aidan is a Brisbane based dietitian who prides himself on staying up-to-date with evidence-based approaches to dietetic intervention. He has long been interested in all things nutrition, particularly the effects of different dietary approaches on body composition and sports performance. Due to this passion, he has built up an extensive knowledge base and experience in multiple areas of nutrition and is able to help clients with a variety of conditions. One of Aidan’s main strengths is his ability to adapt plans based on the client's desires. By having such a thorough understanding of optimal nutrition for different situations he is able to develop detailed meal plans and guidance for clients that can contribute to improving the clients overall quality of life and performance. He offers services both in-person and online.