The body's most basic need is for energy. To get energy it needs food as a fuel and oxygen to burn it. The amount of energy foods can produce is measured in units called calories. A food calorie, or kilocalorie, is the amount of heat required to raise the temperature of 1 kilogram (2.2 pounds) of water 1 degree Celsius (1.8 degrees Fahrenheit).
The body changes the calories in food into energy, which is necessary for every act from blinking an eye to running a race. Energy is also used for the growing process, for rebuilding damaged cells, and for regulating body systems.
The number of calories needed each day depends upon how much energy an individual's body uses. An active child can need more calories than an adult who works at a desk. The body needs more calories in cold weather to stay at an even temperature.
If a person takes in more food than required to meet the body's needs, the excess calories are converted to fat--a stored form of energy. That causes weight gain. Eating too little causes weight loss because the fat is used for energy. One pound (0.5 kilogram) of stored fat contains about 3,500 calories.
If weight loss is advisable, the best way to lose is by eating less of high-calorie foods and getting more exercise. For most people a safe limit for losing weight is 2 pounds a week. Strict dieting to lose weight should be attempted only under a physician's care. So-called quick and easy weight-loss diets are fads. Most are unbalanced, emphasizing one type of food and excluding others. The more unbalanced the diet, the more dangerous it is. Knowledge of the body's nutritional needs helps to guard against promoters of fad diets and special "health" foods that have no advantages and that may even be harmful.
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