At the most basic level, weight loss occurs when your body uses more calories than you consume. Your body burns calories at rest, just to keep your various physiological systems functioning correctly. You burn calories through activity and using your muscles. You also burn calories by digesting and metabolizing the food you eat. Even thinking, which requires cellular communication between the neurons in your brain, burns calories. You take calories in, of course, through food. Therefore, the process of losing weight almost always begins with reducing the number of calories you eat and increasing the number of calories you burn, thus creating a "caloric deficit." The most readily variable method of burning calories is physical activity, so exercise goes hand-in-hand with healthy nutrition in losing weight.
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When your body faces a caloric deficit, it must turn to stored sources of energy to meet its caloric needs. Most of the body's excess calories are stored as fat, and the goal of most people in losing weight is to lose fat. As the body needs more energy than it takes in through food, it turns to these reserves (in addition to glycogen/sugar and sometimes protein reserves), and fat stores begin to deplete. As a rule of thumb, the body must have a caloric deficit of about 3,500 calories to lose one pound of stored fat. This translates to one pound fat loss per week if your daily caloric deficit through decreased intake and increased output is 500 calories.
Fat is stored in many individual cells, called adipocytes. The body usually maintains the same number of adipocytes, regardless of the amount of fat stored in each one. When weight gain occurs very quickly, however, the body may create more adipocytes to accommodate the drastically increased need for storage space. As weight loss occurs, the amount of fat in each adipocyte decreases, but the body usually doesn't destroy adipocytes once they have been created.
Effects of Exercise
Exercise (or more generally, physical activity) is usually the easiest way to increase the body's demand for calories. A workout itself burns a significant number of calories. An aerobic workout such as walking or jogging usually burns more overall calories than a strength (resistance) workout, but both are important for various reasons. Combining them, as in many forms of circuit training, can be a very effective way to get the best of both worlds.
There is evidence that exercise causes an "after-burn" effect (known as excess post-exercise oxygen consumption), which continues to keep the body's caloric demands slightly elevated after the workout is over. This is due to the tissue repair that occurs when the muscles are recovering from exercise. It's much more pronounced following a resistance training workout than an aerobic workout. It's also more pronounced in deconditioned people. Once you become more fit, your body needs less additional energy to recover.
Exercise, especially resistance training, also increases the amount of muscle tissue in the body. Muscle tissue is very metabolically active; this means it takes a lot of energy (calories) for the body to keep the tissue alive, healthy and functioning correctly. Therefore, if you increase your muscle mass through exercise, your body burns a few extra calories, even at rest. Be careful, however, not to take exercise as a license to eat more. Remember, you are creating a caloric deficit. Furthermore, losing weight can actually reduce your metabolic rate because your body doesn't have to work as hard to move around and function correctly when it carries less weight.
The many other health benefits of exercise also help with weight loss. For instance, less stress, better immunity, better overall organ function, better sleep and more resistance to injury all help avoid the typical obstacles to an effective weight-loss program.
Weight loss is as much a mental process as physical. American culture provides plenty of opportunities to consume excess calories, and plenty of excuses and distractions that prevent us from burning them off. Re-training your mind to think differently about your consumption of food and your physical activity level is crucial in making changes that result in long-term weight loss. Optimal weight loss occurs slowly through lifestyle changes that can be sustained throughout a person's lifetime, rather than a quick diet or a plan to exercise every day for two months. The process of losing weight requires patience and a psychological commitment to permanent change, in addition to a healthy diet and an active lifestyle.