Flexible Dieting (IIFYM): What It Is, How to Calculate Your Macros and What the Science Says

Flexible dieting is not a diet with a fixed menu but a way of organising one: instead of a list of forbidden foods, you work with a daily budget of calories and macronutrients (protein, fat and carbohydrate) that you spread however you like. If a food fits that budget, you can eat it. The acronym stands for «If It Fits Your Macros». If macronutrients are new to you, start with what macronutrients are.

What the studies say: flexible versus rigid

Research has not studied «IIFYM» as a label so much as two ways of controlling what you eat: flexible control (making up for an excess by eating a bit less afterwards, without all-or-nothing rules) and rigid control (bans, skipping meals, «all or nothing»). This is what the main studies found:

How to read this table. First: almost all are cross-sectional questionnaire studies, which show association rather than cause; the only randomised trial (Conlin, 2021) had 23 participants and found no difference in weight loss. Second: «flexible control» on a questionnaire is not exactly the same as counting macros; Smith (1999) found that calorie counting was associated with overeating alone, so the number on its own protects you from nothing. Third: in people with binge eating disorder, the distinction did not hold up (Masheb and Grilo, 2002). The honest conclusion: all-or-nothing rigidity is associated with more binges and a worse relationship with food, and flexibility loses the same amount of weight when the calories are the same. As the ISSN summarises (Aragon, 2017), fat loss is driven by a sustained calorie deficit, very different approaches work similarly, and long-term success depends on whether you can stick with the diet.

How to calculate your macros, step by step

To do flexible dieting you need three numbers (protein, fat and carbohydrate) that add up to your target calories. You can do it directly with the macro calculator or by hand like this:

• Calories: start from your daily expenditure (you can estimate it with the TDEE calculator). To lose fat, subtract; the Conlin trial used a deficit of about 20%.
• Protein first: if you do strength training, the ISSN considers 1.4-2.0 g per kg of body weight a day sufficient (Jäger, 2017), and the Morton (2018) meta-analysis saw no further muscle gain above about 1.6 g/kg on average. Without training, EFSA’s reference for adults is 0.83 g/kg. Each gram of protein is 4 kcal.
• Fat: EFSA sets a reference intake range of 20 to 35% of calories. Each gram is 9 kcal.
• Carbohydrate: the remaining calories, divided by 4. EFSA puts its reference intake range at 45-60% of calories.
• Fibre: it is not one of the IIFYM macros, but give it a floor. EFSA considers 25 g a day adequate for adults.

Example: a 75 kg person who trains and wants to lose fat on 2,000 kcal a day. Protein: 130 g (1.73 g/kg), which is 520 kcal. Fat: 65 g, 585 kcal, 29% of the total. Carbohydrate: (2,000 − 520 − 585) / 4 = 224 g, rounded to 225 g, 45%. All three numbers fall within the EFSA and ISSN ranges. If you only want to fine-tune protein, there is the protein calculator, and the full explanation is in how to calculate your macros.

A flexible dieting day that adds up (2,000 kcal, 130 g of protein)

This is a real day for the macros in the example, ice cream included, calculated food by food with USDA FoodData Central (USDA carbohydrates include fibre):

• Breakfast: 50 g of rolled oats cooked in 250 ml of semi-skimmed (2%) milk with 100 g of banana.
• Lunch: 170 g of chicken breast (raw weight), grilled, with 150 g of cooked brown rice, 200 g of broccoli and 10 g of olive oil.
• Snack: 250 g of plain nonfat strained Greek yogurt with 15 g of walnuts and a 150 g apple.
• Dinner: omelette of 2 eggs and 100 g of egg whites with 100 g of spinach, 150 g of baked potato with skin, a salad of 150 g of tomato with 5 g of oil, and 30 g of whole-wheat bread.
• Treat: 100 g of vanilla ice cream (207 kcal, 10% of the day).

The day lands 11 kcal from the target and provides about 30 g of fibre, above EFSA’s 25 g. The ice cream fits because the rest of the day is built on protein, vegetables, fruit and whole grains. That is the so-called 80/20 rule: most of the calories (80-90%) come from minimally processed foods and the rest is free. It is a practical rule, not a figure from a study, but it sums up what separates flexible dieting from «eating anything as long as the number adds up».

The limits of flexible dieting

• Fibre is not among the three macros. You can hit your protein, fat and carbohydrate with ultra-processed food and fall well short on fibre. In the Reynolds review (Lancet, 2019), the people who ate the most fibre had 15 to 30% lower mortality and incidence of coronary heart disease, stroke, type 2 diabetes and colorectal cancer than those who ate the least, and the largest risk reduction was seen at 25-29 g a day.
• Neither are micronutrients. Two days with identical macros can be very different in vitamins and minerals: the base of fruit, vegetables, legumes and whole grains is what covers them.
• Counting is not exact measurement. Labels, eyeballed portions and any estimate have a margin; flexible dieting works with trends over several days, not with the exact gram.
• Counting may not be for you. If you have or have had an eating disorder, or counting makes you anxious, talk to a healthcare professional before tracking what you eat.

Flexible dieting relies on knowing what you are eating without turning it into an exam. With Renzy you take a photo of the plate and get an estimate of calories and macros that you can correct, and the barcode scanner adds packaged foods; that way you can see at a glance whether today’s treat fits your macros.