Registered dietitian, exercise physiologist, and NASM-certified personal trainer Gabbi Berkow, MA, CDN, told POPSUGAR that gaining fat on your body results from eating more calories than you burn. So, anything eaten in excess whether that's protein, carbs, or fat, can be stored as fat if the calories you eat are greater than the calories you burn. Rondel King, MS, CSCS, exercise physiologist at the NYU Langone Sports Performance Center, further explained: "Let's say, for instance, you're eating an excess of 1,000 calories than your resting metabolic rate is calling for. That needs to go somewhere. What the body does as a defense mechanism, it stores those excess calories as fat."
Gaining muscle is a lot harder and requires a lot more work, Gabbi noted. "You have to progressively overload your muscles by lifting weights and continually challenging yourself over time," she said. "After every workout your muscles build more fibers so that they're better equipped to handle the stress of the next workout." But, you eventually get used to the load, which is why you have to keep increasing the intensity.
It's also about what you eat. Rondel explained that protein is necessary because it's essentially the building blocks to develop more muscle mass so you can become stronger and withstand the stresses that you've placed on your body. Since muscle is made of protein, Gabbi said, you have to consume plenty of protein, around one gram per pound of bodyweight every day, for your body to be able to build muscle. (Eating more protein will also boost your metabolism.)
Since building more muscle fibers uses up a lot of calories, you'll build the most muscle if you eat more calories than you burn, Gabbi said, so you can gain fat while building muscle. Building muscle without gaining fat, what she called a "clean bulk" where you put on muscle without eating more than your burn, is possible, but it requires "strict attention to calories and protein, and you won't put on as much muscle as you would if you ate more calories than you burned." Additionally, she said you can't significantly bulk up while you're focused on losing weight (aka, fat) due to the amount of calories required to build muscle. She discusses why muscle is important for weight loss, though, ahead.