OBESITY
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)
BMI
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.
Causes
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.
Dietary
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.
Genetics
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.
Microbiological
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.
Treatment
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