There are several kinds of diets:

• Weight-loss diets restrict 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.

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). This is accomplished by metabolism and blood circulation, by shivering to stay warm, and by sweating to stay cool.^[3]

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), or about 6,500 kilojoules (1,440 kilocalories) per day or 1 kilocalorie each minute.

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. The exact figure is 14 kilocalories burned per pound of muscle at rest. Since muscles are more dense than fat, muscle loss results in little loss of physical bulk compared with fat loss. To determine whether weight loss is due to fat, various methods of measuring body fat percentage have been developed.

Muscle loss during weight loss can be restricted by regularly lifting weights (or doing push-ups and other strength-oriented calisthenics) and by maintaining sufficient protein intake. According to the National Academy of Sciences, the Dietary Reference Intake for protein is "0.8 grams per kilogram of body weight for adults."

Those on low-carbohydrate diets, and those doing particularly strenuous exercise, may wish to increase their protein intake which is necessary. However, there may be risks involved. According to the American Heart Association, excessive protein intake may cause liver and kidney problems and may be a risk factor for heart disease.^[4] There is no conclusive evidence that moderately high protein diets in healthy individuals are dangerous, however; it has only been shown that these diets are dangerous in individuals who already have kidney and liver problems.

Proper nutrition

Food provides nutrients from six broad classes: proteins, fats, carbohydrates, vitamins, dietary minerals, and water. Carbohydrates are metabolized to provide energy. Proteins provide amino acids, which are required for cell, especially muscle, construction. Essential fatty acids are required for brain and cell membrane construction. Vitamins and trace minerals help maintain proper electrolyte balance and are required for many metabolic processes. Dietary fiber is another food component which influences health even though it is not actually absorbed into the body.

Any diet that fails to meet minimum nutritional requirements can threaten general health (and physical fitness in particular). If a person is not well enough to be active, weight loss and good quality of life will be unlikely.

The National Academy of Sciences and the World Health Organization publish guidelines for dietary intakes of all known essential nutrients.

Sometimes dieters will ingest excessive amounts of vitamin and mineral supplements. While this is usually harmless, some nutrients are dangerous. Men (and women who don't menstruate) need to be wary of iron poisoning. Retinol (oil-soluble vitamin A) is toxic in large doses. As a general rule, most people can get the nutrition they need from foods (there are specific exceptions; vegans often need to supplement vitamin B12). In any event, a multivitamin taken once a day will suffice for the majority of the population.

A sensible weight-loss diet is a normal balanced diet; it just comes with smaller portions and perhaps some substitutions (e.g. low-fat milk, or less salad dressing). Extreme diets may lead to malnutrition, and are less likely to be effective at long-term weight loss in any event.

How the body gets rid of fat

All body processes require energy to run properly. When the body is expending more energy than it is taking in (e.g. when exercising), the body's cells rely on internally stored energy sources, like complex carbohydrates and fats, for energy. The first source the body turns to is glycogen (by glycogenolysis). Glycogen is a complex carbohydrate (in total about 2000 kcal). 65% is stored in skeletal muscles and the rest in the liver. It is created from the excess of injested macronutrients, mainly carbohydrates. When those sources are nearly depleted, the body begins lipolysis, the mobilization and catabolism of fat stores for energy. In this process, fats, obtained from adipose tissue, or fat cells, are broken down into glycerol and fatty acids, which can be used to make energy. The primary by-products of metabolism are carbon dioxide and water; carbon dioxide is expelled through the respiratory system.

Fats are also secreted by the sebaceous glands (in the skin). When losing weight one must be careful as to not begin to burn muscle. When the body runs of out of fats and carbohydrates to burn, it will begin to burn muscle which will be harmful for the body.

Dangers

Extreme calorie restriction, medication or unusual patterns of eating (i.e. restricting food consumption to a single fruit or meal) can be dangerous. This can indicate Anorexia Nervosa and/or Bulimia which are common eating disorders and can even be fatal.