Dieting is the practice of ingesting food in a regulated fashion to achieve or maintain a controlled weight. In most cases dieting is used in combination with physical exercise to lose weight in those who are overweight or obese. Some athletes however aspire to gain weight (usually in the form of muscle). Diets can also be used to maintain a stable body weight.
Diets to promote weight loss are generally divided into four categories: low-fat, low-carbohydrate, low-calorie, and very low calorie. A meta-analysis of six randomized controlled trials found no difference between the main diet types (low calorie, low carbohydrate, and low fat), with a 2–4 kilogram weight loss in all studies. At two years all calorie-reduced diet types cause equal weight loss irrespective of the macronutrients emphasized.
Types of diets
Low-fat diets
Main article: Low-fat dietLow-fat diets involve the reduction of the percentage of fat in one's diet. Calorie consumption is reduced but not purposely so. Diets of this type include NCEP Step I and II. A meta-analysis of 16 trials of 2–12 months' duration found that low-fat diets resulted in weight loss of 3.2 kg (7.1 lb) over eating as normal.
Low-carbohydrate diets
Main article: Low-carbohydrate dietLow carbohydrate diets such as Atkins and Protein Power are relatively high in fat and protein. They are very popular in the press but are not recommended by the American Heart Association. A review of 107 studies did not find that low-carbohydrate diets cause weight loss, except when calorie intake was restricted. No adverse effects from low carbohydrate diets were detected. Low carbohydrate diets do not support exercise very well, due to the fact that carbohydrates are the main source of energy during anaerobic exercise, such as sprinting or resistance training, and supply 30% or more during aerobic exercise.
Low-calorie diets
Main article: Calorie restrictionLow-calorie diets usually produce an energy deficit of 500–1,000 calories per day, which can result in a 0.5 kilogram (1.1 lb) weight loss per week. They include the DASH diet and Weight Watchers among others. The National Institutes of Health reviewed 34 randomized controlled trials to determine the effectiveness of low-calorie diets. They found that these diets lowered total body mass by 8% over 3–12 months.
Very low-calorie diets
Main article: Very low calorie dietVery low calorie diets provide 200–800 kcal/day, maintaining protein intake but limiting calories from both fat and carbohydrates. They subject the body to starvation and produce an average weekly weight loss of 1.5–2.5 kilograms (3.3–5.5 lb). These diets are not recommended for general use as they are associated with adverse side effects such as loss of lean muscle mass, increased risks of gout, and electrolyte imbalances. People attempting these diets must be monitored closely by a physician to prevent complications.
Fat loss versus muscle loss
Weight loss typically involves the loss of fat, water and muscle. Overweight people, or people suffering from obesity, typically aim to reduce the percentage of body fat. Additionally, as muscle tissue is denser than fat, fat loss results in increased loss of body volume compared with muscle loss. Reducing even 10% body fat can therefore have a dramatic effect on a person's body shape. To determine the proportion of weight loss that is due to decreased fat tissue, 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. Those on low-carbohydrate diets, and those doing particularly strenuous exercise, may wish to increase their 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."
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, it has only been shown that these diets are dangerous in individuals who already have kidney and liver problems.
Energy obtained from food
The energy intake 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, while efficiency of utilization is affected by individual factors, including body weight and hormones.
The effects of chewing, especially in elderly people, have been shown to affect the intake of micronutrients. However, there was no significant effect on the intake of macronutrients, such as sugars, fats, and proteins.
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 construction, especially for the construction of muscle cells. 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. In any event, a multivitamin taken once a day will suffice for the majority of the population.
Weight-loss diets which manipulate the proportion of macronutrients (low-fat, low-carbohydrate, etc.) have not been found to be more effective than diets which maintain a typical mix of foods with smaller portions and perhaps some substitutions (e.g. low-fat milk, or less salad dressing). Extreme diets may, in some cases, lead to malnutrition.
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, where 65% of it is stored in skeletal muscles and the rest in the liver (totaling about 2000 kcal in the whole body). It is created from the excess of ingested 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).
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.
Cognitive Behavior Therapy has been effective in producing long term weight loss . Judith S. Beck has been one of the most prominent practitioners and writers to bring this method to a popular audience.
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.
The use of drugs to control appetite is also common. Stimulants are often taken as a means to suppress hunger in 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 gr
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