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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.
In the broadest sense, at least some targeted dieting has clearly existed since prehistoric times for various social, religious, and biological reasons.
See Luigi Cornaro for a 16th century treatise on dieting. Throughout the 17th and 18th centuries, physicians and patients regulated their food carefully, in order to prevent disease. In the 19th century, as the scientific classification of foods took shape, doctors and scientists began experimenting with targeted diets.
William Banting is one of the first people known to have successfully lost weight by developing a targeted diet, circa 1863, by targeting carbohydrates. The low carbohydrate diet, sometimes marketed today as the Atkins Diet, remains popular today.
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 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 long-term failure rate of 98%.
- 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 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.
In Children and 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 (namely energy and lipids in the above examples).
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 versus 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.
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), 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).
Energy obtained 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.
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.
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.
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.
In a meta-analysis of 11 randomized controlled trials that compared low fat versus low carbohydrate diets, low fat diets achieved greater reduction in low density lipoprotein but less weight loss and less increase in high density lipoprotein. A more recent cohort study comparing low fat versus low carbohydrate diets for mortality concluded "overall low-carbohydrate-diet and low-fat-diet scores were not associated with total mortality. Unhealthy low-carbohydrate-diet and low-fat-diet scores were associated with higher total mortality, whereas healthy low-carbohydrate-diet and low-fat-diet scores were associated with lower total mortality".
Psychological Aspects of Weight-Loss Dieting
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.
Weight Loss Groups
There exist both profit-oriented and non-profit weight loss organizations who assist people in their weight loss efforts. An example of the former is Weight Watchers ; examples of the latter include Overeaters Anonymous, as well as a multitude of non-branded support groups run by local churches, hospitals, and like-minded individuals.
These organizations' customs and practices differ widely. Some groups are modeled on twelve-step programs, while others are quite informal. Some groups advocate certain prepared foods or special menus, while others train dieters to make healthy choices from restaurant menus and while grocery-shopping and cooking.
Most groups leverage the power of group meetings to provide counseling, emotional support, problem-solving, and useful information.
Popular Weight-Loss Diets
Popular diets (sometimes pejoratively called "fad diets") usually derive their popularity from the personalities of their proponents. These proponents include "diet gurus" and celebrity converts. "Diet books" are the primary means of communicating the specifics of popular diets.
Most popular diets experience short-lived popularity, partly because new diet books are continuously being published.
Judging the effectiveness (and nutritional merit) of popular diets can be especially difficult. Diet proponents often locate medical professionals to back up their work. Some diets are so controversial that they divide the medical community.
Many popular diets advocate the combination of a specific technique (such as eliminating a certain food, or eating only certain combinations of foods) with reduced caloric intake, with the goal being to accelerate weight loss. Others ignore traditional science altogether.
Low-fat diets were popular during the 1980s and 1990s, encouraging people to eat foods low in fat (or without fat altogether) and instead eat foods high in carbohydrates. The general public came to believe, partly due to information from low-fat diet proponents, that carbohydrates were "energy food" and that only fat made people fat. This led to high consumption of low-fat foods high in refined carbohydrates (notably corn syrup), which may have contributed towards increased weight gain as carbohydrates (particularly refined carbohydrates) have a low nutrient density and high in calories. Some low-fat diets like the Pritikin diet focus on whole grains, vegetables and lean meats.
The Atkins diet was developed by Dr. Robert Atkins' and intended to control blood sugar by reducing the number of carbohydrates consumed (particularly refined carbohydrates) while replacing them with significant quantities of fat and protein. The Atkins diet was originally designed for diabetes patients who wanted to manage their insulin levels more effectively. The short-term changes experienced by individuals on the Atkins diet include some rapid weight-loss as the body's glycogen stores were depleted, reducing fasting levels of triglycerides and an increasing blood-bound ketones. The diet also causes acidosis and mild fatigue.
Since the advent of controversial diets such as Atkins, various diets that stress the eating habits of "natural humans" have been developed. The Paleolithic Diet imitates the way people ate during the Stone Age. These eating plans include basically natural foods (those not processed by humans). Whereas the Paleolithic Diet excludes milk and grain-foods, The Evolution Diet excludes human-made ingredients such as partially hydrogenated oils but allows some processed foods such as whole-grain crackers and dairy products. Anthropologists who focus their research on human evolution, however, are quick to point out that the diet of Paleolithic peoples was most likely opportunistic. That is, these early humans would most likely eat whatever edible foods were available at any given moment in that particular area (e.g. vegetables, termites, meat) and not restrict their intake of any food. Until recent human history, starvation has been a far greater threat than over-consumption.
There is a growing body of evidence that vegetarian diets can prevent obesity and lower disease risks.
According to the American Dietetic Association, "Vegetarians have been reported to have lower body mass indices than nonvegetarians, as well as lower rates of death from ischemic heart disease; vegetarians also show lower blood cholesterol levels; lower blood pressure; and lower rates of hypertension, type 2 diabetes, and prostate and colon cancer."
Vegetarians on average weigh 10 percent less than non-vegetarians. And in a year-long study comparing Dean Ornish's vegetarian diet to Weight Watchers, The Zone Diet, and The Atkins Diet, subjects on The Atkins Diet achieved the most weight loss (on average). Strict vegetarian diets like veganism may result in certain vitamin and mineral deficiencies if attention isn't paid to nutrition.
Weight Watchers has two programs. The program offers a wide variety and foods. Each food has a point value. They encourage a well rounded diet, low in fat and high in fruits and vegetables. The core plan focuses more on portion control and natural foods. According to Weight Watchers, the act of keeping track of what one eats is very helpful in reducing overeating or eating for reasons other than hunger.
Dangers of Dieting
Extreme calorie restriction, medication or unusual patterns of eating (i.e. restricting food consumption to a single fruit or meal) can be dangerous.
ACC/AHA/ESC 2006 Guidelines for Management of Patients With Ventricular Arrhythmias and the Prevention of Sudden Cardiac Death - Obesity, Dieting, and Anorexia (DO NOT EDIT) 
|"1. Life-threatening ventricular arrhythmias in patients with obesity, anorexia, or when dieting should be treated in the same manner that such arrhythmias are treated in patients with other diseases, including ICD and pacemaker implantation as required. Patients receiving ICD implantation should be receiving chronic optimal medical therapy and have reasonable expectation of survival with a good functional status for more than 1 y. (Level of Evidence: C) "|
|"1. Prolonged, unbalanced, very low calorie, semistarvation diets are not recommended; they may be harmful and provoke life-threatening ventricular arrhythmias. (Level of Evidence: C)"|
|"1. Programmed weight reduction in obesity and carefully controlled re-feeding in anorexia can effectively reduce the risk of ventricular arrhythmias and SCD. (Level of Evidence: C)"|
Certain medications can be prescribed to assist in weight loss. Some, like amphetamines, are dangerous now banned for casual weight loss. Others, including those containing vitamins and minerals, are not effective for losing weight.
Diuretics induce weight loss through the excretion of water. These medication or herbs will reduce the amount that a body weighs, but will have no effect on an individual's body fat. Diuretics can thicken the blood, cause cramping, kidney and liver damage.
Stimulants such as ephedrine (now illegal in the United States due to an FDA ban) or synephrine work to increase the basal metabolic rate and reduce appetite. Stimulants can cause kidney and liver damage, sudden heart attacks, addiction, and ischemic strokes.[dubious ]
In June 2006, the European Union approved the sale of the diet drug rimonabant, marketed under the trade name Acomplia. This new class of diet pills shows some promise in assisting physician-prescribed diets.
Dangers of Fasting
Lengthy fasting can be dangerous due to the risk of malnutrition and should be carried out under medical supervision. During fasting, low-carbohydrate or very low calorie diets a lack of blood glucose, the preferred energy source of the brain, causes the body to metabolize sugars from protein, which can lead to muscle wasting.
Dieting, especially extreme food-intake reduction and rapid weight loss, can have the following side effects:
- Prolonged hunger
- Reduced sex drive
- Sinus problems (especially post-nasal drip)
- Muscle atrophy
- Bloodshot eyes
- Gallbladder disease
- Weight gain on discontinuation
- Malnutrition, possibly leading to death
- http://news.bbc.co.uk/2/hi/health/6933686.stm Diet food 'may fuel obesity risk in young
- http://cluster3.biosci.utexas.edu/courses/herpetology/ryan/thermoreg.html Thermoregulation
- "High-Protein Diets". American Heart Association. Retrieved 2007-05-24.
- Mansoor N, Vinknes KJ, Veierød MB, Retterstøl K (2016). "Effects of low-carbohydrate diets v. low-fat diets on body weight and cardiovascular risk factors: a meta-analysis of randomised controlled trials". Br J Nutr. 115 (3): 466–79. doi:10.1017/S0007114515004699. PMID 26768850.
- Seidelmann SB, Claggett B, Cheng S, Henglin M, Shah A, Steffen LM; et al. (2018). "Dietary carbohydrate intake and mortality: a prospective cohort study and meta-analysis". Lancet Public Health. 3 (9): e419–e428. doi:10.1016/S2468-2667(18)30135-X. PMC 6339822. PMID 30122560.
- Hooper L, Martin N, Abdelhamid A, Davey Smith G (2015). "Reduction in saturated fat intake for cardiovascular disease". Cochrane Database Syst Rev (6): CD011737. doi:10.1002/14651858.CD011737. PMID 26068959.
- Bloomfield HE, Koeller E, Greer N, MacDonald R, Kane R, Wilt TJ (2016). "Effects on Health Outcomes of a Mediterranean Diet With No Restriction on Fat Intake: A Systematic Review and Meta-analysis". Ann Intern Med. 165 (7): 491–500. doi:10.7326/M16-0361. PMID 27428849.
- Zipes DP, Camm AJ, Borggrefe M, Buxton AE, Chaitman B, Fromer M; et al. (2006). "ACC/AHA/ESC 2006 Guidelines for Management of Patients With Ventricular Arrhythmias and the Prevention of Sudden Cardiac Death: a report of the American College of Cardiology/American Heart Association Task Force and the European Society of Cardiology Committee for Practice Guidelines (writing committee to develop Guidelines for Management of Patients With Ventricular Arrhythmias and the Prevention of Sudden Cardiac Death): developed in collaboration with the European Heart Rhythm Association and the Heart Rhythm Society". Circulation. 114 (10): e385–484. doi:10.1161/CIRCULATIONAHA.106.178233. PMID 16935995.