There's a fear amongst the hardcore fitness Culture that comes around once, maybe twice a year maybe more scarcely when dieting "cutting", whatever you want to call it.
That fear is getting small when in a fat Loss Phase, Yu often hear people who are quite lean go on about how they've lost muscle this time around dieting, OR, those about to diet constantly fretting about their quads suddenly atrophying and shriveling away the minute they eat below maintenance.
Unfortunately (or should I say, fortunately)... It is not that simple.
You CAN lose muscle whilst in a calorie deficit from a severe catabolic state (in extreme scenarios) leading to a process known as gluconeogenesis. This is the synthesis of glucose (carbs) from nonglucose based substrates such as lactate, pyruvate & Amino acids (where our concern lies as this is essentially using muscle for fuel).
It occurs in the kidneys & a small bit in the liver but it is highly endergonic (it takes a lot of energy out of us to do so).
But if you have been dieting for 12 weeks and you certainly feel like you have lost some muscle? , you look and feel smaller even though leaner. Unless your in a severe carb deprived state with low protein and extreme exercise, it’s unlikely you're losing muscle.
The harsh reality is you didn’t have the muscle built up beneath fat stores, to begin with, which re-enforces the timely process which is hypertrophy and how long/hard it does take to add significant amounts of it (unless you're completely new to resistance training of course)
Your body will start to use triglycerides (Fat) and other fuel sources as much as possible before looking to convert amino acids (muscle) into carbohydrates for fuel.
It is possible, just not easy People Can quickly look to it for an excuse from not dedicating enough time to building some quality muscle or having it there in the fist place.