Obesity in absolute terms is an increase of body fatty tissue mass. In a practical setting it is difficult to measure this directly, and obesity is typically measured by BMI (body mass index) and in terms of its distribution through waist circumference or waist-hip circumference ratio measurements.[5] In addition, the presence of obesity needs to be evaluated in the context of other risk factors and comorbidities (other medical conditions that could influence risk of complications)


Body mass index or BMI is a simple and widely used method for estimating body fat mass. BMI was developed in the 19th century by the Belgian statistician and anthropometrist Adolphe Quetelet. BMI is an accurate reflection of body fat percentage in the majority of the adult population, but is less accurate in situations that affect body composition such as in body builders and pregnancy

BMI is calculated by dividing the subject's weight by the square of his/her height, typically expressed either in metric or US "Customary" units:

Metric: BMI = kg / m2

Where kg is the subject's weight in kilograms and m is the subject's height in metres.

US/Customary and imperial: BMI = lb * 703 / in2

Where lb is the subject's weight in pounds and in is the subject's height in inches

The most commonly used definitions, established by the WHO in 1997 and published in 2000, provide the following values:

  • A BMI less than 18.5 is underweight
  • A BMI of 18.5–24.9 is normal weight
  • A BMI of 25.0–29.9 is overweight
  • A BMI of 30.0–34.9 is class I obesity
  • A BMI of 35.0-39.9 is class II obesity
  • A BMI of > 40.0 is class III obesity or severe / morbidly obese
  • A BMI of 35.0 or higher in the presence of at least one other significant comorbidity is also classified by some bodies as morbid obesity.


Most researchers agree that a combination of excessive calorie consumption and a sedentary lifestyle are the primary causes of obesity in the majority of the population. Other less well established or minor influences include genetic causes, medical and psychiatric illnesses, and microbiological causes.

A 2006 review identifies ten other possibly under investigated causes for recently increasing rates of obesity:

(1) Insufficient sleep

(2) Endocrine disruptors - food substances that interfere with lipid metabolism,

(3) decreased variability in ambient temperature,

(4) decreased rates of smoking, which suppresses appetite,

(5) increased use of medication that leads to weight gain,

(6) Increased distribution of ethnic and age groups that tend to be heavier,

(7) Pregnancy at a later age,

(8) Intrauterine and intergenerational effects,

(9) Positive natural selection of people with a higher BMI,

(10) Assortative mating, heavier people tending to form relationships with each other.


Despite the widespread availability of nutritional information in schools, doctors' offices, on the internet and on product packaging, it is evident that overeating remains a substantial problem. In the period of 1971-2000, obesity rates in the United States increased from 14.5% to 30.9% of the population. During the same time, an increase occurred in the average amount of calories consumed. For women, the average increase was 335 calories per day (1542 calories in 1971 and 1877 calories in 2004); while for men the average increase was 168 calories per day (2450 calories in 1971 and 2618 calories in 2004). Most of these extra calories came from an increase in carbohydrate consumption rather than an increase in fat consumption. Dietary trends have also change with reliance on energy-dense fast-food meals tripling between 1977 and 1995, and calorie intake from fast food quadrupling over the same period.

Sedentary lifestyle

An increasingly sedentary lifestyle plays a significant role in obesity. There has been a trend toward decreased physical activity due in part to increasingly mechanized forms of work, changing modes of transportation, and increasing urbanization. Studies in children and adults have found an association between the number of hours of television watched and the prevalence of obesity. Driving one's children to school also decreases the amount of exercise that these children get and has led to calls for reduced car use around schools. An association between leisure time activity and obesity has been found. For example in Canada, 27.0% of sedentary men are obese as opposed to 19.6% of active men.


Like many other medical conditions, obesity is the result of interplay between genetic and environmental factors. Polymorphisms in various genes controlling appetite, metabolism, and adipokine release may predispose to obesity when sufficient calories are present. Obesity is a major feature in a number of rare genetic conditions: Prader-Willi syndrome, Bardet-Biedl syndrome, MOMO syndrome, leptin receptor mutations, and melanocortin receptor mutations. In a people with early-onset severe obesity (defined by an onset before ten years of age and body mass index over three standard deviations above normal), 7% harbour a single locus mutation. Apart from the above syndromes, an association has been found between an FTO gene polymorphism and weight. The 16% of adults in the study who were homozygous for this allele weighed about 3 kilograms more then those who had not inherited this trait and subsequently had a 1.6 fold greater rate of obesity. A study of 5092 identical twin found that childhood obesity has a strong (77%) inherited component, suggesting that many genetic influences underpinning the development of obesity are yet to be discovered.

On a population level, the thrifty gene hypothesis postulates that certain ethnic groups may be more prone to obesity when exposed to an equivalent environment. Their ability to take advantage of rare periods of abundance by storing energy as fat would be advantageous during times of varying food availability. Individuals with greater adipose reserves would be more likely survive famine. This tendency to store fat however would be maladaptive in societies with stable food supplies.

Medical illness

Certain physical and mental illnesses and the pharmaceutical substances used to treat them can increase one's risk of obesity. Medical illnesses that increase obesity risk include several rare genetic syndromes (listed above) as well as some congenital or acquired conditions: (1) hypothyroidism, (2) Cushing's syndrome, (3) growth hormone deficiency, and (4) eating disorders such bulimia nervosa, binge eating disorder and compulsive overeating.

Certain medications may cause weight gain and or negative changes in body composition, such as steroids, atypical antipsychotics, some fertility medication, insulin and sulfonylureas.


The role of bacteria colonizing the digestive tract in the development of obesity has recently become the subject of investigation. Bacteria participate in digestion (especially of fatty acids and polysaccharides), and alterations in the proportion of particular strains of bacteria may explain why certain people are more prone to weight gain than others. Human digestive tract bacteria are generally either member of the phyla of bacteroidetes or of firmicutes. In obese people, there is a relative abundance of firmicutes (which cause relatively high energy absorption), which is restored by weight loss. From these results it cannot be concluded whether this imbalance is the cause or effect of obesity.

Social determinants

The correlation between social class and BMI is inconsistent. Comparing net worth with BMI found obese Americanians approximately half as wealthy as thin ones. When income differentials were factored out, the inequity persisted. A tendency to rely on fast food is seen as one of the reasons why this result occurred. Another study found women who married into a higher social class are thinner than women who married into a lower social class. The 2004 Canadian Health Survey however found the exact opposite. It found that men from lower middle income households were less likely to be obese than were those in the highest income households and women from middle income households had the highest rates of obesity.


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Thursday, July 21, 2011


Physical exercise
A physical exercise is any bodily activity that enhances or maintains physical fitness and overall health. It is often practiced to strengthen muscles and the cardiovascular system, and to hone athletic skills. Frequent and regular physical exercise boosts the immune system, and helps prevent diseases of affluence such as heart disease, cardiovascular disease, Type 2 diabetes and obesity. It also improves mental health and helps prevent depression.

Types of exercise
Exercises are generally grouped into three types depending on the overall effect they have on the human body:
  • Flexibility exercises such as stretching improve the range of motion of muscles and joints.
  • Aerobic exercises such as cycling, walking, running, hiking or playing tennis focus on increasing cardiovascular endurance.
  • Anaerobic exercises such as weight training, functional training or sprinting increase short-term muscle strength.

Exercise benefits
  • Physical exercise is important for maintaining physical fitness and can contribute positively to maintaining a healthy weight, building and maintaining healthy bone density, muscle strength, and joint mobility, promoting physiological well-being, reducing surgical risks, and strengthening the immune system.
  • Frequent and regular aerobic exercise has been shown to help prevent or treat serious and life-threatening chronic conditions such as high blood pressure, obesity, heart disease, Type 2 diabetes, insomnia, and depression. Strength training appears to have continuous energy-burning effects that persist for about 24 hours after the training, though they do not offer the same cardiovascular benefits as aerobic exercises do.
  • There is conflicting evidence as to whether vigorous exercise (more than 70% of VO2 Max) is more or less beneficial than moderate exercise (40 to 70% of VO2 Max). Some studies have shown that vigorous exercise executed by healthy individuals can effectively increase opioid peptides (aka endorphins, a naturally occurring opiate that in conjunction with other neurotransmitters is responsible for exercise induced euphoria and has been shown to be addictive), positively influence hormone production (i.e., increase testosterone and growth hormone),[6] benefits that are not as fully realized with moderate exercise.
  • Exercise has been shown to improve cognitive functioning via improvement of hippocampus-dependent spatial learning, and enhancement of synaptic plasticity and neurogenesis. In addition, physical activity has been shown to be neuro protective in many neurodegenerative and neuromuscular diseases. For instance, it reduces the risk of developing dementia. Furthermore, anecdotal evidence suggests that frequent exercise may reverse alcohol-induced brain damage.
  • Physical activity is thought to have other beneficial effects related to cognition as it increases levels of nerve growth factors, which support the survival and growth of a number of neuronal cells.
  • Both aerobic and anaerobic exercise also work to increase the mechanical efficiency of the heart by increasing cardiac volume (aerobic exercise), or myocardial thickness (strength training, see Organ hypertrophy).
  • Not everyone benefits equally from exercise. There is tremendous variation in individual response to training: where most people will see a moderate increase in endurance from aerobic exercise, some individuals will as much as double their oxygen uptake, while others will never get any benefit at all from the exercise. Similarly, only a minority of people will show significant muscle growth after prolonged weight training, while a larger fraction experience improvements in strength. This genetic variation in improvement from training is one of the key physiological differences between elite athletes and the larger population. Studies have shown that exercising in middle age leads to better physical ability later in life.

Common misconceptions
Many myths have arisen surrounding exercise, some of which have a basis in reality, and some which are completely false. Myths include:
  • That excessive exercise can cause immediate death. Death by exercise has some small basis in fact. Water intoxication can result from prolific sweating (producing electrolyte losses) combined with consumption of large amounts of plain water and insufficient replenishment of electrolytes, especially salt and potassium (e.g. when running a marathon). It is also possible to die from a heart attack or similar affliction if overly intense exercise is performed by someone who is not at an appropriate level of fitness for that particular activity. A doctor should always be consulted before any radical changes are made to a person's current exercise regimen. Rhabdomyolysis is also a risk. Other common dangers may occur from extreme overheating or aggravation of a physical defect, such as a thrombosis or aneurysm.
  • That weightlifting makes you short or stops growth. One caveat is that heavy weight training in adolescents can damage the epiphyseal plate of long bones.

Targeted fat reduction
Spot reduction is a myth that exercise and training a particular body part will preferentially shed the fat on that part; for example, that doing sit-ups is the most direct way to reduce subcutaneous belly fat. This is false: one cannot reduce fat from one area of the body to the exclusion of others. Most of the energy derived from fat gets to the muscle through the bloodstream and reduces stored fat in the entire body, from the last place where fat was deposited. Sit-ups may improve the size and shape of abdominal muscles but will not specifically target belly fat for loss. Such exercise might help reduce overall body fat and shrink the size of fat cells. There is a very slight increase in the fat burnt at the area being exercised (e.g. abs) compared with the rest of the body, due to the extra blood flow at this area.

Muscle and fat tissue
Some people incorrectly believe that muscle tissue will turn into fat tissue once a person stops exercising. This is not literally true — fat tissue and muscle tissue are fundamentally different — but the common expression that "muscle will turn to fat" is truthful in the sense that catabolism of muscle fibres for energy can result in excess glucose being stored as fat. Moreover, the composition of a body part can change toward less muscle and more fat, so that a cross-section of the upper-arm for example, will have a greater area corresponding to fat and a smaller area corresponding to muscle. This is not muscle "turning into fat" however — it is simply a combination of muscle atrophy and increased fat storage in different tissues of the same body part. Another element of increased fatty deposits is that of diet, as most trainees will not significantly reduce their diet in order to compensate for the lack of exercise/activity.

Excessive exercise
  • Exercise is a stressor and the stresses of exercise have a catabolic effect on the body - contractile proteins within muscles are consumed for energy, carbohydrates and fats are similarly consumed and connective tissues are stressed and can form micro-tears. However, given adequate nutrition and sufficient rest to avoid overtraining, the body's reaction to this stimulus is to adapt and replete tissues at a higher level than that existing before exercising. The results are all the training effects of regular exercise: increased muscular strength, endurance, bone density, and connective tissue toughness.
  • Too much exercise can be harmful. The body parts exercised need at least a day of rest, which is why some health experts say one should exercise every other day or 3 times a week. Without proper rest, the chance of stroke or other circulation problems increases, and muscle tissue may develop slowly. It has also been noted by the medical field that expectant mothers should never exercise two days consecutively.
  • Inappropriate exercise can do more harm than good, with the definition of "inappropriate" varying according to the individual. For many activities, especially running, there are significant injuries that occur with poorly regimented exercise schedules. In extreme instances, over-exercising induces serious performance loss. Unaccustomed overexertion of muscles leads to rhabdomyolysis (damage to muscle) most often seen in new army recruits. Another danger is overtraining in which the intensity or volume of training exceeds the body's capacity to recover between bouts.
  • Stopping excessive exercise suddenly can also create a change in mood. Feelings of depression and agitation can occur when withdrawal from the natural endorphins produced by exercise occurs. Exercise should be controlled by each body's inherent limitations. While one set of joints and muscles may have the tolerance to withstand multiple marathons, another body may be damaged by 20 minutes of light jogging. This must be determined by each individual.
  • Too much exercise can also cause a female to miss her period, a symptom known as amenorrhea.

Nutrition and recovery
  • Proper nutrition is at least as important to health as exercise. When exercising, it becomes even more important to have a good diet to ensure that the body has the correct ratio of macronutrients whilst providing ample micronutrients, in order to aid the body with the recovery process following strenuous exercise.
  • Proper rest and recovery are also as important to health as exercise; otherwise the body exists in a permanently injured state and will not improve or adapt adequately to the exercise. Hence, it is important to remember to allow adequate recovery between exercise sessions.
  • The above two factors can be compromised by psychological compulsions (eating disorders such as exercise bulimia, anorexia, and other bulimias), misinformation, a lack of organization, or a lack of motivation. These all lead to a decreased state of health.
  • Delayed onset muscle soreness can occur after any kind of exercise, particularly if the body is in an unconditioned state relative to that exercise.

Exercise and brain function
In the long term, exercise is beneficial to the brain by:
  • Increasing the blood and oxygen flow to the brain
  • Increasing growth factors that help create new nerve cells and promote synaptic plasticity
  • Increasing chemicals in the brain that help cognition, such as dopamine, glutamate, norepinephrine, and serotonin

Categories of physical exercise
  • Aerobic exercise
  • Anaerobic exercise
  • Strength training
  • Agility training

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