The food you eat contains energy, measured in calories, as well as mass, chemicals and compounds used to build and maintain the cells and tissues of your body. How you grew your mass to its present state, and how you transform that big, fat mass of yours into a lean, energy-burning machine, depends on the energy you take in and the energy you expend.
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Just as a car requires fuel to run, your body uses energy to fuel its daily operations. Many of your calories are spent on operating your skeletal muscles for motor movement, but a great many are also expended on other daily functions such as breathing, respiration, operating your cardiovascular system and cell repair. About 20 percent of your caloric intake is spent running your brain, and about 10 percent spent chewing, swallowing, digesting and eliminating the food you eat.
You get the calories needed to fuel all these bodily operations by eating. When you consume more calories than required for your immediate needs, your body stores the excess calories for future energy use. If you eat fewer calories than you need for daily operations, then you tap energy stores to get fuel. Energy balance refers to the number of calories you consume relative to the number you burn. A negative balance means you burn more than you consume.
The energy in food takes the form of fat, carbohydrates, protein and alcohol. During digestion, your body physically and chemically pulverizes your food, extracting compounds and chemicals it needs. The energy in food is transformed chiefly into a carbohydrate, glucose, and absorbed through your intestinal lining into your bloodstream. Your muscles, organs, tissues and cells throughout your body obtain much of their fuel from blood glucose, but your body can only use and tolerate so much glucose. Excess glucose is transformed into a carbohydrate, glycogen, and packed into temporary stores in muscle tissue and in your liver. As your glycogen stores get filled, your body transforms the additional excess glucose into triglycerides and stores it in fat cells that are distributed throughout your body under your skin and in a fatty sheath of tissue that hangs off your stomach called the omentum. Your genes determine the location of your fat cells, and the number is set by adolescence. Your diet determines how full those fat cells are.
When cells need energy, they generally use glucose in your blood. As your glucose levels get low, your body taps your temporary glycogen stores and also extracts triglycerides from your fat cells. The triglycerides are broken down into fatty acids, which are then distributed through your bloodstream to energy-hungry cells throughout your body. When glucose and glycogen supplies are low, your cells will also use protein from muscle tissues for fuel. These raw fuel sources are taken in by cells, where mitochondria, microscopic power plants, use them to power cell operations.
Tipping the Balance
You have limited control over most of your metabolic activity. The areas over which you have the greatest discretionary control are the number of calories you take in and the number of calories you burn through muscular activity. To lose weight, you must create a negative energy balance. You must eat fewer calories or burn more calories through greater activity. Aerobic exercise, such as walking, running, swimming and jogging, uses your biggest muscles and so burns more calories than other forms of exercise. The longer and more intensely you exercise, the greater the calorie burn. Resistance exercise burns carbohydrates and fat and builds lean muscle, which requires more calories to operate and maintain, even when you’re at rest. Regular aerobic and resistance exercises enhance your fitness, so you burn calories more efficiently. By combining a calorie-reduced diet with moderately intense exercise, you force your body to exhaust temporary carbohydrate stores and tap fat stores.