Probably one of the most amazing but misunderstood concepts circulating among bodybuilders and fitness enthusiasts in general, is IIFYM.
What started as a phrase from a user named Erik Stevens on bodybuilding.com, quickly caught on and proceeded to cause an uproar of internet stupidity everywhere.
IIFYM stands for "If It Fits Your Macros" and it was originally phrased on the bb.com forums as a fast way to respond to the overwhelming number of questions about whether or not someone could eat a particular food item without having to worry about getting fat, or not gaining muscle, or (insert other negative effect).
Here are some examples of the questions that would come up: "Hey can I eat fruit on a cut?" "Hey is it okay for me to have oatmeal?" "Hey I had a cookie but I still stayed at my calorie and macro goals, is that okay or will that hurt my progress?" (The answer being "Yes, you can eat it if it fits your macros
IIFYM literally means to hit your calorie and macronutrient targets by end of day choosing foods that you enjoy eating. The concept is completely bastardized because idiots across the internet continually come up with scenarios that don't exist in real life, to try and blow a hole in the idea that IIFYM is a sound practice.
Here are some examples of the strawmen arguments that show up in an attempt to discredit IIFYM: "You're telling me you can just eat straight table sugar for your carbs, and drink olive oil for your fat, and use whey protein and you'll have a good physique?". "Hey have fun eating pizza and donuts all day". "Brb just eating cake, IIFYM".
Good luck with that. (You'll note that you typically can't hit your macros eating chips and donuts all day and if you CAN, your macros are probably horsesh*t to begin with and you've then got bigger problems. The point here is that IIFYM most certainly isn't a disregard for health or nutrient sufficiency, but people will often create and knock down that strawman). What IIFYM is not:
1) It is not eating cake and chips all day.
2) It is not disregarding micronutrients and fiber and general intelligence with regards to food choice.
3) It is not a specific macro setting. There is a website out there that has the IIFYM label that includes a calorie calculation tool and unfortunately several people on MFP believe that doing "IIFYM" means eating those specific macros. This is false. IIFYM is a philosophy about food selection with the belief that body composition changes are primarily a function of nutrient intake and energy balance rather than a function of individual food sources.
When practicing IIFYM, it is recommended that you choose mostly whole and nutrient dense foods to comprise the majority of your intake. Fresh vegetables, fruits, meats, fish, etc, and at the same time, leaving some room for a discretionary intake. A common and very reasonable recommendation would be about 80/20. That is to say, that if you've got a calorie target of 2500, you'd eat approximately 2000 calories of whole and nutrient dense foods with a calorie bank of 500 to eat whatever you would like while still hitting your calorie and macronutrient targets by end of day
It's a flexible approach, and it works.
Layne Norton on IIFYM:
See here for an example of how it could be done: http://www.myfitnesspal.com/topics/show/925464-fitting-it-in-giggity