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>IIFYM is probably not a bad diet for a teenage boy or steroid user trying to pack on weight while doing a high volume of work. Everyone else will put on a lot of unwanted fat.

That would depend on both your total calorie count and your macro breakdown. IIFYM determines what percentage of your calories should come from each of carbs, fats and proteins. So if your calorie count is suitable for your height, weight and activity level, then your macro breakdown isn't going to cause you to gain weight. If your breakdown is out of whack, then you're going to end up feeling tired, hungry or otherwise malnourished, but it shouldn't cause you excessive weight gain outside of water retention. The calorie counting needs to be more accurate than the macro breakdown.

Why are they trying to gain weight? They can't convert donut into muscle. Donuts for a sugar feed while working out, sure.

There are many body functions besides muscle growth and fat burning. These functions require vitamins.