The IIFYM Diet - Success or Failure?
Tim Klein
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Diets Premium Medical Expert • 22.03.2026
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Scientifically reviewed by a clinical dietitian:
This article has been peer-reviewed for adherence to IŻŻ, WHO, and PTGiP clinical nutritional standards.
| CLINICAL KNOWLEDGE MAGAZINE Dietetics | min read 6 min |
| Diets Premium Medical Expert Tim Klein | Scientifically reviewed by a clinical dietitian 22.03.2026 |
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Key Takeaways
The IIFYM diet is becoming increasingly popular. What it encompasses and what it is? In this article, we will delineate its fundamental rules and potential outcomes that can be attained through it. Judge for yourselves.
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Diet IIFYM – What is its fundamental principle?
IIFYM is an acronym for 'if it fits your macros'. This diet is also known as flexible dieting. As the name suggests, the main objective is to meet the body's demand for the three main nutrients – protein, fat, and carbohydrates. This method of eating allows for a great deal of freedom as it does not necessitate providing adequate amounts of vitamins and minerals. It suffices to provide an appropriate, pre-calculated level of energy during the day that matches the three mentioned nutrients. Individuals who follow this diet are required to count calories, yet there are no restrictions regarding the sources from which they should be obtained. The IIFYM diet also does not specify the number of meals per day. By utilizing various applications that aid in calculating the nutrients, the diet can be controlled according to the guidelines established at the outset. It is beneficial to commence with calculating the appropriate calorie intake. The simplest method is to use the Harris-Benedict formula – calculate the basal metabolic rate (BMR), followed by multiplying the obtained result by the coefficient of physical activity (PAL). The basal metabolic rate is the number of calories essential for maintaining the basic life functions of the body.
For women who desire to become acquainted with their energy necessities:
PPM = 655.1 + (9.563 × body mass [kg]) + (1,85 × stature [cm]) – (4.676 × [age]).
For men who desire to investigate their personal energy requirements:
PPM = 66, 5 + (13, 75 × body mass [kg]) + (5, 003 × growth [cm]) – (6, 775 × [age]) The physical activity coefficient can range from 1, 4 (for low activity) to 2, 4 (for peak performance athletes). To avoid overestimation, a person who does not engage in high-intensity sports, but exercises for half an hour a day, should calculate a PAL of approximately 1, 6. The next step is to calculate the protein requirement for a healthy individual, which is about 0, 8–1 g/kg body weight. 1 g protein delivers 4 kcal. It is also important to consider that fats in a balanced diet provide about 30% of the energy. 1 g fat delivers 9 kcal. Therefore, 30% of the total energy requirement must be calculated and divided by 9 – this will give the amount of fat (in grams) that should be considered in the diet. (e.g. 30% of 2000 kcal is 600 kcal, divided by 9 gives 67 g fat). The same procedure should be applied to carbohydrates. Carbohydrates meet the remaining energy needs and typically make up 50-60% of the daily energy requirement. 1 g carbohydrates delivers 4 kcal.
Obtained Results
This notion is relatively new and there are no scientific studies to determine its long-term consequences on the body and its influence on body weight. In theory, if someone consumes approximately 500 kcal less than their total caloric requirement daily for at least a few weeks, the calorie deficit would assist in reducing body weight. However, everything would be significantly more straightforward if there were no other factors that affect metabolism.
Complex carbohydrates as the main component of diet - facts
The primary energy source in the diet should be carbohydrates. The concept of glycemic index (GI) is certainly familiar to those who take care of proper nutrition. The lower the GI of a product, the slower the blood sugar level increases after consumption. Unexpected spikes in blood sugar levels, caused by the intake of simple sugars (such as sucrose, or white sugar, or fructose), lead to a more significant release of insulin, which inhibits fat burning and promotes its depletion. If we satisfy our carbohydrate needs with highly processed products (which typically have a high GI), we expose our body to constant fluctuations in blood sugar levels (the faster it increases, the faster it decreases), leading to increased appetite. Not all calories are equal.
Appropriate calcium intake can decrease the risk of obesity – proven
Research indicates that the lower the intake of calcium, the higher the likelihood of obesity. This applies not only to adults but also to children. Moreover, increasing calcium consumption by around 400 mg each day can further accelerate fat reduction in individuals who restrict calories in their diet. However, calcium is not the only factor, as the consumption of dairy products can also decrease the risk of being overweight. This may be caused by both vitamin D and the content of branched amino acids, medium-chain fatty acids, and CLAs. Conclusion? The content of mineral components and other physiologically active compounds may play a significant role in the process of weight reduction.
During weight loss, calorie deficit is more important than diet quality – a myth we need to expose
In this article, we will challenge this myth and conduct a critical review of IIFYM's assumptions. In a study published in the Journal of the American Medical Association, it was proven that without counting calories, by adhering to high-quality dietary rules, you can lose weight. The study was done on a large scale, involving more than 600 overweight and obese adults. The participants were divided into two groups, one on a low-fat diet and the other on a low-carbohydrate diet for 12 months. After this period, the average weight loss in both groups was similar, with the first group losing 5.3 kg and the second group losing 6 kg. There were no calorie-related restrictions during the study, and participants were instructed on which foods to eat and which to limit. Both diets were varied and focused on nutritious foods. It may seem that losing a few kilograms after 12 months is insignificant, but a gradual weight loss is safe and usually sustainable, which is the main goal of weight loss.
Is the IIFYM diet typically deficient in vitamins and minerals – unlikely?
There is such a risk, but there is a lack of studies that have been able to provide a definitive answer to this question. One of them, published earlier this year in the International Journal of Sport Nutrition and Exercise Metabolism, suggests that IIFYM does not necessarily lead to a low intake of nutrients. A comparison of the nutritional value of diets of bodybuilders using IIFYM and a strict diet revealed no significant differences between the two groups in nutrient intake of vitamins and minerals, fiber, saturated fatty acids, and sugar. Paradoxically, women using IIFYM were found to have a higher intake of vitamins K, C, and E. However, the diet of most of the participants did not meet the recommended amounts of individual minerals.
Tim Klein
Diets Premium Medical Expert
Member of the Diets Premium clinical editorial board. Specializes in medical diet therapy, metabolic health, and nutritional planning.
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