This is accomplished by metabolism and blood circulation, by shivering to stay warm, and by sweating to stay cool.In addition to thermoregulation, humans expend energy keeping the vital organs (especially the lungs, heart and brain) functioning. Except when sleeping, our skeletal muscles are working, typically to maintain upright posture. The average work done just to stay alive is the basal metabolic rate, which (for humans) is about 1 watt per kilogram of body mass (0.45 W/lb). Thus, an average man of 75 kilograms (165 lb) who just rests (or only walks a few steps) burns about 75 watts (continuously).

Physical exercise:Physical exercise is an important complement to dieting in securing weight loss. Aerobic exercise is also an important part of maintaining normal good health, especially the muscular strength of the heart. To be useful, aerobic exercise requires maintaining a target heart rate of 50 percent above one's resting heart rate for 30 minutes, at least 3 times a week.citation needed Brisk walking can accomplish this. For example, if the resting heart rate is 70 bpm (beats per minute), the target heart rate would be 105 bpm. Always consult a physician before beginning any exercise program, and ask what the appropriate target heart rate for your physical condition and age should be.The ability of a few hours a week of exercise to contribute to weight loss can be somewhat overestimated.

Those 10 kilograms (22 lb) converted to work are equivalent to about 350 megajoules (84,000 kilocalories). (We use an approximation of the standard 37 kilojoules or 9 kilocalories per gram of fat. Anyway, since the weight loss is also due to water loss, the actual loss will be slightly greater.) Now assume that his chosen exercise is stairclimbing and that he is 20 percent efficient at converting chemical energy into mechanical work (this is within measured ranges). To lose the weight, he must ascend 70 kilometers. A man of normal fitness (like him) will be tired after 500 meters of climbing (about 150 flights of stairs), so he needs to exercise every day for 140 days (to reach his target). However, exercise (both aerobic and anaerobic) would increase the Basal Metabolic Rate (BMR) for some time after the workout. This ensures more calorific loss than otherwise estimated.The minimum safe dietary energy intake (without medical supervision) is 75 percent of that needed to maintain basal metabolism.

citation needed For our hypothetical 100-kilogram man, that minimum is about 5,700 kilojoules (1,300 kilocalories) per day. By combining daily aerobic exercise with a weight-loss diet, he would be able to lose 10 kilograms in half the time (70 days).There are also some easy ways for people to exercise, such as walking rather than driving, climbing stairs instead of taking elevators, doing more housework with fewer power tools, or parking their cars farther and walking to school or the office. Fat loss versus muscle loss:Weight loss typically involves the loss of fat, water and muscle. A dieter can lose weight without losing much fat. Ideally, overweight people should seek to lose fat and preserve muscle, since muscle burns more calories than fat. Generally, the more muscle mass one has, the higher one's metabolism is, resulting in more calories being burned. Approximately 14 kilocalories of fat are burned per pound of muscle at rest.

Dieting

Dieting is the practice of ingesting food in a regulated fashion to achieve or maintain a controlled weight. In most cases the goal is weight loss, but some athletes aspire to gain weight (usually in the form of muscle) and diets can also be used to maintain a stable body weight. Types of dieting:There are several kinds of diets:Weight-loss diets restricts the intake of specific foods, or food in general, to reduce body weight. What works to reduce body weight for one person will not necessarily work for another, due to metabolic differences and lifestyle factors. Also, for a variety of reasons, most people find it difficult to maintain significant weight loss over time — among individuals that have lost 10% or more of body weight, only 20% are able to maintain that weight loss for a full year. Many professional athletes impose weight-gain diets on themselves. American football players may try to "bulk up" through weight-gain diets in order to gain an advantage on the field with a higher mass.

Individuals who are underweight, such as those recovering from anorexia nervosa or starvation, may adopt weight-gain diets which, unlike those of athletes, have the goal of restoring normal levels of body fat, muscle, and stores of essential nutrients. Many people in the acting industry may choose to lose or gain weight depending on the role they're given.As more cultures scrutinize their diets, many parents consider putting their children on restricted diets that actually do more harm than good. This is extremely deleterious to a young child's health because a full and balanced diet (fats, carbohydrates, protein, vitamins, minerals, fiber, etc.) is needed for growth. A doctor should be consulted before putting any child on a specialized diet.Research also shows that putting children on diet foods can be harmful. The brain is unable to learn how to correlate taste with nutritional value, which is why such children may consistently overeat later in life despite adequate nutritional intake.

In children and young adults: This section does not cite any references or sources. (October 2007)Please help improve this section by adding citations to reliable sources. Unverifiable material may be challenged and removed. Receiving adequate nutrition through a well-balanced diet is critical during childhood and adolescence. Unless a doctor says otherwise, low-carb, low-fat, or other specialty diets for children who are not heavily obese are unhealthy because they deprive the body of the building blocks of cells (namely energy and lipids in the above examples).Children who diet could actually be doing worse things for themselves and their bodies. Alison Field from Harvard Medical School stated that "Our study found that dieting was counterproductive- children who dieted gained more, not less, weight than non-dieters." Scott,R.,Jennifer "Dieting Kids May Gain Weight" 12,July,2006 Children who diet tend to develop habits of overeating, or binge eating as a result of attempting to casually diet.

It is suggested that children should just attempt to eat healthy instead of trying to casually diet. Thermoregulation:According to the principles of thermoregulation, humans are endotherms. We expend energy to maintain our blood temperature at body temperature, which is about 37 °C (98.6 °F).