Nutrition and food science: energy balance

Nutrition in general is a concern for adolescents, who are entering a stressful, confusing, and sometimes frightening time of social, emotional, and physical development. Healthy diet and regular physical activity help children and adults feel better, learn and work more effectively, and avoid developing a variety of risk factors for disease. The key to weight control or weight management is keeping energy intake (food) and energy output (physical activity) in balance; that is energy balance.
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When you consume only as many calories as your body needs, your weight will usually remain constant. If you take in more calories than your body needs, you will put on excess fat. If you expend more energy than you take in you will burn excess fat. The relationship of energy balance to body weight can be summarized by the following equations:

Energy Intake = Energy Output = Weight Maintenance
Energy Intake > Energy Output = Weight Gain
Energy Intake < Energy Output = Weight Loss
Weight management means keeping your body weight at a healthy level. Regular exercise and a healthy diet are a must when it comes to controlling your weight. A weight management plan depends on whether you are overweight or underweight. Many people mistakenly believe that they only “burn calories” when they exercise. In fact, your body is burning calories all of the time (yes, even when sleeping!). Calories are used to keep
basic body functions going, to metabolize the foods you eat, and to do any form of physical activity. Exactly how many calories people need varies, depending on such factors as gender, current body size, activity level and body weight goals a wise choice to achieve a healthy weight. A safe, tried-and-true method for long-term weight loss is to reduce calories by decreasing portion sizes when people tend to eat. When trying to lose weight or hold steady at a desired one, there’s no need to turn to the latest “diet” or outcast your favorite foods. Small changes to your diet and exercise routine can make a big difference.
A healthful eating plan can include all your favorite foods if they are in reasonable amounts and balanced out with daily physical activity. Aerobic physical activity, if no health prohibitions, will assist in increasing muscle tissue and also in burning calories. However, care should be taken not to exercise more frequently and more intensely that is required for good health or to compete well.
Physical activity should be balanced with diet to maintain a desired weight. Experts have come to believe that this approach of weight management is reasonable and promising. No proven side effects, however, success of weight efforts should be evaluated according to improvements in chronic disease risk factors or symptoms and by the adoption of healthy lifestyle habits, not just by the number of pounds lost/gain.
But if you are over 40, have been inactive for some time, suffer from shortness of breath or weakness that interferes with daily activities, or suffer from a chronic condition, you should consult a physician before you begin any effort to reduce your weight or increase your activity level. Education may be necessary for an understanding of energy balance and basic nutrition principles.
REFERENCE
Atkins, R. (1981). Dr.Atkins: Nutrition breakthrough. New York, U.S.A: Bantom Books.