Eating this way gives your body what it needs to burn fat, maintain your metabolism, and fuel your muscles, which are exactly what are needed for massing and getting lean. If you are eating a lot of calories, but a lot of junk food, and not enough protein, your body cannot build muscle tissue, and it ends up with fat.
Plus, Perry points out, you need to eat enough protein and calories when trying to build muscle in order to promote muscle growth. If you are eating lots of protein, but not enough calories in total, you are going to have trouble being able to work out to build more muscle. If you are adding lots of cardio, which burns calories, and restricting too much what you are eating, your body is going to seek out energy from your muscles. If you are at a caloric deficit, then your body may breakdown protein in your muscles to give you energy.
You are going to be eating a lot more calories (and protein) on weight-training days, so your body is using these calories and nutrients to power muscle repair, which then leads to muscle growth. Taking in maintenance calories on a cardio day will make sure that you are at a small enough deficit to encourage fat loss, but not so big that your body starts using muscle tissue for fuel. Foods that supply protein, carbohydrates, and fat are important, as is getting a sufficient number of calories during the day. Making sure to have a good source of lean protein available all day is crucial if you are looking to increase your muscle stores.
While the recommended daily protein intake is under a half-gram per pound of bodyweight, you need to double down on this and go up to one-gram per pound to gain muscle. You will have to see each individual client individually for their recommended caloric intake for a period of muscle-building, but typically, adding between 20 and 30 grams of added protein daily is a good guideline. If you are used to eating meals that have little protein and lots of carbohydrates — say, a large pasta dish with just a minuscule amount of meat sauce — and you want to build lean muscle, you need to change up those proportions. By spreading out calories across, say, six meals, spread about three hours apart, you avoid that full-belly feeling that makes you feel slow, and you ensure your muscles are constantly being fed protein and carbohydrates.
You will feel fuller for longer, as the protein keeps you full, and you will prevent muscle break down that happens when calories are cut. Your body is smarter than you might think, and keeping an eye on your diet (specifically, when to eat what) and training, you absolutely can lose fat and build muscle at the same time. The truth is, every kind of change in body composition, such as losing fat or gaining muscle, is just as, if not more, dependent on your diet as your usual training regimen. Regardless of whether carbohydrates or fat are your energy of choice, what is more important is determining your clients overall daily caloric needs, and keeping your dietary intake somewhat lower (with protein still abundant) to ensure the body stays in negative energy balance, with the least amount of muscle catabolism possible.