Dieting is the practice of ingesting food in a regulated fashion to achieve a particular objective. In many 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.
There are several kinds of diets:
1.)WEIGHT LOSS 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 very difficult to maintain significant weight loss over time. There is some thought that losing weight quickly may actually make it more difficult to maintain the loss over time. It is also possible that cutting calorie intake too low slows or prevents weight loss. The National Institutes of Health notes that the commonly recommended program of reduced caloric intake along with increased physical activity has a failure rate of 98%.
2.)WEIGHT GAIN DIETS
Many professional athletes impose weight-gain diets on themselves. Sport players may try to "bulk up" through weight-gain diets in order to gain an advantage on the field with a higher mass.
3.)Individuals who are underweight, such as those recovering from anorexia nervosa or from starvation, may undergo weight-gain diets which, unlike those of athletes, has 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.
CHILDREN & YOUNG ADULTS
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 (energy and lipids).
It is especially notable that, as more cultures scrutinize their diets, many improperly educated mothers 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.Vegetarian diets can work for children as long as all needed nutrients are received. 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.
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.
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 (2.2 lbs) of body mass. Thus, an average man of 75 kilograms (165 lbs) who just rests (or only walks a few steps) burns about 75 watts (continuously), or about 6,500 kilojoules (1,440 Calories) per day or 1 Calorie each minute.
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 above 50 percent of one's resting heart rate for 30 minutes, at least 3 times a week. Brisk walking can accomplish this.
The ability of a few hours a week of exercise to contribute to weight loss can be somewhat overestimated. To illustrate, consider a 100-kilogram (220 lbs) man who wants to lose 10 kilograms (22 lbs) and assume that he eats just enough to maintain his weight (at rest), so that weight loss can only come from exercise. Those 10 (22 lbs) kilograms converted to work are equivalent to about 350 megajoules. (We use an approximation of the standard 37 kilojoules or 9 Calories per gram of fat.) 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. For our hypothetical 100-kilogram man, that minimum is about 5,700 kilojoules (1,300 calories) 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). Of course, the described regime is more rigorous than would be desirable or advisable for many persons. Therefore, under an effective but more manageable weight-loss program, losing 10 kilograms (about 20 pounds) may take as long as 6 months.
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 VS MUSCLE LOSS
It is important to understand the difference between weight loss and fat 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, even 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.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.
ENERGY FROM FOOD
The energy humans get from food is limited by the efficiency of digestion and the efficiency of utilization. The efficiency of digestion is largely dependent on the type of food being eaten. Poorly chewed seeds are poorly digested. Refined sugars and fats are absorbed almost completely. Chewing does not compensate for the calorie content of a food that is eaten; even celery, which is primarily indigestible cellulose, contains enough sugars to easily compensate for the cost of chewing it.
Humans require essential nutrients from six broad classes: proteins, fats, vitamin, dietary minerals, and water. Essential amino acids (protein) are required for cell, especially muscle, construction. Essential fatty acids are required for brain and cell membrane construction.
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.
BODY & 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), body cells rely on internally stored energy sources, like complex carbohydrates and fats, for energy. The first source the body turns to is glycogen, which is a complex carbohydrate created by the body. When that source is nearly depleted, the body begins lipolysis, the metabolism of fat for energy. In this process, fats, obtained from 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).
Diets affect the "energy in" component of the energy balance by limiting or altering the distribution of foods. Techniques that affect the appetite can limit energy intake by affecting the desire to overeat.
Consumption of low-energy, fiber-rich foods, such as non-starchy vegetables, is effective in obtaining satiation (the feeling of "fullness").
Exercise is also useful in controlling appetite as is drinking water and sleeping. (Extreme physical fatigue, such as experienced by soldiers and mountain climbers, can make eating a difficult chore.)
The use of drugs to control appetite is also common. Stimulants are often taken as a means to suppress (normal, healthy) hunger by people who are dieting. Ephedrine (through facilitating the release of adrenaline and noradrenaline) stimulates the alpha(1)-adrenoreceptor subtype, which is known to act as an anorectic. L-Phenylalanine, an amino acid found in whey protein powders also has the ability to suppress appetite by increasing the hormone cholecystokinin (CCK) which sends a satiety signal to the brain.