Your body burns calories every day just carrying out your day-to-day activities -- from breathing to keeping your heart beating to staying active -- and the balance between the calories you burn and the calories you eat controls your weight. Eat too little, and your body will start burning fat as an alternative energy source, which is why people reduce their calorie intake when they want to lose weight. Eat too many calories, though, and your body will store the excess. In most cases, excess calories get stored as fat, but, if you follow a healthy diet and exercise program, some of that energy will get used to make muscle.
Most Excess Calories Stored as Subcutaneous Fat
When you start eating more calories than you need, you'll likely notice a difference in how your clothes fit and how you look. That's because the majority of the fat in your body -- about 90 percent -- is in the form of subcutaneous fat, which is located right under your skin. As you gain weight, your subcutaneous fat cells swell in size as they start storing more and more fat molecules, so you'll notice visible fatty deposits developing under your skin. Where you see the most subcutaneous fat accumulate depends on your genetics. In addition, women tend to carry the excess weight in their hips and thighs, while men tend to carry weight in their midsections. While excess subcutaneous fat affects how you look, it's not the most harmful type of fat.
Some Excess Calories Stored as Visceral Fat
Excess calories also can get stored as visceral fat -- a type of fat that makes up about 10 percent of the total fat level in most people, according to Harvard Medical School. Unlike subcutaneous fat, which sits under the skin, visceral fat lies within your abdominal cavity, surrounding your internal organs. It pushes your abdominal muscles outward and creates a hard, protruding belly that feels different from "squishy" subcutaneous fat.
Since it's deep in your abdomen, visceral fat has more access to your body's blood supply, and it secretes hormones called cytokines that contribute to inflammation and cardiovascular disease. High visceral fat levels are also linked to other illnesses, including dementia and colorectal cancer. When you start to lose weight, you'll begin by primarily burning visceral fat, then start burning subcutaneous fat for energy.
Extra Calories Help Build Muscle, Too
If you're following a healthy lifestyle aimed at "bulking up," your body will use at least some of your extra calories to build muscle. When you're bulking, a relatively large proportion of your calories come from protein, which is a source of amino acids. During digestion, your body absorbs the amino acids and sends them to your muscle tissue, where you'll use them to build new muscle fibers. That's why many bodybuilders eat large amounts of protein; they want to ensure they're getting enough amino acids to support muscle growth. However, your body won't build new muscle tissue for no reason; you need to challenge your muscles with strength-training workouts to trigger new muscle growth.
Staying Healthy on Excess Calories
When you're eating a calorie surplus, you'll typically want to optimize your diet and exercise program so that a significant part of that extra energy gets used to make muscle, not just add body fat. To do that, plan for slow weight loss that gives your body time to build new muscle -- a half-pound to one pound per week. Ensure that you're getting high-quality lean proteins, such as soy, beans and lentils, fish, lean meat and eggs. And pair your diet with a progressive strength-training program that challenges your muscles just a bit more with each workout, so you're constantly triggering muscle growth through exercise.
If you're trying to gain mass but worry you're gaining too much fat, talk to a professional; he or she can help troubleshoot your program and tailor it for your unique needs, so you can achieve your health goals.