Why Flexible Dieting (IIFYM) Isn't Actually Flexible

If it fits your macros (IIFYM) is supposed to give you more flexibility in your diet. Learn how the general population misuses this dieting approach.

Two definitions from Merriam-Webster Dictionary provide some context for this article:


  1. Capable of being readily changed;
  2. Not bound by rigid standards


  1. Food and drink regularly provided or consumed;
  2. A regimen of eating and drinking sparingly so as to reduce one’s weight

Logically, then, “flexible dieting” should describe a way of eating that can be personalized and subject to change. The goal may or may not be weight loss.

However, as we will discuss, “flexible dieting” (sometimes used interchangeably with “IIFYM” or “If It Fits Your Macros”) is most often used to describe a way of eating that is not flexible at all.

Instead, it describes an approach that is unchanging and strict – the opposite of flexible.

Let’s consider this disconnect between theory and practice and how we might be able to resolve it.

If you have experience following a flexible dieting approach, you might already know what I’m talking about.

What Flexible Dieting is Supposed to Do

In theory, flexible dieting is a great approach to allow for flexibility in the foods we eat. The basic idea is that we do not need to stress about every single item of food that we eat. The nutrient density of every food we ingest is not important if the entire dietary pattern is nutrient-dense.

Related: 3 Things You Can Learn From IIFYMers

In other words, we can meet our fitness and health goals by eating highly palatable foods like French fries and Pop-Tarts if the overall pattern of our diet contains whole, nutrient-dense foods like whole grains, lean protein, fruit, vegetables, and low-fat dairy.

Regarding overall health, research suggests that it is fine to stick to any “diet” you like, e.g. Paleo, Atkins, Vegetarian, or Mediterranean if the overall dietary pattern is high in minimally processed, plant-based food1.

M&S Athlete Prepping Vegetables

This covers the “not bound by rigid standards” part of the definition we described above. But remember there was another piece to the definition. The first aspect of the flexible dieting definition requires that it be readily changeable.

The changeability aspect of flexible dieting is critical because a diet that can easily be adapted to any situation (1) makes it easier for us to stick to our diets and (2) benefits our mental health.

In theory, flexible dieting should be easy to adhere to, and we know that adherence is key to dietary success regardless of which diet we choose to follow2. Often a diet that is easy to adhere to is one that allows us to say “yes” to eating out or eating homemade meals. Flexible dieting allows us to fit such meals into our daily nutrient guidelines.

Other than this adherence factor, flexible dieting is not necessarily superior for physical health or fat loss, but it is superior for its mental health benefits. In fact, the freedom to choose your diet alone may not improve weight loss at all3.

However, the stress reduction that comes from not having to measure and track every item you eat to the exact gram, or turn down friends, family, and colleagues when they offer you homemade treats or invite you to social gatherings, is priceless.

Flexible Dieting is Not Flexible

“Flexible dieting” and “IIFYM” are both terms used to describe a way of eating that involves eating whatever you want as long as overall daily macronutrient and calorie goals are met. The typical approach is to track macronutrients for every item that we eat or drink to the exact gram value.

We use “flexible” to refer to the variety of the foods we eat as opposed to the amounts of the same foods. Many flexible dieters weigh and track every item they eat before it goes into their mouth.

As the definitions at the beginning of this article notes, the first component of flexible dieting is a diet that is “capable of being readily changed.” Not planning your day of eating to incorporate the foods that you have access to is the opposite of this definition.

M&S Athlete Eating out of Tupperware in the Gym

Meticulously planning, measuring, and tracking days of eating that are full of variety and a balance of “healthy” and “unhealthy” foods can give us the illusion that we are dieting flexibly. But if we refuse to give in to reasonable opportunities to change our carefully planned menus, how are we being flexible?

Eating a donut or pizza because we have measured and tracked the exact quantity is not what flexible dieting should be about.

Why There is a Disconnect

The disconnect between theory and practice comes from the fact that we seem to believe that an all-inclusive diet that allows for donuts or pizza is what makes flexible dieting “flexible.”

However, we must remember that flexible means changeable and adaptable as well. Flexible does not mean calculating the macronutrient content of a donut and fitting it into our nutrient goals for the day.

There is a lack of clear definition of flexible dieting in scientific literature, which further complicates the practical applications of flexible dieting.

In research, flexible control has been associated with lower body mass index, lower self-reported energy intake, and fewer binging episodes4. There is evidence that flexible dieting is not associated with the same disordered habits as is rigid dieting5.

Related: How to Properly Incorporate Cheat Meals Into Your Diet

But we cannot apply such research findings to our everyday life if we are not using the same definitions used in the studies. As referenced above, we do not typically define “flexible dieting” how it is meant to be defined.

Both by definition and in research, “flexible dieting” is not what we colloquially use to describe an “IIFYM” approach in the fitness industry. Far too often, things are taken so far out of context from the scientific literature and incorrectly applied, interpreted, and spread throughout the general population.

Let’s not make that mistake when it comes to something as individualized and helpful as flexible dieting.

The Best Approach

Flexible dieting requires a certain level of nutrition knowledge, which means that as flexible dieters we can make educated decisions about what to eat and drink without having to weigh everything to the exact gram or obsessively search for what we want to order at a restaurant in a food-tracking app.

There is a difference between nutrition self-efficacy and obsessively and frantically researching nutrition information on Google.

Let’s consider two contrasting approaches:

1. Classic macronutrient tracking: Involves meticulously measuring and tracking every morsel of food. If you cannot weigh or otherwise measure your food, you will need to research nutrition data for every meal. Misleadingly known as “flexible dieting,” this is considered by many to be the most successful and best approach to dieting.

2. True flexible dieting: Involves a basic understanding of human nutrition and of your own nutrient requirements. The goal is to meet daily nutrient requirements by eating whatever foods you have access to. Measuring and tracking food intake are optional.

Each of these approaches has its benefits. If your goal involves decreasing or increasing body weight, the first approach is the tried and true method. Energy balance is like an equation, and you can’t solve an equation without using numbers.

However, if you have trouble sticking to the first approach, it won’t get you anywhere. The second approach is superior not only in its mental health benefits but also in its potential for success with dietary adherence.

The purpose of this article is not to say that a strict approach to dieting is bad. Rather, such an approach is just not flexible, and I don’t think we should pretend that it is.

“Flexible dieting” can describe an approach that maximizes the opportunity for mental health to thrive (e.g. #2 above) or, alternately, that justifies disordered eating habits (e.g. #1 above).

For those of us who tend to be too restrictive or strict with our diets, flexible dieting can encourage us to eat foods that we might not otherwise eat or enable us to enjoy social gatherings where food is involved that we might not otherwise consume. For others of us, being flexible can be an excuse to fit too many nutrient-poor foods in our diet.

As always, moderation is key, and the best approach for you may not be the best approach for the next person.

  1. Katz DL, Meller S. Can we say what diet is best for health? Annu Rev Public Health. 2014;35:83-103.
  2. Johnston BC, Kanters S, Bandayrel K, Wu P, Naji F, Siemieniuk RA, Ball GD, Busse JW, Thorlund K, Guyatt G, Jansen JP, Mills EJ. Comparison of weight loss among named diet programs in overweight and obese adults: a meta-analysis. JAMA. 2014 Sep;312(9):923-33.
  3. Yancy WS Jr, Mayer SB, Coffman CJ, Smith VA, Kolotkin RL, Geiselman PJ, McVay MA, Oddone EZ, Voils CI. Effect of Allowing Choice of Diet on Weight Loss: A Randomized Trial. Ann Intern Med. 2015 Jun;162(12):805-14.
  4. Westenhoefer  J, Stunkard A, Pudel V. Validation of the flexible and rigid control dimesnsions of dietary restraint. Int J Eat Dis. 1999 Jul;26(1):53-64.
  5. Stewart TM, Williamson DA, White MA. Rigid vs. flexible dieting: association with eating disorder symptoms in nonobese women. Appetite. 2002 Feb;38(1):39-44.