is the practice of ingesting food in a regulated fashion to achieve
or maintain a controlled weight. 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
several kinds of diets:
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.
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
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
in the acting industry may choose to lose or gain weight depending
on the role they're given.
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
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. 
children and young adults
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).
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.
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.
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 calories) per day or 1 calorie each minute.
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.
of a few hours a week of exercise to contribute to weight loss
can be somewhat overestimated. To illustrate, consider a 100-kilogram
(220 lb) man who wants to lose 10 kilograms (22 lb) and assume
that he eats just enough to maintain his weight (at rest), so
that weight loss can only come from exercise. Those 10 kilograms
(22 lb) converted to work are equivalent to about 350 megajoules
(84,000 calories). (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
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.
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.
loss versus muscle 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 calories 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.
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."
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.
obtained from food
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.
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.
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.
Academy of Sciences and the World Health Organization publish
guidelines for dietary intakes of all known essential nutrients.
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.
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.
the body gets rid of fat
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, which is a complex carbohydrate
stored in the liver, created from the excess which is ingested.
When that source is 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.
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.
aspects of weight-loss dieting
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
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 that
experienced by soldiers and mountain climbers, can make eating
a difficult chore.)
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.
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
customs and practices differ widely. Some groups are modelled
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.
leverage the power of group meetings to provide counseling,
emotional support, problem-solving, and useful information.
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.
medications can be prescribed to assist in weight loss. Some,
like amphetamines, are dangerous and are now banned for casual
weight loss. Some supplements, including those containing vitamins
and minerals, may not be effective for losing weight.
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.
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.
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 lack blood glucose,
the preferred energy source of the brain, causing the body to
metabolize sugars from protein, which over a prolonged fast
can lead to muscle wasting.
Dietetic Association. 2003. Position paper on vegetarian
diets. J Am Diet Assoc. 103:748-765.
M.L., Gleason, J. L., Griffith, J.L., et al., "One Year
Effectiveness of the Atkins, Ornish, Weight Watchers, and
Zone Diets in Decreasing Body Weight and Heart Disease Risk",
Presented at the American Heart Association Scientific Sessions
November 12, 2003 in Orlando, Florida.)
B. and Melina, V. 2000. Becoming Vegan. pg. 22.
- Wansink, B. Mindless
Eating: Why We Eat More Than We Think, New York:
Bantam Dell (2006).