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To supply the calories our bodies need for energy, we must eat food. As food passes through our digestive systems, it is mechanically and chemically broken down into nutrients (amino acids, simple sugars, fatty acids, and monoglycerides) that our bodies use for growth, maintenance, and repair. When these simplified nutrients -- especially the simple sugars and fats -- reach the cells, they are metabolized as fuel. This releases heat, which is measured in calories. With calories, the body works on a supply-and-demand system. If the daily calorie supply from food you've eaten meets the daily demand, all the calories from fats, proteins, and carbohydrates are converted to energy. If the daily supply exceeds the demand, the excess calories are stored in fat cells. These fat cells serve as energy warehouses for fat molecules, allowing your body to draw upon the stored fat when your demand for calories exceeds the daily supply from the food you consume. Despite the recent negative publicity about fatty foods, we all need some fat in our diets. It's a good source of the calories we require to fuel our bodies and to keep us going when our energy demands suddenly increase. One gram of fat provides nine calories of energy, while one gram of protein or carbohydrate offers only four. Fat also gives texture and flavor to foods. It helps us feel full and satisfied after we eat. In addition, it protects our organs, aids in the development of cell membranes and hormones, and insulates our bodies. However, we should eat fat sparingly -- it should make up no more than 30 percent of our daily caloric intake. Excess fat in our bodies is linked to health problems such as hardening of the arteries and heart disease, to name just two. Throughout a day, a month, a year, and even a lifetime, the body's supply of and demand for fat changes. Demand increases with activity and decreases in sedentary times. We can modify our caloric intake to meet these changing conditions. We can also manage the body's fat supply through exercise, which uses fat as fuel. So, to maintain a healthy balance in our bodies, we need to monitor our food intake and energy demands and add a daily dose of exercise.