Dr. Andrew Chappell, Performance Nutritionist

BSc (Hons), MSc, PhD, RNutr (Sport)

Take home points

  • IIFYM or Flex Dieting involves setting macronutrient targets, tracking calories and fitting food items into your diet to hit those macro targets
  • There are different versions of IIFYM, some will eat a high-quality balanced diet, others will opt to eat junk food so long as it fits into their macros
  • Flex Dieting is similar to IIFYM, however the fibre target forces people to make sensible food choices
  • IIFYM and Flex Dieting is an effective strategy for weight loss and gain, although may not always be suitable for physique competition preparation
  • Lack of precision in food analysis and composition tables make it nearly impossible to match foods for macronutrient content
  • This lack of precision can lead to day to day variations in energy intake that has consequences for physique competition
  • Two food items with similar macronutrient content, can have very different effects on the body’s physiology and appearance. E.g. liquid vs solid meals, salt or fibre content
  • IIFYM dieting likely leads to higher dietary adherence and reduced dietary fatigue compared to hardcore/clean eating approaches
  • In the offseason for physique athletes IIFYM dieting is a viable alternative to clean eating and hardcore dieting.

What is it?

IIFYM – involves setting a set target for macro nutrients and then consuming foods to hit those macros. For example, say your target for the day is 200g protein, 400g of carbohydrate and 90g of fat. So long as you meet those targets, you can essentially eat however you want. There are different variations on this approach, some will try to hit a fibre target alongside the macros, others may set targets based around 80 % “clean eating”. The basic premise however is calculate your macros then track your food intake using an App like MyFitnessPal, and go for it, whether it be weight gain or weight loss. Flexible dieting is essentially another name for the same approach, although like IIFYM there are nuanced interpretations.

Is it a fad?

An IIFYM approach is nothing new. Slimming World and Weight Watchers, popular weight loss support groups, uses a similar dietary approach. A points system where dieters are allotted so many points per day and meals are prescribed a value. Like most things IIFYM has simply been repackaged and rebranded, in the same way that not tracking calories and simply eating until your full is now termed “initiative eating”, or “Atkins” is now “Keto”.  Athletes and Joe public have been doing IIFYM for years. In fact almost any lifestyle magazine you pick up will no doubt contain a diet based around a variety of foods, presented in a menu with calorie and macro targets to be consumed on over a week. This provides healthy and balanced diet rich in vitamins and minerals from different sources. All based around the central theme of either helping people lose weight or gain muscle for athletic performance. However most Joes and a lot of athletes at least at amateur level won’t weigh food or count calories to the same extent as a bodybuilder or fitness model. Menus however often include portion sizes, just don’t expect to have the same precision when you don’t weigh things. For most though this isn’t an issue and there’s certainly no need to weigh your food it’s possible to meet your fitness or health goals by simply being aware of portion sizes.

An IIFYM approach is nothing new. Slimming World and Weight Watchers, popular weight loss support groups, uses a similar dietary approach.

Where IIFYM or Flex Dieting goes wrong is where people set a calorie or macro goal and take the stance that you can eat what you want. A “calorie is a calorie” is a favourite phrase and some will use this as an excuse to eat junk food, limited in nutrients and high in sugars and saturated fats. Not all Flex or IIFYM dieters will eat “dirty” so they shouldn’t all be tarred with the same brush, but there are limitations with the calorie is a calorie approach.

Problems with assuming foods are equal.

Any self-respecting food scientist worth their weighing balance will tell you that employing an IIFYM approach in the real world is almost impossible. This is largely down to the inherent difficulties in determining what’s in food e.g what is the macronutrient and overall calorie content. Two food items may claim to have the same macronutrient content, but ultimately those are only ballpark numbers, and that’s one of the major limitations of this type of dieting for bodybuilding competition preparation.

Reference food tables from which the nutrient composition of any given food is obtained are inherently flawed and inaccurate. If you do a quick search around the web you can easily find multiple different calorific values for something as simple as an apple. There are multiple reasons for this. Differences in analytical techniques used to determine nutrient composition which use either direct or indirect methods to assess the nutrient composition make matching difficult. Moreover, the source from which the apple was obtained, New Zealand vs Spain for example, growing conditions, variety, processing and the list goes on.

The likely hood of being able to pick different foods or meals on any day that match your macros because of this is almost impossible.

It’s also worth pointing out that when you start to look at processed food, ready meals and the likes or pizza’s, and burgers the waters become even muddier. I’ve seen duplicate diet analysis of food items matched to food tables where foods had as much as an extra 500 kcals or 30g more fat compared to what’s on the label.  Ridiculously, you even get anomalies in food tables where no fat data is available for process meat, so the contribution is excluded from a meal’s nutrient breakdown.

A miscalculation of 150 kcals per day in either direction is worth around 1000 kcals a week, that’s 3 – 4 extra meals  you’re having or missing.

You may wonder how they could get away with your nutrient analysis being so wrong. Well recall the horsemeat scandal.  The FSA lack funding to police such things like recording and comparing the composition of every food item available in the UK. Although they do have a program dedicated to just this. Combine that with the fact that a lot of companies don’t employ full time food scientists, so nutrient composition analysis might only be outsourced every other year. Or a foods composition might be determined simply by using food tables for as many as 50 ingredients.  The likely hood of being able to pick different foods or meals on any day that match your macros because of this can be almost impossible. Fitting things to your macros is a nice idea, but it doesn’t work practically. Sure, you might say that you’ll be a little off, or in the ballpark with your macros, but when it comes to competition prep you want to be bang on. Besides the more processed foods you eat, the more likely you’ll be well off with your macros and overall energy intake.

How off could I my calories be?

When you’re involved in such a number driven sport like bodybuilding, depending how flexible you diet it’s easy to see how you could be over, or under eating on a daily basis. A miscalculation of 150 kcals per day in either direction is worth around 1000 kcals a week, that’s 3 – 4 extra meals  you’re having or missing. Something I’d certainly think of as a loose approach.  Which is where following a more structured plan has distinct advantages. The same pit falls still apply, but at least the day to day variation in intake will be significantly less. This matters when you are removing 50 – 100kcals per week from a diet. During a diet you make small changes like this can help keep the fat burning process going, keep your training intensity high, keep you full, and spare muscle tissue. This is all particularly important when it comes to the business end of the diet.  The consistent clean dieting approach works well, and stops you having to be drastic with cuts to the diet. It does require you to be meticulous, but the evidence of its effectiveness is plain to see in the top natural bodybuilders who have competed over the past 10 years. Names like Rob Hope, Dave Kaye, Ben Tennessen spring to mind.

Although you do see people who struggle with meticulous clean eating or “hardcore dieting” and you will see shredded IIFYM dieters; for me a IIFYM dieting plan based around contest prep would benefit from structure, to ensure consistent over a week (planning foods for certain days for example). And anyone who understands the fundamentals behind dieting theory will know consistent intake over a week is better than one that fluctuates wildly like you tend to see in the general population.

Different foods Change the way you look

Bodybuilding and physique sports is very a case of how you look, and different foods can make you look very different. Whole foods with lower GI, and less processed ingredients will give a physique a much cleaner look. Compare that to foods highly processed, with large amounts of sodium, sugars and saturated fats, which tend to make a physique appear watery or cloudy. Certainly not an ideal place to be when trying to assess progress and another reason why IIFYM dieters would better suited to eating quality foods.

Beverages also have different effects to whole foods, and if you blended your lunch compared to eating it you can bet you’ll feel a lot hungrier with the blended one.

Another idea I’ve seen perpetuated is that there is no difference between two meals if they contain the same nutrient breakdown.  This is unlikely as I mentioned already, however assuming they do have the same nutrients, the effect on your physiology can be very different. There are simply too many variables for the response to be the same. I read recently Layne Norton stating that so long as the fibre content was high enough it didn’t really matter what you ate for IIFYM (this obviously forces people to be sensible).  However, I feel this betrays his own fundamentals, the idea that an active ingredient can influence a foods value to a bodybuilder i.e leucine. 

Foods that fit but aren’t the same

Foods with the same nutrient content, may have completely different arabinoxylan content, resistant starch, starch gelatinisation, amylopectin ratio, amino content, creatine content, fat ratios (trans fat, SCFA, MUFA, PUFA) and viscosity will all effect how a food performs in the body. In turn these factors influencing things like protein synthesis, carbohydrate response elements, leptin, insulin, CCK, prostaglandins, cytokines, resolvins, PPAR’s etc. This can also change your body’s appearance from one that’s crisp to a watery mess. All of the above can be influenced via food processing, additives and simple factors like cooking.  You can even take two identical foods, increase the molecular weight of an active ingredient  (3g high MW vs 3g low MW) and you’ll see a different hormonal response. Beverages also have different effects to whole foods, and if you blended your lunch compared to eating it you can bet you’ll feel a lot hungrier with the blended one.


For the everyday fitness enthusiast, I think a healthy flex IIFYM approach works well if you want to track your food intake. For most everyday people I would point them towards portion size guidelines, since I’m not a big fan of people weighing food and counting calories.  People taking part in physique sports where the goal is to get super lean a planned IIFYM approach with variation would be my recommendation, although the meticulous clean eating approach is one, I think always works best. There are however many roads to London. There’s a lot different ways you can get in shape. One of the things I haven’t discussed is the effect on the individual’s psychology, adherence and dietary fatigue, which are extremely important for any dieter. I think there are distinct psychological and physiological advantages an IIFYM and flex dieting approach over clean eating/hardcore dieting. So, I always recommend a version of flex/IIFYM dieting in the offseason for bodybuilders. Whatever your goal, do your research, make yourself a plan, record your progress and be sure to give it a fair shot.