The Skinny on IIFYM



IIFYM or If It Fits Your Macros is a way of eating, referred to as flexible dieting or following the 80/20 rule. It’s fairly self-explanatory – if it fits your macros you can eat it. But in this article we’ll remove your rose-tinted glasses when it comes to IIFYM by debunking some myths with real nutrition science, to prove that IIFYM isn’t all it’s cracked up to be.


So first of all, what is IIFYM?


IIFYM is essentially based on the concept that you can control your body weight by controlling your caloric intake: if you want to lose weight, you should eat in a caloric deficit – meaning you are eating less than you are expending. If you want to gain weight, you eat in a calorie surplus, meaning you eat more than you are taking in.

Once you have decided what your goals are (gain or lose weight), you then do some simple calculations (or let an IIFYM calculator do it for you) to work out your specific macronutrient targets.

According to IIFYM, as long as you are hitting those targets, it doesn’t really matter what you are eating because your calories are within the right range for your goals. While this may have started off positively, it quickly became bro-science.


The flexible dieting community wanted their diets to be just that – flexible. This is a change from the typical dieting message of ‘go hard or go home,’ living off plain chicken breast and steamed broccoli, we get it.


That is the positive side of IIFYM. You can reach your fitness goals without having to be so stringent with your nutrition, and in fact, it is a mindset that has been proven to have better adherence, better mental health, and better satisfaction, as explored in a 1999 study on flexible vs. rigid dieting.


But as with everything, people took it too far. The 80/20 flexible dieting rule dictates that as long as 80% of your diet is whole, nutrient-dense, non-processed foods, then 20% of your diet can be whatever you like.


In reality, if your caloric target is 2,000 calories per day, only 400 are yours to play with, which equates to that Starbucks coffee with syrup and creamer that you had this morning. Can you see the potential issue?


But IIFYM doesn’t work – here’s why


You just have to do a quick ‘#IIFYM’ search on Instagram to find millions of posts that include carefully weighed and measured pancake stacks covered in chocolate bars and syrup, fried chicken with zero-calorie sodas and protein bar after protein bar.


Let me break it down for you – these foods aren’t healthy. They may fit your macros but they are not healthy and should not be consumed.


According to the IIFYM community, it’s all about calories in versus calories out. But 100 calories of veggies is not the same as 100 calories of candy. Here are some reasons why.


Fructose and glucose are the two main simple sugars in your diet and gram for gram they provide the same number of calories, but the way they are metabolized in the body is worlds apart.


Firstly, glucose is metabolized by all of your body’s tissues whereas fructose can only be metabolized by the liver. When you consume too much fructose, your liver can’t keep up which leads to a fat accumulation in the liver, due to both increased lipogenesis and impaired fat oxidation, as explored in a study by Jensen et al.


Too much fructose consumption over time causes non-alcoholic fatty liver disease (NAFLD), which is becoming more common in individuals of a normal weight due to the mass consumption of sodas.


Ghrelin, the hunger hormone, tells your brain when to eat and when to stop eating. Fructose increases ghrelin levels meaning it stimulates hunger despite satiety, which can not be stimulated by glucose. Glucose produces increases in circulating satiety hormones, making you feel full and to stop eating, whereas fructose does not.

A study published in the Journal of Clinical Investigation gave one group a fructose-sweetened beverage and another group a glucose sweetened beverage and the same caloric intake for 10 weeks and unsurprisingly, the fructose group gained fat and decreased insulin sensitivity, whereas the glucose group did not.


Within the same fructose group, different food items can also be metabolized differently. For example, a banana is not metabolized the same as a soda, the fiber coating of a banana means it is broken down very slowly by the body and does not spike insulin levels like a soda. Whereas a soda goes straight into the bloodstream, spikes insulin levels and does not stimulate satiety hormones.


This brings us on to the next point: each food within the same macronutrient group has a different glycemic index. White bread is just as much a carbohydrate like sweet potato, but they have very different glycemic indexes.


The glycemic index is the scale of how much a food spikes your insulin levels. Frequent insulin spikes has been linked in research to weight gain, highly fluctuating energy levels, heart disease, decreased human growth hormone (HGH) production, chronic hormonal disruption and more.


The glycemic index of a food item also impacts consumption. A study by Ludwig et al gave a group of teenage boys high-GI versus low-GI foods found that the high-GI group ate on average 81% more. This is because high-GI foods are more palatable, have less fiber and do not release the ‘full’ signal via leptin and ghrelin.


Finally, if you’re living off an IIFYM diet of protein bars, protein shakes and junk food you’re not consuming many if any, nutrients. Nutrients are not just something your mom tells you to eat, they actually play a huge role biochemically.


Nutrients help to raise your productivity levels by 20%, they improve gut health, help to regulate the hormones associated with feeding,  help to regulate body weight, help you to gain and retain muscle mass and so much more.


This is just touching on the surface of the science that completely discounts the validity of IIFYM. A calorie is not just a calorie and calories in versus calories out is not the full picture of weight management. If you want an excuse to not fix your diet, then, by all means, follow IIFYM, but if you want to take your health and fitness to the next level then you need to forget this weak trend.


Read more articles by John Smith

Join the discussion

Do not miss any update from us

What did we write about?

Scroll to Top