What Is The “If It Fits Your Macros” Diet—And Should You Try It?

#IIFYM (or If It Fits Your Macros) is the most buzzed-about eating plan on social media, blowing up as a “non-diet” diet that helps physique competitors and bodybuilders reach their goals by tracking their macros (aka macronutrients) intake. With no food off the table as long as you hit the right final numbers, proponents tout IIFYM as a science-backed advance in flexible eating, while critics call it the “Pop-Tart diet.”

In the end, what you get out of “If It Fits Your Macros”—from healthy weight loss, better energy, and bigger muscles to nutritional deficiencies and food obsession—depends on how you choose to play it.

All About Macros

IIFYM really hones in on the ‘macros’ you consume. These macronutrients—carbohydrates, protein, and fat—contain all of the calories that you consume and burn for energy, and are the nutrients the body needs in the greatest amounts to function properly, explains Jessica Swift, M.S., R.D.

Each macronutrient has its own benefits and roles in the body. Carbohydrates serve as your main form of energy and power your muscles during high-intensity exercise, explains Kamal Patel, M.P.H., director of Examine.com. Protein triggers the release of powerful satiety hormones within the gut, helping you feel full, and breaks down into the amino acids that form your muscles. Fat slows the release of carbohydrates into the bloodstream and aids in the production of hormones, including muscle-building testosterone. Carbs and protein contain four calories per gram while fat contains nine.

While all three macronutrients are necessary, the average American diet contains far more carbs than are needed for energy, says Patel. This can cause blood sugar and insulin spikes, and lead to weight gain over time. Meanwhile, fat intake is often one of two extremes. Too much calorie-dense fat can send your total daily calorie intake through the roof, Patel says. But ‘low-fat’ food products often pack more sugar and calories than their full-fat counterparts, leading you to consume more than you realize. All of these factors can result in caloric surpluses and weight gained, not lost. The premise of IIFYM is to find the right balance of these macronutrients for your body and goals.

How IIFYM Works

To start IIFYM, you’ll first calculate the number of calories you burn per day, which is also how the number of calories you would need to eat per day to maintain your current weight. The interwebs are chock-full of calculators ready to determine your total energy expenditure—IIFYM.com even has its own calculator.

The IIFYM calculator also determines how many of those calories should come from each macronutrient, based on your goals—and how aggressively you want to pursue them. If weight loss is your end-game, IIFYM—like all other diets—recommends a caloric deficit, explains Patel. That means consuming fewer calories than you expend each day. IIFYM suggests a caloric deficit of 15 to 20 percent to lose fat fast—without also losing lean muscle.

On the flip side, if you’re trying to build muscle, IIFYM suggests a caloric surplus (consuming more calories than burned) of five to 10 percent. It is possible to build muscle while burning fat, but caloric deficits do reduce muscles’ tendency to effectively absorb and use amino acids to grow.

Related: 5 Myths About Your Metabolism—Busted

For example, let’s consider a 30-year-old, 180-pound, 5’10” man, working a desk job and exercising for an hour five days per week. According to IIFYM’s calculator, to get down to 160 pounds, he would need to consume 2,108 calories, 153 grams of protein, 68 grams of fat, and 221 grams of carbs each day. To gain 20 pounds of muscle, however, he would need to consume 2,728 calories, 177 grams of protein, 67 grams of fat, and 348 grams of carbs each day.

IIFYM first calculates optimal protein and fat intake, and then bases carb intake on whatever calories are left. While recommended fat intake, according to IIFYM, is pretty stable from person to person, recommended protein intake increases for those trying to build muscle.

However, many people who follow the flexible-dieting approach like to calculate their carbs and protein intake and leave fat as the “leftover” macro instead, says Patel. When that’s the case, carb intake is largely contingent on how much you exercise, and at what intensity. The more and harder you exercise, the more carbs you need to fuel those workouts.

Your calories—and which macros you get them from—undoubtedly affect your weight and body composition, but the IIFYM site explains that these numbers are estimates, not absolutes.

Keep in mind that most nutritionists recommend getting 40 percent of your daily calories from carbs, 30 percent from protein, and 30 from fat for overall health. Sure, those percentages may vary based on activity levels, current metabolic health, and goals, but it’s a pretty good starting point for most people.

Weighing the Good and the Bad

IIFYM says that you can fill your plate with whatever you want, as long as your total daily food intake adds up to the right amount of protein, carbs, fat, and total calories.

On the positive side, IIFYM’s flexibility definitely makes it easier to stick to than many of the stricter diets and elimination strategies out there. Plus, a JAMA meta-analysis shows that, since pretty much all diets lead to similar weight-loss results when you follow them to a T, the most effective diet for weight loss is simply the one that you can stick with.

Just don’t get too excited about the idea of filling up on junk as a great weight-loss strategy. “You can create a caloric deficit and lose weight while still eating in a way that negatively impacts your cholesterol and other health markers,” Swift says. “For example, you could eat pizza, soda, and potato chips as long as it fits your macro goals. This way of eating can be reckless because it doesn’t account for lean versus fatty proteins, good fats versus bad fats, or calorie sources.”

Related: How To Eat Carbs And Still Lose Weight

But, according to Patel, IIFYM can also help dieters become more aware of their eating habits—which can help them clean it up. Since you have to track every calorie and gram of carb, protein, and fat, IIFYM can help you become a pro food label reader. “IIFYM can often be a gateway to researching and becoming more aware of good nutrition in general,” Patel says. Still, while it’s important to learn about nutrition and to be informed about the foods you put in your body, at a certain point, all of the macro- and calorie-counting may create an obsessive and unhealthy relationship with food, he says.

It’s important to remember that we don’t eat macros in isolation. They come packaged alongside vitamins and minerals (a.k.a. micronutrients) and, if you’re eating processed foods, alongside manmade chemicals, Swift says. If you follow IIFYM, you still want to choose whole foods over highly processed ones, whole-grain carbs over refined ones, and healthy fats over trans fats, she says. After all, whether you are vying for fat loss, muscle gain, or all of the above, your health still matters.

Related: Find pantry staples and seasonings to help you whip up healthy meals.