Can I lose fat and gain muscle at the same time?
Yes, especially if you are newer to lifting, returning after time off, carrying more body fat, or improving training and protein intake from a low baseline. This is usually called body recomposition: losing fat while gaining or preserving lean mass. It is less likely if you are already lean, well-trained, and dieting aggressively.
Recomposition requires two things at once: a fat-loss environment and a muscle-building signal. That usually means maintenance calories or a modest calorie deficit, high protein, consistent resistance training, and enough recovery to adapt. In one trial, higher protein combined with strenuous training during an energy deficit produced better lean-mass outcomes than lower protein.
Low calories do not make muscle gain impossible, but they make it more challenging. Muscle growth requires a training stimulus, amino acids, and enough energy to support muscle protein synthesis, repair, and adaptation. The larger the deficit, the more the body is biased toward conserving energy rather than building new tissue. If your main goal is muscle gain, do not run an aggressive fat-loss diet. If your main goal is fat loss, accept that muscle gain may be slower and focus on preserving strength, technique, and training quality.
The practical plan is not exotic: train progressively, keep protein high, avoid reckless calorie cuts, sleep enough, and judge progress with more than scale weight. If the scale is stable but waist size is down, strength is up, and photos improve, recomposition may be happening.
If your primary goal is maximum muscle gain or maximum fat loss, a dedicated phase may eventually work better. Recomposition is real, but it is usually slower than focusing one major direction at a time.