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Can you Lose Weight with IIFYM?

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There are so many different types of diets and eating plans that it is difficult to know which one will work best for you. Many people feel that when they begin a diet, the place to start is by clearing out all of the poor quality food choices and focus on eating so called ‘clean’ foods like chicken, broccoli and rice.

But not all approaches follow this mentality. In this article we’ll take a look at how the IIFYM method differs from other approaches, and whether or not it can help you with weight loss.

We’ll cover:

  • What is IIFYM?
  • The science – does it help with weight loss?
  • Other considerations
  • Summary

What is IIFYM?

As a biological organism we require energy to complete our day to day tasks – we get this energy from three macronutrients, or ‘macros‘ – carbohydrates, fat and protein.

‘If it fits your macros’ (IIFYM) refers to a flexible dieting approach where the individual follows a diet based solely on macro targets and goals. This is different from so called ‘rigid’ dietary approaches where emphasis is placed on the eating of only ‘clean’ foods to achieve a total calorie requirement, and with minimal or no allowance for cheat foods.

It is a common when wanting to lose weight or burn fat that you eat less calories than you burn off – a process termed ‘negative energy balance‘. It uses principles of thermodynamics to suggest that if we don’t give our body quite enough energy to complete daily tasks, it will dip into the body’s fat stores – thus leading to fat loss. It is a well-evidenced concept with many studies to back it up.

Rather than focusing on an overall calorie count in order to fall into a negative calorie balance, flexible dieting focuses more on hitting specific amounts of macronutrients. 

When done correctly, a more flexible diet approach allows you to balance food choices, but with a certain amount of ‘wiggle room’ and moderation. By following the flexible diet you can eat whatever foods you like, as long as they balance with your daily macro values. For example if you had 200kcal of carbohydrates to eat you could obtain them from either brown rice or from donuts.

Whether you obtain your pre-determined daily carbohydrates from wholesome foods like brown rice, or whether they are from donuts and cakes instead, the calories are the same, therefore the same negative energy balance is created – just without restricting food choices.

The key thing is that the overall macro values do not change, nor does the total daily calorie intake. 


Key Point: Rather than focusing on an overall calorie count, flexible dieting focuses more on hitting specific amounts of macronutrients to create a negative energy balance.

The science – can IIFYM help with weight loss?

The differences between rigid and flexible eating patterns have been investigated widely in the last 20 years. Here is a breakdown of the research you need to know:

Firstly it is worth noting that a lot of this research is about not only losing weight but being able to build positive relationships with food that turn into long-term behavior. ‘Restraint theory’ predicts that intentional efforts to lose weight through rigid restraint are very cognitively and emotionally taxing and will likely fail eventually [1].

Meule et al [2] studied the effects of both rigid and flexible dieting by issuing an online survey to 616 volunteers. They found that those who adopted a more flexible diet were more successful in their diets. It also suggested that they suffered less food cravings than the rigid groups.

These results were echoed by Smith et al [3] in their study into dieting and behavior outcome. They recruited 223 men and women – half of which were overweight, and calculated the effects of dietary restraint on a number of behaviors.

They found that in flexible dieters, there was a higher absence of overeating, lower body mass and lower levels of depression and anxiety. Additionally they also reported that flexible eaters were less likely to be concerned with calorie counting and obsessive thoughts about food.

Dietary restraint through rigid eating patterns has been found to increase the chances of eating disorders, mood disturbances and concern with body shape [4]. On top of this, those that followed a flexible eating diet had a lower body mass index. 

There is definitely evidence here to suggest that by allowing some adjustments to the diet, positive behaviors can be formed that will increase diet success long term. This provides an interesting rationale for the use of IIFYM as a weight loss tool.


Key Point:Research suggests that an IIFYM approach can help with weight loss, whilst at the same time reducing food cravings and negative emotions associated with dietary restrictions.

Are there any other considerations?

Whilst evidence shows that free-choice dieting can be effective for weight loss, it doesn’t not take into account the wider health implications of choosing poor quality foods – it is purely based on short-term body composition changes.

It is important that a flexible diet isn’t seen as a green light to make poor nutritional choices throughout day – it is essential that you still obtain all of the necessary vitamins and minerals too. This means aiming to gain the majority of your daily calorie needs from good-quality choices, and allowing some small flexibility with less nutritious items in your diet as necessary – not the other way around.

From a logical point of view it is much harder to monitor and count macros when food choices are varied – so called clean foods typically have fewer ingredients. By adopting a rigid diet, individuals may find it easier to monitor and record food choices. It is also much harder to make measurement errors with a smaller, more rigid diet. 

Summary – can flexible dieting help with weight loss?

If it fits your macros (IIFYM) is a flexible approach to weight loss dieting where an individual has the freedom to eat whatever they want as long as it doesn’t go over their daily total calories, or pre-determined macro numbers. When done correctly, this approach allows you to balance food choices, but with a certain amount of adjustment and moderation.

Evidence suggests that by following a flexible diet, you can lose weight, whilst at the same time reducing your chances of eating disorders associated with rigid eating routines. You can also reduce food cravings, and modify long-term behavior as well.

However, it is important when following a flexible diet that the majority of foods come from healthy, nutritious foods, aiming to gain the majority of your daily calorie needs from good-quality choices. This approach should then allow some small flexibility with less nutritious items in your diet as necessary – not the other way around.

Bottom Line: If you can manage to find a balance during flexible eating diets you can increase your chances of not only losing weight, but modifying positive behavior that will help you keep it off too.


  1. Stotland, S et al. Restraint, moderation and the stages of weight self-regulation: Implications for CBT for obesity. Canadian Journal of Diabetes. 201135(2): 154
  2. Meule, A et al. Food cravings mediate the relationship between rigid, but not flexible control of eating behavior and dieting success. Appetite. 2011; 57(3): 582-584
  3. Smith, CF et al. Flexible vs. Rigid dieting strategies: relationship with adverse behavioral outcomes. Appetite. 1999; 32(3): 295-305
  4. Stewart, TM et al. Rigid vs. flexible dieting: association with eating disorder symptoms in nonobese women. Appetite. 2002; 38: 39-44